US 6331414 B1
Human insulin-like growth factors is synthesized in recombinant cell culture by host cells transformed with expression vectors bearing DNA encoding human insulin-like growth factors.
1. A process for producing human IGF-I comprising preparing a replicable expression vector capable of expressing the DNA sequence encoding human IGF-I in a prokaryotic host cell, transforming a prokaryotic host cell culture with said vector to obtain a recombinant host cell, culturing said recombinant host cell culture under conditions permitting expression of said human IGF-I-encoding DNA sequence to produce human IGF-I, and recovering said human IGF-I.
2. The process of claim 1 wherein the prokaryote is E. coli.
3. The process of claim 1 wherein the IGF-I is recovered from refractile bodies.
4. The process of claim 1 wherein the prokaryotic host cell is a bacterial host cell.
5. A method for producing human IGF-I comprising preparing a replicable expression vector capable of expressing in prokaryotic cells a DNA sequence encoding a fusion protein comprising the amino acid sequence of mature human IGF-I and a bacterial protein, transforming prokaryotic cells with said vector, culturing said transformed cells under conditions permitting expression of said DNA sequence to produce the fusion protein, recovering the fusion protein from the culture, and cleaving the fusion protein to obtain mature human IGF-I, wherein the prokaryotic cells are capable of such expression and of processing the IGF-I.
6. The method of claim 5 wherein the IGF-I is recovered from refractile bodies.
7. The method of claim 5 wherein the prokaryotic cells are bacterial cells.
8. A process for producing mature human IGF-I comprising culturing a recombinant prokaryotic host cell transformed with a replicable expression vector capable of expressing a DNA sequence encoding human IGF-I, under conditions permitting expression of said encoding DNA sequence, and recovering therefrom mature human IGF-I having the proper amino terminus (gly).
9. A process for producing mature human IGF-I comprising culturing a recombinant prokaryotic host cell, transformed with a replicable expression vector capable of expressing in a suitable host cell a DNA sequence encoding a fusion protein comprised of human IGF-I fused at the N-terminus of the IGF-I to amino acid sequence exogenous to human IGF-I, under conditions permitting expression of the DNA sequence, and cleaving the fusion protein to release mature human IGF-I having the proper amino terminus (gly).
10. A process according to claim 9 wherein the cleavage is effected with an enzyme.
This is a continuation of application Ser. No. 06/506,078 filed on Jun. 20, 1983, abandoned which is a continuation-in-part of Ser. No. 06/501,353 filed Jun. 6, 1983, now abandoned, which applications are incorporated herein by reference and to which applications priority is claimed under 35 USC §120.
This application is related to commonly assigned applications filed concurrently herewith under U.S. Ser. Nos. 506,077 abandoned and 506,098 abandoned and their parents U.S. Ser. Nos. 501,351 abandoned and 501,352 abandoned.
This invention relates to the preparation of human IGF (insulin-like growth factor), in various forms, via recombinant DNA technology. Notably, the present invention provides for the preparation of human IGF as a mature protein product of expression, processing, and secretion in a recombinant DNA modified host organism. This invention thus provides for the production, isolation, and use of human IGF, in its various forms, as well as to the associated recombinant DNA technology by which it is prepared. In addition, the present invention relates to the similar preparation of a related protein, human EGF (Epidermal Growth Factor).
The present invention arises in part from the discovery of a novel system by which human IGF can be prepared by a recombinant host organism in the form of a discrete, mature protein. This is accomplished according to one aspect of the present invention by an expression system which permits the expression of the amino acid sequence of human IGF fused with at least a portion of the yeast alpha factor signal sequence, followed by processing of said signal sequence, and secretion of mature human IGF protein into the medium supporting the host organism. Thus, this novel aspect of the present invention, it is believed for the first time, permits the preparation, isolation, and utilization of human IGF as a discrete, mature protein. The present invention, in its broad compass, however, covers the preparation of the amino acid sequence of human IGF in other recombinant systems including bacteria and cell culture and includes, therefore, the expression of human IGF DNA sequences providing not only mature human IGF but also fusion product derivatives containing the amino acid sequence of IGF as the essential component. All such products have been found to be biologically active, hence useful as intended.
The publications and other materials hereof used to illuminate the background of the invention, and in particular cases, to provide additional details concerning its practice are incorporated herein by this reference and for convenience, are alphabetically and numerically referenced in the following text and respectively grouped in the appended bibliography.
A. Human IGF (Insulin-like Growth Factor)
Human IGF has been the subject of a fair amount of intensive study by past workers. A body of literature has been developed related to various aspects of this protein or series of proteins (see references A through L).
Insulin-like growth factors I and II have been isolated from human serum (A). The designation “insulin-like growth factor” or IGF was chosen to express the insulin-like effects and the insulin-like structure of these polypeptides which act as mitogens on a number of cells. The complete amino acid sequences of IGF-I and IGF-II have been determined (D,E). They are both single-chain polypeptides with three disulphide bridges and a sequence identity of 49 and 47 percent respectively, to human insulin A and B chains. The connecting peptide or C region is considerably shorter than the one of proinsulin and does not show any significant homology to it. (For a summary of earlier studies on the biological efforts of IGF, see Reference F).
IGF-I and IGF-II are growth promoting polypeptides occurring in human serum and human cerebral spinal fluid. Their structure is homologous to proinsulin. IGF-I seems to be produced by the liver along with a specific IGF-binding protein both of which are under control of growth hormone. Thus, human IGF is considered to be an active growth promoting molecule that mediates the effect of human growth hormone.
It was perceived that the application of recombinant DNA and associated technologies would be a most effective way of providing the requisite large quantities of high quality human IGF for applied use to human beings as a growth factor. The goal was to produce human IGF either as biologically active fusion protein, or more importantly, as a mature protein, as products of recombinant DNA technology from a host organism. Such materials would exhibit bioactivity admitting of their use clinically in the treatment of various growth affected conditions.
B. Recombinant DNA Technology
Recombinant DNA technology has reached the age of some sophistication. Molecular biologists are able to recombine various DNA sequences with some facility, creating new DNA entities capable of producing copious amounts of exogenous protein product in transformed microbes and cell cultures. The general means and methods are in hand for the in vitro ligation of various blunt ended or “sticky” ended fragments of DNA, producing potent expression vehicles useful in transforming particular organisms, thus directing their efficient synthesis of desired exogenous product. However, on an individual product basis, the pathway remains somewhat tortuous and the science has not advanced to a stage where regular predictions of success can be made. Indeed, those who portend successful results without the underlying experimental basis, do so with considerable risk of inoperability.
DNA recombination of the essential elements, i.e., an origin of replication, one or more phenotypic selection characteristics, an expression promoter, heterologous gene insert and remainder vector, generally is performed outside the host cell. The resulting recombinant replicable expression vehicle, or plasmid, is introduced into cells by transformation and large quantities of the recombinant vehicle are obtained by growing the transformant. Where the gene is properly inserted with reference to portions which govern the transcription and translation of the encoded DNA message, the resulting expression vehicle is useful to actually produce the polypeptide sequence for which the inserted gene codes, a process referred to as expression. The resulting product may be obtained by lysing, if necessary, the host cell, in microbial systems, and recovering the product by appropriate purification from other proteins.
In practice, the use of recombinant DNA technology can express entirely heterologous polypeptides—so-called direct expression—or alternatively may express a heterologous polypeptide fused to a portion of the amino acid sequence of a homologous polypeptide. In the latter cases, the intended bioactive product is sometimes rendered bioinactive within the fused, homologous/heterologous polypeptide until it is cleaved in an extracellular environment. See references (M) and (N).
Similarly, the art of cell or tissue cultures for studying genetics and cell physiology is well established. Means and methods are in hand for maintaining permanent cell lines, prepared by successive serial transfers from isolated normal cells. For use in research, such cell lines are maintained on a solid support in liquid medium, or by growth in suspension containing support nutriments. Scale-up for large preparations seems to pose only mechanical problems. For further background, attention is directed to references (O) and (P).
Likewise, protein biochemistry is a useful, indeed necessary, adjunct in biotechnology. Cells producing the desired protein also produce hundreds of other proteins, endogenous products of the cell's metabolism. These contaminating proteins, as well as other compounds, if not removed from the desired protein, could prove toxic if administered to an animal or human in the course of therapeutic treatment with desired protein. Hence, the techniques of protein biochemistry come to bear, allowing the design of separation procedures suitable for the particular system under consideration and providing a homogeneous product safe for intended use. Protein biochemistry also proves the identity of the desired product, characterizing it and ensuring that the cells have produced it faithfully with no alterations or mutations. This branch of science is also involved in the design of bioassays, stability studies and other procedures necessary to apply before successful clinical studies and marketing can take place.
The present invention is based upon the discovery that recombinant DNA technology can be used successfully to produce human IGF and related protein, human EGF, preferably in direct form and in amounts sufficient to initiate and conduct animal and clinical testing as prerequisites to market approval. The products human IGF and EGF are suitable for use in all of their forms as produced according to the present invention, viz. in the prophylactic or therapeutic treatment of human beings for various growth associated conditions or diseases. Accordingly, the present invention, in one important aspect, is directed to methods of treating growth conditions in human subjects using human IGF or human EGF, and suitable pharmaceutical compositions thereof, prepared in accordance with the methods and means of the present invention.
The present invention further comprises essentially pure, mature human IGF, as a product of expression, processing, and secretion in a recombinant host organism. Such human IGF is free from association with N-terminus amino acid sequence derivable from the expression systems that can be employed to prepare the material. Thus, while the present invention is directed to the preparation of polypeptides comprising the amino acid sequence of IGF, a notable aspect of the present invention involves the production of the mature human IGF directly into the medium of the recombinant host organism employed. The present invention is also directed to replicable DNA expression vehicles harboring gene sequences encoding human IGF and human EGF in expressible form, to microorganism strains or cell cultures transformed with such vehicles and to microbial or cell cultures of such transfomants capable of producing amino acid sequences of human IGF and human EGF. In still further aspects, the present invention is directed to various processes useful for preparing said genes sequences, DNA expression vehicles, microorganisms and cell cultures and specific embodiments thereof. Still further, this invention is directed to the preparation of fermentation cultures of said microorganisms and cell cultures.
FIG. 1 represents the chemically synthesized DNA strands used in the construction of expression vectors for human IGF.
FIG. 2 shows the completed double stranded DNA of FIG. 1.
FIG. 3 show the fragments of DNA of FIG. 2 after restriction by EcoRI and PstI and BamHI.
FIG. 4 depicts the ligation of parts 1 and 2 of FIG. 3 into PBR322.
FIG. 5 show parts 3 and 4 of IGF-I right half.
FIG. 6 depicts the ligation of parts 3 and 4 of FIG. 5 into the vector of FIG. 4.
FIG. 7 shows a sequence of DNA and deduced fusion protein containing IGF-I.
FIG. 8 shows a sequence of DNA and deduced short fusion protein containing IGF-I.
