c-Raf-1, Insulin, PP1-cat alpha, EGF, AKT(PKB), PP1-cat, MEK2(MAP2K2), Insulin receptor, c-Cbl, PI3K cat class IA, GAB1, PtdIns(3,4,5)P3, MEK1(MAP2K1), eIF2AK3, GRB2, EGFR, DYRK2, PKR, H-Ras, DYRK1a, IRS-1, PI3K reg class IA, eIF2S1, Casein kinase II, alpha chains, 220.127.116.11, Erk (MAPK1/3), eIF2AK1, SOS, IRS-2, eIF2, GSK3 alpha/beta, PDK (PDPK1), eIF2B5, eIF2B, GCN2, Shc, Casein kinase I, Casein kinase II, beta chain (Phosvitin), PtdIns(4,5)P2
Regulation of eIF2
Eukaryotic translation initiation factors ( eIF2 ) is a G-protein that is composed of 3 non-identical subunits, eIF2S1, eIF2S2 and eIF2S3, and catalyzes the first regulated step of protein synthesis initiation, promoting the binding of the initiator tRNA to 40S ribosomal subunits. Phosphorylation of eIF2S1 modulates rate of formation of the complex eIF2/GTP/tRNA .
Key pathways of eIF2 activity regulation are phosphorylation of eIF2S1 and phosphorylation of Eukaryotic initiation factor 2 ( eIF2B ) by some protein kinases.
Four distinct protein kinases inhibit protein synthesis in eukaryotic cells via phosphorylation of eIF2S1 at serine-51. There are Eukaryotic translation initiation factor 2-alpha kinase 1, 2, 3 and 4 ( EIF2AK1, PKR, EIF2AK3 and GCN2 ), that belong to the serine/threonine family of protein kinases. Phosphorylation of eIF2S1 results in shutdown of protein synthesis.
EIF2AK1 is activated under conditions of heme deficiency, predominantly in immature erythroid cells, and its activity is inhibited by heme .
PKR acts as an antiviral machinery of type I Interferons. Expression of PKR is induced by interferon, and its kinase activity is stimulated by low concentrations of double-stranded RNA. PKR -mediated inhibition is neutralizated via direct dephosphorylation and monomerization of PKR by Alpha catalytic subunit protein phosphatase 1 ( PP1 ) . Also, PP1 can dephosphorylates eIF2S1 thereby activating eIF2 formation .
EIF2AK3 is activated under conditions of Endoplasmic reticulum (ER) stress. ER stress is caused by unfolded or misfolded proteins, which are accumulared by extracellular or intracellular stimuli .
GCN2 is activated by amino acid starvation phosphorylates. GCN2 inhibites EIF2S1. It is one of the eukaryotic initiation factors that have a role in eukaryotic peptide chain initiation process .
Exchange of GDP for GTP on eIF2 is stimulated by eIF2B. eIF2B is a heteromeric guanine nucleotide exchange factor that plays an important role in regulating mRNA translation. It is composed of 5 subunits termed eIF2B1-5 in order of increasing size .
eIF2B has multiple phosphorylation sites in the largest, catalytic, subunit eIF2B5.
Kinases, which phosphorylate of the eIF2B5- subunit of eIF2B, are Casein kinase I and II, Glycogen synthase kinase 3 alpha and beta isoforms ( GSK3 alpha/beta ), and Dual specificity tyrosine-phosphorylated and -regulated kinases ( DYRK1a and DYRK2 ). elF2B5 phosphorylation by casein kinases enhances eIF2B activity, whereas phosphorylation by GSK3 has an inhibitory effect . Phosphorylation by GSK3 requires previous elF2B5 phosphorylation that is catalyzed by DYRKs .
Phosphotase PP1 deviates inhibitory effect of GSK3 via dephosphorilation of eIF2B . Activity of PP1 and GSK3 is regulated by different extracellular stimulus.
For example, Insulin receptor and Epidermal growth factor receptor ( EGFR ) activate the enzymatic activity of Phosphatidylinositol 3-kinase class I via recruitment of regulatory subunit ( PI3K reg ) either directly or via adaptor proteins (e.g. Insulin receptor substrates 1 and 2 ( IRS-1 and IRS-2 ), GRB2-associated binding protein 1 ( GAB1 ) or Signal transduction protein CBL ( c-Cbl )) , , .
Active Phosphatidylinositol 3-kinase class I catalytic ( PI3K cat ) converts Phosphatidylinositol 4,5-biphosphate ( PtdIns(4,5)P2 ) to Phosphatidylinositol 3,4,5-triphosphate ( PtdIns(3,4,5)P3 ) . PtdIns(3,4,5)P3 is a second messenger. It recruits and activates V-akt murine thymoma viral oncogene homolog 1 