FIG. 9 depicts a plasmid used in the present construction.
FIG. 10 shows the DNA and protein sequence of IGF-I fused with alpha factor pre-pro sequence.
FIG. 11 is a vector containing alpha factor promotor and pre-pro sequence fused to IGF-I.
FIG. 12 shows the yeast invertase signal fused to IGF-I.
FIG. 13 shows the parental plasmid containing the yeast PGK promotor.
FIG. 14 depicts a yeast expression vector containing PGK promotor, invertase signal and human IGF-I gene.
FIG. 15 is the synthetic DNA used to construct the coding sequence of mature human EGF.
FIG. 16 depicts the yeast alpha factor “pre-pro” sequence fused to the human EGF coding sequence.
FIG. 17 depicts the yeast invertase signal sequence fused to the human EGF coding sequence.
FIG. 18 shows the coding sequence for human IGF-II.
FIG. 19 illustrates the structure of pools of synthetic oligonucleotides used as hybridization probes to isolate the gene for α-factor.
FIGS. 20A and 20B illustrates the results of electrophoresis of DNA fragments obtained using the probes of FIG. 19.
FIGS. 21A and 21B and 22 are the nucleotide sequences of α-factor genes.
FIGS. 23A and 23B illustrates the scheme for joining the gene for human interferon D with the gene for the α-factor promoter and signal sequence.
FIG. 24 illustrates the scheme for construction of a yeast/E. coli shuttle vector for use as a starting plasmid herein for expression of heterologous genes supplying the α-factor promoter and signal polypeptide gene sequences.
As used herein, “human IGF” and “human EGF” denotes human insulin-like growth factor and human epidermal growth factor, produced by microbial or cell culture systems and bioactive forms comprising the amino acid sequence corresponding to human IGF and human EGF otherwise native to human tissue. The human IGF and EGF proteins produced herein have been defined by means of DNA, gene, and deductive amino acid sequencing. It will be understood that inasmuch as natural allelic variations exist and occur from individual to individual, as demonstrated by (an) amino acid difference(s) in the overall sequence or by deletions, substitutions, insertions, inversions, or additions of one or more amino acids of said sequences, the present invention is intended to embrace all of such allelic variations of the two molecules involved. In addition, the location of and the degree of glycosylation depend upon the nature of the recombinant host organism employed and such variations as may occur are included within the ambit of this invention. Finally, the potential exists in the use of DNA technology for the preparation of various derivatives of human IGF and human EGF by simple modification of the underlying gene sequence for such molecules. Such modifications could be accomplished by means of site directed mutagenesis of the underlying DNA, as an example. All such modifications resulting in derivatives of human IGF and human EGF are included within the scope of the present invention so long as the essential characteristic human IGF and human EGF activities remain unaffected in kind.
“Essentially pure form” when used to describe the state of human IGF or human EGF produced by this invention means that the proteins are free of proteins or other materials normally associated with human IGF or human EGF when produced by non-recombinant cells, i.e. in their “native” environments.
“Expression vector” includes vectors which are capable of expressing DNA sequences contained therein, where such sequences a operably linked to other sequences capable of effecting their expression, i.e., promotorloperator sequences. In sum, “expression vector” is given a functional definition: any DNA sequence which is capable of effecting expression of a specified DNA code disposed therein. In general, expression vectors of utility in recombinant DNA techniques are often in the form of “plasmids” which refer to circular double stranded DNA loops which in their vector form are not bound to the chroymosone. In the present specification, “plasmid” and “vector”, are used interchangably as the plasmid is the most commonly used form of vector. However, the invention is intended to include such other forms of expression vectors which function equivalently and which become known in the art subsequently.
“Recombinant host cells” refers to cells which have been transformed with such vectors. Thus, the human IGF and human EGF molecules produced by such cells can be referred to as “recombinant human IGF” and “recombinant human EGF”.
B. Host Cell Cultures and Vectors
The vectors and methods disclosed herein are suitable for use in host cells over a wide range of prokaryotic and eukaryotic organisms.
In general, of course, prokaryotes are preferred for cloning of DNA sequences in constructing the vectors useful in the invention. For example, E. coli K12 strain 294 (ATCC No. 31446) is particularly useful. Other microbial strains which may be used include E. coli strains such as E. coli B and E. coli X1776 (ATTC No. 31537). The aforementioned strains, as well as E. coli W3110 (F−, λ−), prototrophic, ATTC No. 27325), bacilli such as Bacillus subtilus, and other enterobacteriaceae such as Salmonella typhimurium or Serratia marcesans, and various pseudomonas species may be used. These examples are, of course, intended to be illustrative rather than limiting.
In general, plasmid vectors containing replicon and control sequences which are derived from species compatible with the host cell are used in connection with these hosts. The vector ordinarily carries a replication site, as well as marking sequences which are capable of providing phenotypic selection in transformed cells. For example, E. coli is typically transformed using pBR322, a plasmid derived from an E. coli species (Bolivar, et al., Gene 2: 95 (1977)). pBR322 contains genes for ampicillin and tetracycline resistance and thus provides easy means for identifying transformed cells. The pBR322 plasmid, or other microbial plasmid must also contain, or be modified to contain, promoters which can be used by the microbial organism for expression of its own proteins. Those promoters most commonly used in recombinant DNA construction include the β-lactamase (penicillinase) and lactose promoter systems (Chang et al, Nature, 275: 615 (1978), Itakura, et al, Science, 198: 1056 (1977); (Goeddel, et al Nature 281: 544 1979)) and a tryptophan (trp) promoter system (Goeddel, et al , Nucleic Acids Res., 8: 4057 (1980); EPO Appl Publ No. 0036776). While these are the most connonly used, other microbial promoters have been discovered and utilized, and details concerning their nucleotide sequences have been published, enabling a skilled worker to ligate them functionally with plasmid vectors (Siebenlist, et al, Cell 20: 269 (1980)).
In addition to prokaryotes, eukaryotic microbes, such as yeast cultures, may also be used. Saccharomyces cerevisiae, or common baker's yeast, is the most commonly used among eukaryotic microorganisms, although a number of other strains are commonly available. For expression in Saccharomyces, the plasmid YRp7, for example, (Stinchcomb, et al, Nature, 282: 39 (1979); Kingsman et al, Gene, 7: 141 (1979); Tschemper, et al, Gene, 10: 157 (1980)) is commonly used. This plasmid already contains the trp1 gene which provides a selection marker for a mutant strain of yeast lacking the ability to grow in tryptophan, for example ATCC No. 44076 or PEP4-1 (Jones, Genetics, 85: 12 (1977)). The presence of the trp1 lesion as a characteristic of the yeast host cell genome then provides an effective environment for detecting transformation by growth in the absence of tryptophan.
Suitable promoting sequences in yeast vectors include the promoters for 3-phosphoglycerate kinase (Hitzeman, et al., J. Biol. Chem., 255: 2073 (1980)) or other glycolytic enzymes (Hess, et al, J. Adv. Enzyme Reg., 7: 149 (1968); Holland, et al, Biochemistry, 17: 4900 (1978)), such as enolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isomerase, phosphoglucose isomerase, and glucokinase. In constructing suitable expression plasmids, the termination sequences associated with these genes are also ligated into the expression vector 3′ of the sequence desired to be expressed to provide polyadenylation of the mRNA and termination. Other promoters which have the additional advantage of transcription controlled by growth conditions are the promoter regions for alcohol dehydrogenase 2, isocytochrome C, acid phosphatase, degradative enzymes associated with nitrogen metabolism, and the aforementioned glyceraldehyde-3-phosphate dehydrogenase, and enzymes responsible for maltose and galactose utilization (Holland, ibid.). Any plasmid vector containing yeast-compatible promoter, origin of replication and termination sequences is suitable.
In addition to microorganisms, cultures of cells derived from multicellular organisms may also be used as hosts. In principle, any such cell culture is workable, whether from vertebrate or invertebrate culture. However, interest has been greatest in vertebrate cells, and propagation of vertebrate cells in culture (tissue culture) has become a routine procedure in recent years [Tissue Culture, Academic Press, Kruse and Patterson, editors (1973)]. Examples of such useful host cell lines are VERO and HeLa cells, Chinese hamster ovary (CHO) cell lines, and W138, BHK, COS-7 and MDCK cell lines. Expression vectors for such cells ordinarily include (if necessary) an origin of replication, a promoter located in front of the gene to be expressed, along with any necessary ribosome binding sites, RNA splice sites, polyadenylation site, and transcriptional terminator sequences.
For use in mammalian cells, the control functions on the expression vectors are often provided by viral material. For example, commonly used promoters are derived from polyoma, Adenovirus 2, and most frequently Simian Virus 40 (SV40). The early and late promoters of SY40 virus are particularly useful because both are obtained easily from the virus as a fragment which also contains the SY40 viral origin of replication (Fiers, et al, Nature, 273: 113 (1978) incorporated herein by reference). Smaller or larger SV40 fragments may also be used, provided there is included the approximately 250 bp sequence extending from the HindIII site toward the Bgl I site located in the viral origin of replication. Further, it is also possible, and often desirable, to utilize promoter or control sequences normally associated with the desired gene sequence, provide such control sequences are compatible with the host cell systems.
An origin of replication may be provided either by construction of the vector to include an exogenous origin, such as may be derived from SV40 or other viral (e.g. Polyoma, Adeno, VSV, BPV, etc.) source, or may be provided by the host cell chromosomal replication mechanism. If the vector is integrated into the host cell chromosome, the latter is often sufficient.
C. Methods Employed
If cells without formidable cell wall barriers are used as host cells, transfection is carried out by the calcium phosphate precipitation method as described by Graham and Van der Eb, Virology, 52: 546 (1978). However, other methods for introducing DNA into cells such as by nuclear injection or by protoplast fusion may also be used.
If prokaryotic cells or cells which contain substantial cell wall constructions are used, the preferred method of transfection is calcium treatment using calcium chloride as described by Cohen, F. N. et al Proc. Natl. Acad. Sci. (USA), 69: 2110 (1972).
Construction of suitable vectors containing the desired coding and control sequences employ standard ligation techniques. Isolated plasmids or DNA fragments are cleaved, tailored, and religated; in the form desired to form the plasmids required.
Cleavage is performed by treating with restriction enzyme (or enzymes) in suitable buffer. In general, about 1 μg plasmid or DNA fragments is used with about 1 unit of enzyme in about 20 μl of buffer solution. (Appropriate buffers and substrate amounts for particular restriction enzymes are specified by the manufacturer.) Incubation times of about 1 hour at 37° C. are workable. After incubations, protein is removed by extraction with phenol and chloroform, and the nucleic acid is recovered from the aqueous fraction by precipitation with ethanol.
If blunt ends are required, the preparation is treated for 15 minutes at 15° with 10 units of Polymerase I (Klenow), phenol-chloroform extracted, and ethanol precipitated.
Size separation of the cleaved fragments is performed using 6 percent polyacrylamide gel described by Goeddel, D., et al, Nucleic Acids Res., 8: 4057 (1980) incorporated herein by reference.
For ligation approximately equimolar amounts of the desired components, suitably end tailored to provide correct matching, are treated with about 10 units T4 DNA ligase per 0.5 μg DNA. (When cleaved vectors are used as components, it may be useful to prevent religation of the cleaved vector by pretreatment with bacterial alkaline phosphatase.)
For analysis to confirm correct sequences in plasmids constructed, the ligation mixtures are used to transform E. coli K12 strain 294 (ATCC 31446), and successful transformants selected by ampicillin resistance where appropriate. Plasmids from the transformants are prepared, analyzed by restriction and/or sequenced by the method of Messing, et al, Nucleic Acids Res., 9:309 (1981) or by the method of Maxam, et al, Methods in Enzymology, 65:499 (1980).
The following examples are intended to illustrate but not to limit the present invention.
Enzymes were obtained from the following suppliers:
New England Biolabs: restriction enzymes, T4 DNA ligase
Bethesda Research Labs: restriction enzymes, Bact. Alkaline Phos.
Boehringer-Mannheim: E. coli DNA Polymerase I (Klenow)
P+L Biochemicals: Polynucleotide kinase, Terminal Nucleotidyl Transferase
New England Nuclear:
pBR322 [oligo(dG)-tailed] DNA
BioRad: Bis Acrylamide, Acrylamide, TEMED
Sigma: Ammonium Persulfate
Amersham: 10218 γ32P ATP >5000 Ci/mmol; 10165 α32P dCTP >400 Ci/mmol.
Solutions and Media: 1×TBE: 0.54M Tris Base, 0.54 M Boric Acid, 0.017 M Na2 EDTA.
Difco: Yeast Nitrogenous Base (YNB); Tryptoa , Yeast Extract; Bacto-Agar; Casamino Acids.
Kodak X-0 mat AR XAR-2 Film
0.45-0.50 mM B. Braun Melsungen AG
LB medium (per liter):
10 g NaCl; 5 g yeast extract; 10 g tryptone; 0.17 ml NaOH (50 percent)
LB Agar (per liter):
10 g tryptone; 5 g yeast extract; 0.5 g NaCl; 15 g Bacto-Agar; adjusted to pH 7.5 with NaOH.
Tetracycline (5 μg/ml) in all mediums; Ampicillin (20 μg/ml) in all mediums (plates or liquid)
M9 Medium (per liter):
6 g Na2HPO4 (anhydrous); 1 g NH4Cl; 3 g KH2PO4; 0.5 g NaCl; 1 mM MgSO4; 0.5 percent (w/v) glucose; 0.5 percent (w/v) Casamino Acids; 0.0001 percent Thiamine-HCl.
YNB-CAA (per liter):
6.7 g Yeast Nitrogenous Base (without Amino Acids); 10 mg adenine; 10 mg uracil; 5 g Casamino Acids; 20 g Glucose.
YNB-CAA agar plates (per liter):
Same as YNB-CAA+30 g agar.
Standard Ligation Conditions:
10-fold molar excess of insert (or linker) to vector. 1×T4 DNA ligase buffer and 400-800 U T4 DNA ligase; 14°—12-16 hours.
Standard Kination Conditions:
1×Polynucleotide kinase buffer; 15 U polynucleotide kinase; 37° 60 minutes; followed by reaction termination by heating to 65° for 10 minutes.
70 mM Tris-HCl (pH 7.6); 10 mM MgCl2; 5 mM DTT
1×T4 DNA Ligase Buffer:
50 mM Tris-HCl (pH 7.8); 10 mM MgCl2; 20 mM DTT; 1 mM rATP.
The 1° protein structure of the human IGF-1 molecule has been determined (1). Based upon this protein sequence and the genetic code, a DNA sequence coding for mature human IGF-1 protein, including all possible base substitutions at any one base position was determined by computer analysis (Genentech Untrans Program). Using a restriction site analysis program (Genentech Asearch Program), all potential restriction sites located in all possible DNA sequences consistently coding for the same protein were found. Three sites internal to the coding sequence were selected: PstI, BamHI, and AvaII. Two additional sites were placed at the ends, just outside of the coding sequence of the mature protein: one EcoRI site before the initiation codon, AUG, and the SalI site following the termination codon, TAG, of the coding sequence. The choice of these sites facilitated the cloning of the coding sequence in separate parts, each of which subsequently could be excised and then assembled to form an intact synthetic IGF-1 gene. This construction involved the assembly of 4 parts, 2 parts forming the left half, 2 parts forming the right half. Each part consisted of two single strands of chemically synthesized DNA (see FIG. 1). Proposed synthetic fragments were also analyzed for internal complementarity.
The constructions used to generate these four parts employed the use of DNA Polymerase I repair synthesis of synthetic oligonucleotide substrates having 9-10 bp stretches of complementary sequence at their 3′ termini. In the presence of DNA Polymerase I (Klenow) and the four deoxynucleoside triphosphates, these primer-templates were extended to become full-length double-stranded DNAs. To prevent priming at locations other than the desired portions as well as self-hybridizations, each set of single-stranded DNAs were analyzed by a computer program (Genentech Homology Program), and wherever possible, sequences which would have potentially led to hairpin loops, self-priming, or mis-priming, were eliminated by alternate codon usage. Each of these four double-stranded DNAs were synthesized to include 9-12 additional bp of non-IGF-1 coding DNA at each end (see FIG. 2). This additional DNA was included to allow generation of sticky ends by restriction enzyme digestion. The sticky ends thus formed facilitated the ligation of the double-stranded pieces to contiguous coding section of the synthetic gene or into a cloning vehicle.
The 9-12 extra bp of double stranded DNA beyond the restriction site at the end of each part (see FIG. 2) allowed for the TdT-mediated formation of single-stranded oligodeoxycytidine strands at the 3′ ends of each double-stranded DNA section. These oligodeoxycytidine tailed double-stranded DNAs could then be annealed into a complementary oligodeoxyguanosine tailed PstI site of a cloning vehicle. Once cloned, and sequenced to ensure the correct base sequences, the parts could be easily isolated and ligated following restriction enzyme cleavage at the restriction sites selected at the ends of each of the four parts, to form the intact synthetic IGF-1 gene.
The method used successfully here was similar to that described by Rossi et al. (28); however, attempts at the construction and cloning of the IGF-1 coding sequence using the Rossi et al. method (28) with only two base pairs of extra DNA beyond the restriction enzyme recognition sites repeatedly failed. The method employed here also differs from the Rossi et al. procedure (28) in that restriction sites placed at both ends of a double stranded DNA allow for the convenience of cloning each double stranded DNA fragment, individually, by (dC)-tailing and annealing into a (dG)-tailed. vector, a method which in practice requires less of the double stranded DNA than three-part ligations.
Eight fragments, 43, 43, 46, 46, 46, 46, 54, and 46 bases in length (see FIG. 1), were chemically synthesized according to the method of Crea and Horn (2), the only change being the use of mesitylene nitrotriazole as the condensing agent rather than 2,4,6-trnisopropyl benzenesulfonylchloride tetrazole.
The syntheses of the fragments were accomplished from the appropriate solid support (cellulose) by sequential addition of the appropriate fully protected dimer- or trimer-blocks. The cycles were carried out under the same conditions as described in the synthesis of oligothymidilic acid (see Crea et al., supra). The final polymers were treated with base (aqueous conc. NH3) and acid (80 percent HoAc), the polymer was pelleted off, and the supernatant was evaporated to dryness. The residue, dissolved in 4 percent aq. NH3, was washed with ethyl ester (3×) and used for the Isolation of the fully deprotected fragment.
Purification was accomplished by electrophoresis using 20 percent polyacrylamide gels. The pure oligonucleotide was ethanol precipitated following gel elution.
225-285 pmoles of each chemically synthesized fragment was mixed with an equivalent amount of the complementary single-stranded DNA fragment (i.e. 1L+3L; 2L+4L; 1R+3R; 2R+4R) in the presence of deoxyribonucleoside triphosphates at a final concentration of 200 μM (with the exception of dCTP. dCTP was added to a concentration of 5 μM as a α32P-labeled isotope with a specific activity of 1000-2000 Ci/mmol) to allow easy monitoring of the repair-synthesis reaction product. The reactions were carried out in a buffer containing a final concentration of 50 mM Tris HCl pH 7.5; 20 mM MgCl2; 20 mM DTT and 154 DNA Polymerase I (Klenow) in a reaction volume of 200 μl. Reactions were allowed to proceed at 4° for 12-18 hrs.
Upon completion, EDTA was added to a concentration of 25 mM. Sample buffer containing the mixes were phenol extracted, CHCl3 extracted 2×, and products were etOH precipitated. Pellets were taken up in 0.3 M NaOAc and the DNA was reprecipitated with etOH. After dissolving the pellets in H2O, the 1L+3L and 2L+4L products were then digested separately with PstI in 100 μl reaction mixes containing 1×PstI buffer (50 mM (NH4)2SO4, 20 mM Tris HCl pH 7.5, 10 mM MgCl2), and 70 U PstI. After 4 hrs, EDTA was added to a concentration of 10 mM, and the material was ethanol precipitated. Pellets were then taken up In 0.3 M NaOAc and reprecipitated, then taken up in H2O. The PstI-digested 1L+3L product was digested with EcoRI at 37° in a 100 μl reaction mix 1×EcoRI buffer (150 mM NaCl, 6 mM Tris HCl pH 7.5, 6 mM MgCl2) and 70 U EcoRI. The PstI digested 2L+4L product was digested at 37° with BamHI in a 100 μl reaction mix in 1×BamHI Buffer (150 mM NaCl, 6 mM Tris HCl pH 7.9, 6 mM MgCl2) and 70 U BamHI. After 4 hrs, EDTA was added to both mixtures, and sample buffer was added. They were electrophoresed on a 6 percent polyacrylamide slab gel. Six percent slab gels were cast with a mixture containing 6 percent (w/v) acrylamide (20 to 1 ratio of acrylamide to Bis acrylamide) 1×TBE, 1 percent APS and 0.1 percent TEMED. Reaction products were located on the gel by autoradiography and the band corresponding to the 45 bp EcoRI-PstI digested 1L+3L product (Part 1) (see FIG. 3) and the band corresponding to the 50 bp PstI-BamHI digested 2L+4L. product (Part 2) (see FIG. 3) were excised from the gel, and the material was electroeluted in 0.2×TBE, phenol extracted, CHCl3 extracted, and ethanol precipitated. Parts 1 and 2 were dissolved in H2O.
Cloning vector was prepared by digesting 20 μg pBR322 (15) with 50 U EcoRI and 60 U BamHI, in 1×RI Buffer at 37° for 6 hr. After addition of EDTA to a concentration of 10 mM, sample buffer was added, and the mixture was run on a 5 percent polyacrylamide gel. The gel was developed by staining 10′ in H2O containing 5 μg/ml Et. Bromide, rinsing 2× in H2O and placing upon a UV transilluminator (302 nM). The band corresponding to the ca. 3712 bp EcoRI-BamHI digested pBR322 molecules was cut from the gel. The DNA was electroeluted from the gel slice, phenol extracted, CHCl3 extracted 2×, and ethanol precipitated. The pellet was dissolved in H2O and was ready for ligation.
In a three-part ligation (see FIG. 4), in which the molar ratio of inserts to vector in the ligation reaction was approximately 10 to 1, parts 1 and 2 were ligated into the EcoRI-BamHI digested 322 vector in 1×T4 DNA ligase buffer (cont. 50 mM Tris HCl pH 7.8; 10 mM MgCl2, 20 mM DTT, 1 mM rATP) and ˜800 U T4 DNA ligase (NEB). The reaction was carried out at 14° for 12-16 hrs.
E. coli strain 294 was used as the transformation host, using the procedure of M. Dagert and S. D. Ehrlich (3). The transformed cells were plated on LB-agar plates containing ampicillin (20 μg/ml; LB-Amp-plates) and transformants were screened and grown in LB medium containing ampicillin at 20 μg/ml ampicillin. Transformants were screened using a modification of the rapid miniscreen method of Birnboim and Doly (4). Miniprep DNA prepared as such was digested with EcoRI and BamHI and run on polyacrylamide slab gels. Several transformants which illustrated a ca. 218 bp EcoRI-BamHI insert were grown in large scale and plasmids from each were isolated and sequenced according to the procedure of Maxam and Gilbert (5) to confirm the correct chemical synthesis and construction. The pBR322 vector containing the complete correct left half sequence of IGF-1 was called IGF-1 LH 322 (see FIG. 5).
Using the identical conditions of DNA Polymerase I-mediated repair-synthesis, the two pairs of fragments comprising the right half of the synthetic IGF-1 were converted into double-stranded DNAs. After the DNA Polymerase I reactions, and without enzymatic digestion, the 1R+3R (Part III) and 2R+4R (Part IV) reactions were run on a 6 percent polyacrylamide slab gel. The 83 bp (Part III) and 91 bp (Part IV) bands were located by autoradiography and cut from the gel. After electroelution the ethanol precipitated double-stranded DNAs were dC-tailed (see FIG. 6) using the procedures of Villa-Komaroff et al. (6) and Rowenkamp and Firtel (7). Reactions were carried out in 50 μl vols. of 1×tailing mix (cont. 0.2M Pot. Cacodylate, 25 mM Tris HCl pH 6.9, 2 mM DTT, 0.5 mM CoCl2) and 22 μm dCTP. After prewarming at 37° for 10′, the 150 second reaction was begun by the addition of 10-20 units of terminal nucleotidyl transferase and terminated by addition of EDTA followed by phenol extraction, CHCl3 extraction 2×, and ethanol precipitation.
These oligo (dC) tailed Parts III and IV were then separately mixed with equimolar amounts of oligo (dG)-tailed PstI cut pBR322 vector in 50 μl of 1×annealing buffer (0.1M NaCl; 10 mM Tris HCl pH 7.8, 1 mM EDTA) at a final DNA concentration of 1-2 μg/ml. After heating to 75° C., the mixes were gradually cooled to 4° over a period of 16 hr and the mix transformed into competent E. coli 294 cells prepared according to the procedure of Dagert and Ehrlich (3). Transformed cells were plated on LB-Tetracycline-Agar plates and grown in LB-Tetracycline medium at tetracycline concentrations of 5 μg/ml. Tetracycline resistant transformants were picked and plated onto LB-Ampicillin-Agar plates to check for insertions at the PstI site. Several tetracycline resistant, Ampicillin-sensitive colonies for each Part 3 and 4 were miniscreened and those exhibiting insertions at the PstI locus were grown in large scale and sequenced Dy the Maxam and Gilbert technique (5) to confirm the correct DNA sequences of Parts 3 and 4.
Preparation: Parts 3 and 4.
Parts 3 and 4 were separately removed from their vectors by digestions of 20 μg of each vector with AvaII in 1×AvaII buffer (60 mM NaCl, 6 ml Tris-HCl (pH 8.0); 10 ml MgCl2; 6 ml 2-mercaptoethanol) and 30 U of AvaII. After 6 hr., at 37°, EDTA was added to the 150 μl reactions to a concentration of 15 mM and the material phenol extracted, CHCl3 extracted 2× and ethanol precipitated. The Part 3 pellet was then taken up in 1×BamHI buffer and digested in a volume of 150 μl with 30 U BamHI at 37° for 4 hr. The pellet containing Part 4 was digested with 30 U SalI in 150 μl of 1×SalI buffer at 37° for 4 hr.
Both digests were then run on 6 percent polyacrylamide slab gels and stained. The 51 bp band representing Part 3 and the 62 bp band representing Part 4 were removed from the gels and the DNA was electroeluted, phenol extracted, CHCl3 extracted 2× and ethanol precipitated. Pellets were then taken up in H2O and were ready for ligation.
20 μg of the IGF-1 LH 322 vector was digested with 50 U of BamHI and 50 U of SalI in a 200 μl reaction containing 1×BamHI buffer at 37° for 6 hr. After addition of EDTA to a concentration of 15 mM, the digestion mix was run on a 6 percent polyacrylamide slab gel, ethidium bromide stained and the 3814 bp band excised from the gel.
After electroelution, phenol extraction, chloroform extraction and ethanol precipitation, the DNA pellet was taken up in H2O and was ready for ligation with Parts 3 and 4 in a three-part ligation. The ligation was performed under conditions described above for a three-part ligation (see FIG. 7). Parts 3 and 4 were present in the ligation mix at a 10-fold molar excess of inserts to vector. The mix was transformed into competent E. coil 294 cells prepared according to the Dagert and Ehrlich procedure (3) and plated onto LB-Ampicillin plates. Several transformants were miniscreened and two clones exhibiting a ca. 115 bp BamHI-SalI fragment were grown in large scale and their plasmids prepared. Both strands of the intact synthetic gene were sequenced by the Maxam-Gilbert technique (5) to confirm the correct sequence. The pBR322 plasmid containing the complete correct sequence coding for Human IGF-1 was called pBR322 HuIGF-1.
Initial attempts were to obtain expression of IGF-1 as a fusion protein. To accomplish this, both the pNCV (9) and the pNCVsLE (10) expression vectors were used. (The pNCVsLE expression vector is a derivative of the pNCV vector and was prepared as follows: pNCV was treated with BglII, which cleaves at the 13 codon of the LE fusion. The site was converted to an ECoRI cleavage site using synthetic DNA, to give the expression vector pNCYsLE). The synthetic DNA introduced into the plasmid has the sequence:
and this sequence was introduced into the plasmid:
As a strategy to release the fused human IGF-1 protein from the trp fusion protein, a linker was designed such that an enzymatic proteolys method reported by Wunsch et al. (8) could be applied to this expression system. To accomplish this, a DNA linker:
was chemically synthesized by standard methods (2) which when linked the trp fusion protein and the IGF-1 gene, coded for the amino acid residues Proline and Alanine followed by Glycine and Proline which ay the first two amino acid residues of IGF-1 and preceded by Proline ay Alanine together comprise a recognition site for a collagenase isolated from Clostridium histolyticum (11,12). This enzyme reportedly acts such a site to cleave the alanine-glycine peptide bond.
To construct a DNA sequence coding for a fusion protein with a collagenase cleavage site, 30 μg pBR322 HuIGF-1 plasmid was cleaved 50 U BamHI and 50 U PvuI enzyme in 200 μl 1×BamHI buffer at 37° for 6 hours. After addition of EDTA to a concentration of 15 mM, the reaction mix was chromatographed on a 6 percent polyacrylamide slab gel. The smaller PvuI-BamHI fragment (˜725 bp) was Isolated and digested with 40 U AvaII in 150 μl 1×Sau96I buffer (60 mM NaCl, 6 mM Tris-HCl pH 7.4, 15 mM MgCl2, 6 mM 2-mercaptoethanol). After addition of EDTA to a concentration of 15 mM, the resulting mix chromatographed on a 6 percent polyacrylamide slab gel. The smaller SalI-BamHI fragment (˜86 bp) was extracted from the gel, phenol extracted, chloroform extracted 2×, and ethanol precipitated. This fragment was ready for ligation.
200 pmols of linker fragments were kinased with 100 U polynucleotide kinase in 20 μl of 1×polynucleotide kinase buffer (70 mM Tris-HCl (pH 7.6); 10 mM MgCl2; 5 mM DTT; 1 mM rATP) at 37° for 1 hour. The reaction was terminated by heating to 65 ° C. for 5 minutes. 100 pmols of the kinased linker fragments were ligated to the 86 bpp SalI-BamHI fragment with 400 U of T4 DNA ligase in 30 μl of 1×T4 DNA ligase buffer at 14° for 12-16 hours. The ligation reaction was terminated buff addition of EDTA to a concentration of 15 mM followed by phenol extraction, chloroform extraction 2×, and ethanol precipitation. The pellet was then taken up in 1×BamHI buffer and digested in a 100 μl reaction with 50 U of EcoRI and 50 U of BamHI at 370 for 6 hrs. After terminating the digestion with EOTA, the mixture was chromatographed on a 6 percent polyacrylamide slab gel and the newly created (97 bp) EcoRI-BamHI fragment was extracted from the gel, and prepared for ligation. The vector to receive this new fragment was prepared by digesting 30 μg pBR322 HuIGF-1 with 100 U of each EcoRI and BamHI in 200 μof 1×BamHI buffer at 37° for 8 hr. The reaction was terminated, chromatographed a 6 percent polyacrylamide slab gel and the larger band (˜3830 bp) representing the EcoRI-BamHI digested plasmid was isolated and the plasmid DNA extracted and prepared for ligation as above. In a 30 μl ligation reaction containing a 10-fold molar excess of insert fragmer vector, the EcoRI-BamHI fragment was ligated into the EcoRI-BamHI digested plasmid pBR322 HuIGF-1 under standard ligation conditions mentioned above. Competent E. coli 294, prepared as above (3), were as transformation hosts and the transformed cells were plated onto LB-Ampicillin agar plates. Several transformants were picked, miniscreened as above (4), and two exhibiting an EcoRI-BamHI insertion were grown in large scale and their plasmids purified. Using the Maxam-Gilbert procedure (5) the construction was sequenced to verilfy the correct synthesis and insertion of the EcoRI-Sau96I collagenase linker. This plasmid was called pBR322 HuSynIGF-1-M.
To prepare this EcoRI-SalI IGF-1 coding sequence for insertion into pNCV and pNCVsLE, 30 μg of pBR322 HuSynIGF-1-M was digested with 70 U of SalI in 200 μl of 1×SalI buffer (150 mM NaCl, 6 mM Tris-HCl (pH 7.9); 6 mM MgCl2; 6 mM, 2-mercaptoethanol) at 37° for 6 hours . After addition of EDTA to 15 am, the mixture was phenol extracted, chloroform extracted 2×, and ethanol precipitated.
Using standard chemical synthesis procedures (2) a SalI-EcoRI linker
was synthesized and 400 pmols kinased, as above. 200 pmols of the kinased linker was ligated to the SalI digested pBR322 HuSynIGF-1-M (prepared above) with 800 U T4 DNA ligase in 30 μl of 1×ligation buffer for 12-16 hours at 14° C.
After termination of the reaction with EDTA, the mixture was phenol extracted, chloroform extracted 2×, and ethanol precipitated. The pellet was then taken up in 1×EcoRI buffer and digested with 100 U EcoRI in a volume of 200 μl for 8 hours at 37°. After addition of EDTA to a concentration of 15 mM, the mixture was chromatographed on a 6 percent polyacrylamide slab gel. The gel was stained and the ˜230 bp band corresponding to the EcoRI-EcoRI HuIGF-1 fragment was extracted from gel, phenol extracted, chloroform extracted 2×, and ethanol precipitated. This fragment was ready for ligation into pNCV and pNCVsLE. pNCV and pNCVsLE were prepared for ligation by digestion of 20 μg of each with 100 U EcoRI in 200 μl 1×EcoRI buffer at 37° for 8 hours. After digestion, 200 U of bacterial alkaline phosphatase was added to each reaction and the mixtures were warmed to 65° C. for 2 hours. EDTA was added to a concentration of 15 mM and the mixes were phenol extracted 3×, chloroform extracted 2× and then ethanol precipitated. These expression vectors were prepared for ligation.
Ligations of the EcoRI-EcoRI Human IGF-1 fragment into the two expression vectors were performed in 30 μl reaction volumes in 1×T4 DNA ligase buffer with 800 U T4 DNA ligase at 14° for 12-16 hours. The EcoRI-EcoRI fragment was present at a 10-fold molar excess to vector.
Competent E. coli 294 were prepared (3) (ATCC 31446) and used as transformation hosts for the ligatlons. Transformed cells were plated onto LB-agar plates containing tetracycline (5 μg/ml; LB-Tet-plates) and transformants were miniscreened (4). Miniscreen plasmid DNA from transformants of the pNCV-IGF-1 construction were digested with both PstI and BglII to determine the orientation of the EcoRI fragment insertions. Two clones whose plasmids contained a ˜570 bp BglII-PstI fragment (as opposed to a ˜690 bp fragment) were grown in large scale and their plasmids prepared. The construction was sequenced using the Maxam-Gilbert procedure (5) to confirm the correct insertion at the junction of the trp fusion and IGF-1 protein coding sequences as well retention of the desired reading frame. Plasmids with the correctly inserted IGF-1 fragment were called pNCVLE-IGF-1. Transformants of the pNCV-sLE-IGF-1 construction were also miniscreened by the same procedure (5), and the plasmid DNAs were digested with HindIII and PstI. Two clones exhibiting a ˜150 bp HincII-PstI fragment (as opposed to a ˜105 bp HincII-HincII fragment) were grown in large scale and their plasmids prepared. Using the Maxam-Gilbert techniques (5), the functions of the trp fusion and IGF-1 protein coding sequences were sequenced to ascertain proper orientation and retention of the proper reading frame. Those plasmids possessing the correct insertion and proper reading frame were called pNCV-sLE-IGF-1.
To attempt expression of each of these constructions, two clones, one possessing pNCV-IGF-1 and the other possessing pNCV-sLE-IGF-1, were inoculated into 10 ml M9-Tetracycline culture medium supplemented with 0.5 mg/ml Tryptophan. A clone containing pNCV-LE with no IGF-1 gene insert was also inoculated into culture medium to provide as a negative control in assays.
After 12-16 hours growth at 37° with agitation, 0.5 ml of these cultures were used to inoculate 250 milliliters of M9-Tetracycline culture medium. After growing for 12-16 hours at 37° with agitation, the cells were harvested by centrifugation at 5000 rpm for 10 minutes in a Sorvall GSA rotor. The refractile bodies were purified from the pellete cells by: a) suspending the host cells in a buffered solution of ionic strength suitable to solubilize most of the host protein, b) subjecting the suspension to cell wall/membrane disruption, c) centrifuging the disrupted suspension at low speed to form a pellet, optionally repeating the foregoing steps, and d) recovering the heterologous protein as refractile bodies in the pellet (Reference 13). A small quantity of refractile particles of each of the three preparations was boiled in SDS and 2-mercaptoethanol containing sample buffer and run on SDS-polyacrylamide slab gels according to the Laemmli method (14). The size of the protein expressed by pNCV-IGF-1 (LE-IGF-1) was ˜28,670 Daltons (see FIG. 7), and ˜9770 Daltons for the pNCV-sLE-IGF-1 protein (sLE-IGF-1) (see FIG. 8). These two expressed proteins were subjected to solubilization in 6M Guanidine-HCl followed by 50 -fold dilution with dilute buffers. The final buffer for pNCV-IGF-1 after dilution was 0.12 M Guanidine-HCl; 0.05 M Tris-HCl pH 8, 20 percent glycerol; 0.1 mg/ml BSA; 0.15 M NaCl; 0.1 mM EDTA and the final buffer after dilution of the pNCV-sLE-IGF-1 refractile bodies was 0.14 M Guanidine-HCl; 25 mM Tris-HCl pH 7.6; 10 mM CaCl2. After spinning out particulate matter, the two solutions containing solubilized trp-IGF-1 fusion proteins were assayed by a radioimmune assay procedure of Furlanetto et al. (23), as, modified by Hintz et al. (24). Both fusion proteins demonstrated activity in this assay. A negative control prep was also included in the assay and the control exhibited no measurable activity.
To avoid the necessity of refractile body purification and solubilization, from bacterial cell lysates, yeast expression-secretion systems were sought as an alternative. Aside from the advantage of avoiding protein purification from cell lysates, coupled expression-secretion systems might obviate a subsequent in vitro processing step to remove a fused protein. Available were three yeast expression-secretion systems. These were: 1) yeast a factor (22), employing yeast α-factor promoter and preprosequence; 2) yeast invertase (16) consisting of the invertase promoter and signal sequence; and 3) a hybrid, composed of the PGK promoter (25) and invertase signal (16).
To obtain expression of IGF-1 using the α factor promoter and preprosequence, a plasmid constructed by Singh (22) was used. Plasmnid P65 (FIG. 9) possesses sequences of the α-factor promoter, α-factor preprosequence, yeast 2 micron terminator, the yeast Trp 1 gene, as we as portions of the pBR322 plasmid. Plasmid p65 was obtained by the following method: The 15-mer oligonucleotide probes for the α-factor gene were designed on the basis of the amino acid sequence of the pheromone (232) and yeast codon usage frequencies. The rationale is outlined in FIG. 19 where the last 5 amino acids of the α-factor and all the possible codons and their usage frequencies are given. (The codon usage is the total of 2 different glyceraldehyde-3-phosphate dehydrogenase clones (23b,23c) and of alcohol dehydrogenase I.) The codon usage for these and other genes has recently been summarized (23c). As can be seen from FIG. 19, virtually all possible sequences coding for the 5 amino acids are included in the oligonucleotide sequence
Accordingly, two pools consisting of two oligonucleotides each, and complementary to the above sequence, were synthesized. No other contiguous 5 amino acids in the pheromone could be covered with such a limited set of oligonucleotides.
A. Screening of Recombinant Plasmids
A genomic library, made by insertion of partially Sau3A-digested yeast DNA into the DELHI site of YRp7 (23e), was screened for presence of α-factor gene clones. E. coli transformants were grown on nitrocellulose filter paper (Schleicher and Schuell, BA85) placed on S-agar plates containing 5 g/ml ampicillin. After 6 hours at 37° C., filters were transferred to S-agar plates containing 150 g/ml chloramphenicol. After 15 hours of amplification colonies were tested for hybridization using a modified in situ colony screening procedure (23f). 32P-labeled (5) synthetic oligonucleotides described above were used as hybridization probes. Filters were hybridized overnight at 42° C. in 10 mM Tris (pH 7.5), 6 mM EDTA, 0.1 mM ATP, 1 mM sodium pyrophosphate, 0.8M NaCl, 1×Denhardt's solution, 0.5 percent NP-40, and 0.1 mg/ml E. coli tRNA. Filters were washed 3 times for 20 min. in 6×SSC at 30°. Dried filters were exposed to Kodak XR-2 X-ray film with Dupont: Lightning-Plus intensifying screen at −80°.
B. Identification of Recombinant Plasmids Containing the α-factor Gene
Approximately 4500 bacterial colonies containing recombinant plasmids were tested for In situ hybridization (23f) with 32P-end-labeled oligonucleotide pool I (FIG. 19). Twenty-four plasmids hybridized to varying degrees. Small amounts of plasmid DNAs were prepared from these 24 colonies by the method of Birnboim and Doly (4) and tested for hybridization with the same probes after spotting the DNA samples on a nitrocellulose filter. Two of the 24 plasmids, designated as p51 and p52 respectively, hybridized strongly and were chosen for further study. The p51 and p52 plasmids also hybridized with the oligonucleotide pool II.
C. Subcloning of the Hybridizing Sequences
To characterize the inserts that hybridized with the synthetic probes, plasmid DNA prepared from the p51 and p52 clones was subjected to restriction enzyme analysis with EcoRI, SalI, HindIII, BamHI, and PstI. As seen in FIG. 20A, the 2 recombinant plasmids are quite dissimilar. Only EcoRI and PstI digestions of the two plasmids yielded one common fragment each. In both cases the common fragment is the TRP1 insert and the 1.38 kbp PstI piece is the DNA between PstI sites in the TRP1 and the ampR genes.
The fragments that contained sequences complementary to the probe were identified by the method of Southern (23h). FIG. 20B shows that, except in one case, digestion with all 5 restriction enzymes yielded a fragment that specifically hybridized with the probe. No hybridization was seen with any of the fragments produced by restriction of p52 DNA with HindIII.
The smallest restriction fragments that contained sequences complementary to the synthetic probes were the 1.7 kbp EcoRI fragment from p52 and the 1.8 kbp HindIII fragment from p51. These two DNA fragments were isolated from a preparative agarose gel by electroelution and separately ligated to appropriately cleaved plasmid pBR322 (15) DNA. The ligation mixture was used to transform E. coli 294 and the plasmid DNA from the transformants was analyzed by a quick-screen procedure (4). Two transformants, designated p53 and p56, containing the 1.7 kbp EcoRI and 1.8 kbp HindIII fragment inserts, respectively, were analyzed as follows: Plasmid DNA was prepared from p53 and p56 and digested separately with BamHI, ClaI, PvuI, PstI, and SalI. The resulting DNA fragments were separated on a 1 percent agarose gel, transferred to nitrocellulose filter paper (23h) and tested for hybridization with 32p-labled probes. The analysis of the restriction digests and corresponding hybridization patterns of the p53 DNA, the recombinant plasmid containing the 1.7 kbp yeast DNA as an EcoRI fragment, showed that the yeast DNA in this clone contained one SalI and two PstI sites and that the sequence complementary to the probes was included within a 0.5 kbp PstI-SalI fragment. The HindIII fragment of yeast DNA in the clone p56 lacked recognition sites for these enzymes, and the linearized plasmid, resulting from cleavage at single recognition sites for these enzymes in the pBR322 vector, hybridized with the probes. This plasmid was then digested with a number of additional restriction endonucleases and the digests were analyzed by the method of Southern as described above. It was found that the hybridizing sequences in this plasmid were contained on a 1.3 kbp HindIII-SalI fragment.
The property of growth inhibition of “a” cells by α-factor was used to test whether or not the pheromone gene contained in the cloned 1.7 kbp EcoRI and 1.8 kbp HindIII fragments are functional. If an active α-factor pheromone gene were present in a plasmid, it would be expected significantly more pheromone would be synthesized in cells containing the multi-copy plasmid than in cells containing only the chromosomal copy (or copies) of the gene. The enhanced level of the α-factor could then be detected by an increase in the area of nongrowth in a lawn of responsive “a” cells. The 1.7 kbp fragment, isolated from EcoRI-digested p53 DNA, and the 1.8 kbp fragment, isolated from HindIII-digested p56 DNA, were separately ligated to a pBR322-based vector plasmid which contained the yeast selectable marker TRP1 and the yeast origin of replication from the 2 μm yeast plasmid (23j). Yeast strain 20B-12 was separately transformed with these plasmids and with a control plasmid that lacked DNA sequences coding for the α-factor. The transformants were then compared for pheromone production. The transformants containing MFα1 or MFα2 coding sequences on plasmids produced significantly more α-factor than the same strain transformed with the control plasmid. We concluded that the 1.7 kbp EcoRI (MFα1) and 1.8 kbp HindIII (MFα2) fragments contain active α-factor pheromone genes. The result with MFα1 is consistent with that described by Kurjan and Herskowitz (23k), as this gene corresponds to the gene described by them.
D. DNA Sequence Determination
DNA sequence determination was as previously described (23o). Briefly, DNA sequences were obtained by the chain termination method (23l) using recombinant phages M13 mp8 and mp9 (23o) as the source for single-stranded “template” DNA and a synthetic oligonucleotide for priming E. coli DNA polymerase I (large fragment, Boehringer Mannheim) in the presence of α-32P dCTP (400 Ci/mmole, Amersham). Reactions were electrophoresed on 5 percent polyacrylamide/8M urea “thin” gels (23l). Gels were dried onto 3 MM paper (Whatman) and exposed to X-ray film for 2 to 12 hr.
The nucleotide sequences of large parts of the 1.7 kbp EcoRI fragment and the 1.3 kbp HindIII-SacI fragment are shown in FIG. 3 and FIG. 22, respectively. The p53 sequence contains an open reading frame coding for a protein of 165 amino acid residues which carries 4 internal repeat units within its C-terminal half. Each unit begins with Lys-Arg and ends with the α-factor sequence. Within each unit the pair of basic residues is separated from the α-factor by several Glu (or Asp)-Ala dipeptide repeats. The N-terminal half of the protein starts with a highly hydrophobic sequence of 22 amino acids which probably represents a signal sequence for secretion. The 61 amino acid residues between this hydrophobic sequence and the first repeat unit include 3 possible recognition sites for N-glycosylation (indicated by bars in FIG. 21). The organization of the pheromone gene contained in p53 clone is identical to the MFα gene recently described by Kurjan and Herskowitz (23k). This gene differs from MFα1 at 4 positions. It contains T (instead of C) residues at positions −8 and −7, and 125 and an A (instead of C) residue at position 604. Because of the difference at position 125 there is a TTA (Leu) rather than TCA (Ser) codon at amino acid position 42. We have designated the gene contained in p53 as MEα1.
A different α-factor gene, MFα2, is present in the p56 clone. The organization of this gene (FIG. 22) is similar, but not identical, to the MEα1. The α-factor encoded by this gene is apparently made as a precursor protein of 120 amino acid residues containing two copies of the pheromone. One of the α-pheromone tridecapeptides contained in the putative precursor is identical to the pheromone copies encoded by the MEal gene, whereas the second copy contains a Gln→Asn and a Lys→Arg.
E. Construction of a Plasmid p65 for Expression and Secretion of Human Interferon
The preparation of a plasmid to demonstrate the usefulness of the α-factor promoter and the α-factor presequences for expression and secretion of heterologous gene products is outlined in FIGS. 23-24. The DNA sequences coding for the α-factor peptides were removed from one of the α-factor clones (p53) such that the resulting plasmid, p57, contained the promoter sequences and the sequence corresponding to 89 amino acids of the α-factor “prepro” protein. This sequence was then joined with human interferon D (IFN-α1) gene to form plasmid p58. For this purpose an expression plasmid p65, was constructed as shown in FIG 24. This plasmid, like YEp9T, contains the origins of replication for E. coli and yeast as well as selective markers for selection in each of these two organisms. It also contains a convenient EcoRI site for gene insertion so that any gene that is contained on an EcoRI fragment where the first codon of the gene is immediately preceded by the EcRI site could be tested for the synthesis and secretion of the corresponding protein. Due to the dearth of convenient restriction sites in the α-factor preprosequence, to insert the IGF-1 coding sequence, the identical ˜230 bp EcoRI-EcoRI HuSynIGF-1-M fragment that was ligated into pNCY and pNCYsLE (as mentioned previously in bacterial construction) was used. This EcoRI-EcoRI fragment contained the collagenase recognition site Proline-Alanine-Glycine-Proline, and allowed for collagenase digestion should IGF-1 be secreted as a fusion protein. The protein expressed in this construction (see FIG. 10) consists of the prepro a-factor protein fused to IGF-1.
To insert the ˜230 bp EcoRI-EcoRI fragment, the plasmid P65 was partially digested in 1×EcoRI buffer with EcoRI, and then sized upon a 0.7 percent horizontal agarose gel. The band corresponding to the linearized singularly restricted plasmid was excised, eluted from the gel, and phenol extracted, chloroform extracted 2×, and then ethanol precipitated. This DNA pellet was then taken up in 50 mM Tris-HCl (pH 8) and treated with bacterial alkaline phosphatase under conditions to ensure 100 percent dephosphorylation of the 5′ protruding ends. Following this treatment, the phosphatase activity was removed by first adding EDTA to a concentration of 15 mM, then extracting the DNA with phenol 3×, chloroform extracting 2×, and ethanol precipitating the vector. This material then contained linearized P65 vector, digested with EcoRI in either of two locations: one, either at the EcoRI site upstream of the α-factor promoter and preprosequence, or at another, at the EcoRI site just downstream of the α-factor promoter and preprosequence. The ˜230 bp EcoRI-EcoRI IGF-1 fragment was ligated into the vector. The desired location of insertion was at the EcoRI site just downstream from the α-factor promoter and preprosequence.
The ligation was carried out under standard ligation conditions and the transformation hosts were competent E. coli 294 prepared according to Dagert and Ehrlich (3). The transformed cells were plated onto LB-Amp-Agar plates. Several transformants were miniscreened according to the method of Birnboim and Doly (4), and plasmid DNA prepared as such was digested with both SalI and HindIII in the appropriate buffers. One of several clones which contained a plasmid with an ˜110 bp EcoRI-HindIII fragment was grown in large scale and its plasmid was purified. This plasmid, YEp9T α-factor EcoRI-EcoRI IGF-1 (see FIG. 11), was used to transform competent yeast strain 20B-12 (αtrp pep4) cells according to the Hitzeman modification (19) of Hinnen et al. (17) and Beggs et al. (18) procedures.
Two such transformants, as well as a negative control transformant (with no IGF-1 insertion in the plasmid), were grown in suspension as were those of the yeast pre-invertase-IGF-1 plasmid transformations. Supernates were tested for secreted IGF-1 activity, as measured by the radioimmune assay procedure of Furlanetto et al. (23) as modified by Hintz et al. (24). Both supernates of transformants having plasmids with IGF-1 inserts contained IGF-1 activity and the negative control supernate did not. One of these transformants was grown in large scale in a 10 liter fermenter and the supernate contained secreted IGF-1 activity at a peak level of ˜3 μg/ml. The IGF-1 activity of the fermentation supernate was also demonstrated by a placental membrane radioreceptor assay developed by Horner et al. (26).
Yeast Invertase Promoter Signal IGF-1 Plasmid Construction
Based upon evidence of correct processing and secretion in yeast of proteins with heterologous signal sequences (16), the yeast invertase expression-secretion system became of interest. Attempted first was expression of the yeast invertase signal protein fused to IGF-1 (FIG. 12), coupled with the processing and secretion of IGF-1, using the invertase promoter as a starting point for transcription.
The yeast invertase signal coding sequence was attached to the IGF-1 gene by the use of a NcoI-HindIII (˜400 bp) fragment containing the initiation ATG codon and 5′ end of the signal DNA sequence, and 4 DNA fragments synthesized by standard procedures (2):
The construction began with the isolation of the 90 bp AvaII-BamHI IGF-1 left half fragment by AvaII digestion of a ˜730 bp PvuI-BamHI fragment isolated from PvuI-BamHI digested pBR322-HuSynIGF-1.
After phosphorylation of all four synthetic DNA fragments using standard kination conditions, the four synthetic fragments were mixed with the AvaII-BamHI IGF-1 left half fragment and ligated using stand ligation conditions. Following inactivation of the ligase by phenol and chloroform extraction 2×, the ethanol precipitated DNA pellet was dissolved and digested with HindIII and BamHI in the appropriate buffers. Newly constructed HindIII-BamHI (ca. 140 bp) fragment was isolated and extracted from a 6 percent polyacrylamide gel. This material was then ligated into HindIII-BamHI digested pBR322 vector, which had been first digested with HindIII, then BamHI in the appropriate buffers, followed by purification of the 4014 bp vector fragment from a 6 percent gel.
The transformation host was competent E. coli 294 prepared by standard procedures (3) and the transformed cells were plated onto LB-Ampicillin agar plates. Several transformants were miniscreened by the Birnboim-Doly procedure (4) and their plasmid DNAs digested with EcoRI and BamHI. Two plasmids containing a ˜167 bp EcoRI-BamHI fragment (illustrating the insertion of a 140 bp fragment into the HindIII and BamHI sites) were grown in large scale and their plasmids prepared. Using Maxam-Gilbert sequencing techniques (5), the entire 43 bp HindIII-AvaII section of DNA was sequenced to confirm the correct chemical synthesis and construction. The correctly constructed plasmid was called pBR322-P-I-HuSynIGF HindIII-BamHI (˜4154 bp).
To insert the right half of the IGF-1 gene, this newly created plasmid was digested with BamHI-SalI in the appropriate buffers and the larger fragment (3879 bp) was purified by gel fractionation. pBR322 HuSynIGF was digested with BamHI-SalI in the appropriate buffers and the 115 bp BamHI-SalI fragment corresponding to the right half of the IGF-1 gene was isolated by gel fractionation. This 115 bp BamHI-SalI IGF-1 right half fragment was then ligated into the BamHI-SalI digested pBR322-P-I-IGF-1 LH HindIII-BamHI vector using standard ligation conditions. Competent E. coil strain 294 prepared according to Dagert and Ehrlich (3) were used as transformation hosts and transformed cell were plated onto LB-Amp-Agar plates. Several transformants were miniscreened using standard techniques (4) and plasmid DNA prepared as such was digested with EcoRI and SalI in the appropriate buffers and those plasmids illustrating an insertion of the BamHI-SalI fragment corresponding to the right half of IGF-1 were called pBR322 P-I-HuSynIGF-1 HindIII-SalI. One of the clones containing the pBR322 P-I-IGF-1 HindIII-SalI plasmid was grown in large scale and the plasmid was isolated. This plasmid was then digested with HindIII and SalI in the appropriate buffer to prepare a 255 bp HindIII-SalI fragment containing all of the IGF-1 gene and the 3′ portion of the yeast invertase signal coding sequence. This fragment of DNA was isolated by polyacrylamide gel fractionation and prepared for ligation by standard techniques.
The (˜400 bp) NcoI-HindIII fragment containing the 5′ end of the DNA sequence coding for the invertase signal as well as the yeast invertase promoter was created by NcoI and HindIII digestion of plasmid YIpsp-LeIFA (16) in the appropriate buffers. The YIpsp-LeIFA plasmid was first digested with NcoI to completion in the appropriate buffer, then phenol extracted, chloroform extracted 2× and ethanol precipitated. The linearized molecules were then taken up in 1×HindIII buffer and partially digested to generate the needed NcoI-HindIII (˜400 bp) fragme which contains an internal HindIII restriction site. This NcoI-HindIII fragment was then isolated by gel fractionation and prepared for ligation using standard techniques.
To provide for a vector, plasmid pUC12-YI (EcoRI-BamHI) (16) was digested with NcoI and SalI in the appropriate buffers. After purification by gel fractionation, the ˜2.6 kbp vector was eluted from the gel and prepared for ligation by standard techniques. To perform the final construction, a three-part ligation was arranged using standard ligation techniques. The DNA used in the ligation included the NcoI-SalI-digested pUC12-YI (EcoRI-BamHI) (16), the ˜400 bp NcoI-HindIII and the ˜255 bp HindIII-SalI fragments. After ligation, the material was transformed into competent E. coli 294 cells prepared according to Dagert et al. (3) . Transformed cells were plated onto LB-Amp-Agar plates and several transformants were miniscreened using the procedure of Birnboim and Doly (4). Plasmid DNA prepared as such was digested with NcoI and SalI in the appropriate buffers and one of several clones containing plasmids exhibiting the insertion of a ˜625 bp NcoI-SalI DNA fragment was grown in large scale and its plasmid was purified.
As a final step, this plasmid was linearized by digestion with SalI in the appropriate buffer. SalI-EcoRI linker, prepared as mentioned above, and kinased under standard kination conditions, was ligated to the linearized vector to convert the SalI ends to EcoRI ends using standard ligation conditions. After termination of the ligation reaction by addition of EDTA to 15 mM, phenol extraction, chloroform extraction 2× and ethanol precipitation, the DNA pellet was dissolved in 1×EcoRI buffer, and digested with EcoRI. The EcoRI digestion released a ˜1150 bp EcoRI fragment which contained the yeast invertase promoter, yeast invertase signal coding sequence and the IGF-1 coding sequence in one contiguous sequence. This material was isolated as a ˜1150 bp band from a 6 percent polyacrylamide slab gel after fractionation and prepared for ligation using standard procedures.
The yeast-E. coli shuttle vector to receive this EcoRI fragment was prepared by EcoRI digestion of plasmid YEp9T (16) to linearize the vector, followed by treatment of the EcoRI termini with bacterial alkaline phosphatase using conditions recommended by the manufacturer to produce 100 percent dephosphorylation of the 5′ protruding ends. The phosphatase reaction was terminated by addition of EDTA to 15 mM and the mixture phenol extracted 3×, chloroform extracted 2×, and then the DNA was ethanol precipitated. After redissolving the DNA pellet in 1×ligation buffer, the vector was mixed with the EcoRI ˜1150 bp fragment and ligated under standard ligation conditions. Competent E. coli 294 cells prepared according to Dagert et al. (3) were used as transformation hosts and the transformants were plated onto LB-Amp-Agar plates. To determine the orientation of the insertion, several transformants were miniscreened using the method of Birnboim and Doly (4) and plasmid DNAs purified as such were digested with BamHI in the appropriate buffer. One of several transformants possessing plasmids which produced a 1.3 kb BamHI-BamHI fragment upon BamHI digestion (as opposed to a 475 bp fragment) was grown in large scale and its plasmid was purified. This plasmid, called P.I.IGF-1 EcoRI-EcoRI P.I. Promoter was used to transform competent yeast cells prepared essentially according to the methods of Hinnen, A., et al. (17), and Beggs, J. D. (18), but with the modifications of Hitzeman (19). The yeast strain 20B-12 (αtrpl pep4) was used and was obtained from the Yeast Genetics Stock Center. In this construction, the expression of IGF-1 begins with transcription at the invertase promoter and terminates in the yeast 2 micron sequence. The fusion protein expressed by this construction consisted of the yeast invertase signal fused to the IGF-1 protein, the combined molecular weight of which was 9964 Daltons. Another plasmid with the EcoRI fragment inserted in the reverse orientation was also used to transform competent yeast cells. In this construction, the IGF-1 was not provided with the yeast terminator.
Several transformants were picked and streaked on YNB-CAA agar plates. Among these, three transformants were picked and inoculated into 10 ml of YNB-CAA grow-up medium, in shake flasks. A fourth culture was also started using a colony transformed with the same vector, but with the EcoRI fragment inserted into the vector in the reverse orientation. After 16-20 hours growth at 30°, the cultures were sampled (1 ml) and cleared of cells by spinning 5′ in an eppendorf microfuge. Supernatant were taken off and assayed for secreted activity using the radioimmune assay procedure of Furlanetto et al. (23) as modified by Hintz et al. (24). The supernates of the three transfornants demonstrated activities of 1.7 to 3.3 ng/ml of IGF-1 activity and the negative control showed no activity. To determine intracellular activity, the pellets from 1 ml of culture were washed 1× in 25 mM Tris-HCl (pH 7.6), 1 mM EDTA and then lysed by 3-4 minutes of vigorous vortexing in 0.5 ml of the above Tris-EDTA solution with 0.4 ml of glass beads.
Assay of the cell lysates demonstrated IGF-1 activities of 1.5-2.8 ng/ml in the three IGF-1 secreting transformants and no activity in the negative control transformant. The highest secretor of the three transformants was grown in a 5 liter fermentation and the secreted IGF-1 activity reached a peak of 74 ng/ml of supernate.
One difficulty in the use of the invertase promoter was that it was subject to repression in the presence of glucose. Due to the incompatibility of glucose with high levels of transcription initiation at the invertase promoter, the PGK promoter was sought as an alternative promoter, glucose, being the mainstay carbon source of fermentation processes.
To begin construction of the PGK promoter P.I.IGF-1 construction, it was necessary to clone a fragment containing the entire invertase signal coding sequence. To do this, plasmid pLeIF-A-Invertase Signal (16) was digested with BglII and then BamHI in the appropriate buffers. This digestion released several fragments, one of which was a ˜625 bp BglII-BamHI fragment which was isolated from a 6 percent polyacrylamide slab gel and prepared for ligation using standard techniques. To clone this fragment, the pUC8 vector was chosen as a cloning vehicle. pUC8 plasmid was digested with BamHI in 1×BamHI buffer, treated with bacterial alkaline phosphatase to dephosphorylate the 5′ termini, and then run onto and purified from a 5 percent polyacrylamide slab gel.
After standard preparation for ligation the BamHI digested vector was mixed with the above ˜625 bp BglII-BamHI fragment, and ligated under typical ligation conditions. The mixture was then transformed into competent E. coli 294 prepared by the Dagert et al. method (3) and the transformed culture plated onto LB-Amp-Agar plates. Several transforms were picked and miniscreened using the Birnboim and Doly (4) technique. Miniscreen plasmid DNA was digested with EcoRI and an analytical gel of the digests Illustrated two types of plasmids having EcoRI fragments either ˜260 bp or ˜385 bp in length. One clone containing a ˜260 bp EcoRI fragment was grown in large scale and its plasmid purified. This plasmid was called pUC8 P.I. Promotor-Signal BglII-BamHI.
A clone of this type was chosen because of the desired orientation of the inserted BglII-BamHI fragment. What was needed from this plasmid was an ˜20 bp EcoRI-HindIII fragment containing the ATG initiation codon and 5′ end of the invertase signal coding sequence.
To construct the intact invertase signal coding DNA sequence, ˜150 bp HindIII-BamHI fragment containing the 3′ end of the signal sequence fused to the left half of the IGF-1 gene was isolated from HindIII-BamHI digestion of plasmid pBR322 P.I. IGF-LH HindIII-BamHI (˜4154 bp). Isolation was by polyacrylamide slab gel fractionation, and the DNA band corresponding to the ˜150 bp fragment was excised and prepared for ligation using standard techniques.
To obtain the short (˜20 bp) EcoRI-HindIII fragment, the plasmid pUC8 P.I. Promotor-Signal-BglII-BamHI was digested with EcoRI in 1×EcoRI buffer. This digestion released the ˜260 bp EcoRI-EcoRI fragment which was isolated from a 6 percent polyacrylamide slab gel after fractionation of the digestion mixture. This ˜260 bp fragment was then digested with HindIII in the appropriate buffer, causing the creation of two HindIII-EcoRI fragments, one ˜20 bp and the other ˜240 bp in length. After complete digestion, the digestion was terminated by addition of EDTA to 15 mM and the entire mix phenol extracted, chloroform extracted 2×, and then ethanol precipitated.
A vector was prepared by EcoRI-BamHI digestion of pBR322 (15) in the appropriate buffers followed by purification of the EcoRI-BamHI digested vector from a 5 percent polyacrylamide slab gel. After preparation for ligation using standard techniques, the vector was mixed with the ˜150 bp HindIII-BamHI fragment (3′ end of invertase signal+Left Half IGF-1), and the two HindIII-EcoRI fragments (the ˜20 bp fragment containing the 5′ end of the invertase signal coding sequence), and the entire mixture was ligated under standard ligation conditions. Competent E. coli 294 prepared according to Dagert and Ehrlich (3) were used as transformation hosts for the ligation, and the transformed cells plated onto LB-Amp-Ager plates. Several transformants were miniscreened according to Birnboim and Doly (4) and the purified miniscreen DNAs were digested with EcoRI and BamHI. One of several clones possessing an ˜170 bp EcoRI-BamHI fragment was grown in large volume and its plasmid purified. This plasmid contained the complete yeast invertase signal coding sequence fused to the left half of IGF-1 and was called P.I. IGF-1 L.H. RI-BamHI.
The desired ˜170 bp EcoRI-BamHI fragment was isolated from this plasmid by digestion of the plasmid with EcoRI and BamHI in the appropriate buffers followed by slab gel fractionation of the reaction mix. Using standard techniques, the ˜170 bp band of DNA was prepared for ligation. To complete the construction, the right half of IGF-1 was solated as an ˜120 bp BamHI-EcoRI fragment from the plasmid P.I. IGF-1 EcoRI-EcoRI-P.I. Promoter by digestion with EcoRI and BamHI in the appropriate buffers followed by elutlon from a gel slice after polyacrylamide slab gel fractionation of the digestion mixture. These two fragments, the ˜170 bp EcoRI-BamHI and the ˜120 bp BamHI-EcoRI, were ligated together in vitro under standard ligation conditions, with both fragments present in roughly equimolar concentrations. This ligation mixture was then terminated by the addition of EDTA to ˜15 mM followed by phenol extraction, chloroform extraction 2×, and ethanol precipitation. The DNA pellet was then taken up in 1×EcoRI buffer and digested with EcoRI. The digest was then run on a 6 percent polyacrylamide slab gel and the DNA band staining at ˜290 bp (as opposed to ˜340 bp and 240 bp) was excised and prepared for ligation using standard techniques. This ˜290 bp EcoRI-EcoRI fragment contained the entire yeast invertase signal coding sequence fused to the complete IGF-1 coding sequence.
To express this protein, it was necessary to select a yeast vector with a promoter. The PGK promoter of the plasmid YEp1PT Small (see FIG. 13) was used. YEp1PT Small was constructed as a derivative of YEp1PT (21) by ClaI and PvuII digestion of YEp1PT in the appropriate buffers. The ClaI 5′ protruding end was converted to a blunt end by use of DNA polymerase I (Klenow) under conditions recommended by the vendor. After blunting the (ClaI protruding ends, the blunt ends ClaI and PvuII) of the linearized vector were fused using T4 DNA ligase under standard ligation conditions. The resultant YEp1PT small vector was ˜5.9 kbp in size (or ˜2.7 kbp smaller than YEp1PT). Just as YEp1PT, YEp1PT small possesses the 2 micron origin and terminator, the PGK promoter, the TRP1 gene, and sequences from pBR322, including the β-lactamase gene.
YEp1PT Small was employed as a vector by insertion of the ˜290 bp EcoRI fragment into the unique EcoRI site of the plasmid. EcoRI linearized YEp1PT Small vector was prepared by EcoRI digestion of YEp1PT Small followed by bacterial alkaline phosphatase (BAP) treatment (to prevent religation of the complementary termini). The BAP was removed by phenol extraction 3×, chloroform extraction 2×, and ethanol precipitation. Under standard ligation conditions, the ˜290 bp EcoRI fragment was ligated into the vector.
Competent E. coli 294 prepared according to Dagert and Ehrlich (3) were used as transformation hosts and the transformed culture was plated onto LB-Amp-Agar plates. Several transformants were miniscreened by the Birnboim and Doly procedure (4) and miniscreen plasmid DNAs were digested with HindIII in the appropriate buffer to determine the orientation of the insert. One of several transformants possessing a plasmid with a ˜400 bp HindIII fragment was grown in large scale and its plasmid was purified. This plasmid was called YEp1PT Small P.I. IGF-1 PGK promoter (see FIG. 14) and was used to transform competent yeast strain 20B-12 (ATCC 20626) (αtrp pep4) cells employing the Hitzeman modification (19) of Hinnen et al. (17), and Beggs et al. (18) procedures.
Several yeast transformants were grown in suspension in identical fashion as were those of the P.I. IGF-1 EcoRI-EcoRI P.I. promoter plasmid transformation and supernates were measured for activity determined by a radioimmune assay method of Furlanetto et al. (23) as modified by Hintz et al. (24). Shake flask supernates of three transformants contained activities ranging from 38 to 53 ng/ml of supernate. Similarly, one of these transformants was selected and grown in larger scale, utilizing a 10 liter fermenter and the secreted IGF-1 activity in the supernate reached a peak of ˜780 ng/ml. This fermentation supernate was also subjected to a radioreceptor assay (26) and was demonstrated to contain IGF-1 activity.
To construct a DNA sequence coding for the α-factor pre-pro protein fused to the DNA sequence coding for mature IGF-I, an M-13 in vitro mutagenesis technique was employed. (See Regin et al., Proc. Acad. Science (USA) 75, 4268; Hutchinson, et al., Journal Biological Chem. 253, 6551; Gilliam, et al., Gene 8, 81 and 99; Gillam, et al., Nucleic Acids Research 6, 2973; Adelman, et al., DNA (June, 1983).)
To construct the M-13 plasmid, the plasmid YEp9T α-factor EcoRI-ECoRI IGF-I (FIG. 16) was digested with BglII and SalI and the ca. 1.5 Kbp fragment containing the α-factor promotor-signal fused to IGF-I was isolated by polyacrylamide gel electrophoresis. This fragment was then ligated under standard ligation conditions to an MP-8 (BRL) vector digested with BamHI and SalI, and treated with bacterial alkaline phosphatase. This ligation mix was then transformed into competent JM101 cells prepared according to the method of Dagert and Ehrlich (3). These transformants were then mixed with non-competent JM101 cells grown to log phase, mixed with top agar and plated onto LB agar plates. Several clear plaques were picked and sequenced using the M-13 dideoxy sequencing technique to confirm the presence of an insertion into the SalI-BamHI sites of the vectors.
To perform the deletion according to the method above, a single strand of DNA of the sequence
was chemically synthesized by standard methods (2) and used to delete DNA sequence
just preceding the IGF-1 coding sequence of the α-factor promotor/signal IGF-I fusion sequence. This construction was then isolated as a replicative form, using a large scale plasmid preparation procedure from a JM101 cell culture inoculated with this plasmid containing the deletion.
The isolated replicative form (10 mg) was then digested with SalI. phenol-chloroform extracted and then ethanol precipitated and prepared for ligation. To this replicative form was ligated SalI-ECoRI linkers. After ligation and inactivation of the ligase by phenol, chloroform extraction followed by ethanol precipitation, the material was digested with ˜50 U EcoRI enzyme under standard conditions and then run onto a 6 percent polyacrylamide gel. The ca. 1.5 kbp RI-EcoRI fragment released was isolated from the gel and prepared for ligation using standard conditions.
Yeast vector was prepared by digestion of 10 mg YEP9T plasmid with 50 units of EcoRI followed by treatment with bacterial alkaline phosphatase. The digestion was then repeatedly phenol-chloroform extracted and then ethanol precipitated and prepared for ligation.
The ca. 1.5 kbp EcoRI-EcoRI fragment containing the deletion was then ligated to the EcoRI-EcoRI YEP9T vector and the ligation mix was then transferred into competent 294 cells prepared according to the method of Dagert and Erhlich (3) and miniscreened using the method of Birnboin and Doly (4). DNA prepared was screened by degestion with EcoRI and those DNAs illustrating an insertion of the ca. 1.5 kbp fragment were used to transform competent yeast strain 20B-12 (ATCC 20626) according to the modification of Hitzeman (19) of the Hinner, et al., (17), and Beggs, et al., (18) procedures.
Transformants were then grown in shaker flasks and supernates assayed and shown to have IGF-I activity by the radioimmune assay procedure of Furlanetto, et al., (23) as modified by Hintz, et al., (24).
One of these clones was grown in large scale in a 10-liter fermentor and IGF-I purified from the supernatant of this fermentation. This material was then subjected to amino terminal protein sequencing and shown to be mature IGF-I protein.
Human EGF is prepared in accordance with the invention following analogous procedures as those described above.
In a fashion similar to IGF-1, double stranded DNA (FIG. 15) synthesized either by chemical means or through polymerization reactions was assembled to form a mature EGF coding sequence, with a codon coding for methionine (ATG) just preceding the amino-terminal asparagine found in the mature protein, and a codon (GTC) substituting valine for methionine at residue number 21 from the amino-terminal asparagine. This construction was then attached at the 5′ end to an additional coding sequence, which when expressed in yeast or bacteria produced a fusion protein. This fusion protein was then susceptible to CNBr cleavage at the methionine to release the valine substituted human EGF molecule.
To secrete the mature form of EGF from yeast, the above sequence coding for the mature protein was attached to the α-factor promoter/prepro sequence, the codon coding for valine at residue number 21 was replaced by ATG, and the appropriate deletion was made to bring the coding sequence for mature EGF adjacent to the α-factor signal coding sequence (FIG. 16). This construction was then inserted into the yeast vector Yep9T and transformed into yeast. Transformants produced as such expressed and secreted mature human EGF. In addition, the sequence coding for mature EGF was attached to the preinvertase signal sequence (FIG. 17) and this construction, when inserted into the yeast vector Yep1PT Small containing the PGK promoter, and transformed into yeast, resulted in the expression and secretion of human EGF.
A double stranded DNA sequence coding for mature IGF-II was constructed from a combination of synthetic and natural DNA sequences (FIG. 18). This coding sequence, which did not contain an internal methionine, was attached to the TrpE leader protein coding sequence and was expressed as a fusion protein. Mature IGF-II was chemically cleaved from the purified fusion product by the action of CNBr upon a methionine residue preceding the first residue (alanine) of the mature protein.
The IGF-II coding sequence was also attached to the α-factor promoter/prepro sequence and after the appropriate deletion was made to bring the 3′ end of the α-factor signal coding sequence adjacent to the 5′ end of mature IGF-II coding sequence, the construction was inserted into the Yep9T vector and transformed into yeast. Resultant transformants expressed and secreted mature human IGF-II. In the same manner, the sequence coding for mature IGF-II was attached to the preinvertase coding sequence. The resultant construction was inserted into Yep1PT Small and transformed into yeast. Transformants produced as such expressed and secreted mature human IGF-II.
The compounds of the present invention can be formulated according to known methods to prepare pharmaceutically useful compositions, whereby the human IGF and human EGF or products hereof are combined in admixture with a pharmaceutically acceptable carrier vehicle. Suitable vehicles and their formulation, inclusive of other human proteins, e.g. human serum albumin are described, for example, in Remington's Pharmaceutical Sciences by E. W. Martin, which is hereby incorporated by reference. Such compositions will contain an effective amount of the protein hereof together with a suitable amount of vehicle in order to prepare pharmaceutically acceptable compositions suitable for effective administration.
Notwithstanding that reference has been made to particular preferred embodiments of the present invention, It will be understood that the present invention is not to be construed as limited to such, but rather to the lawful scope of the appended claims.
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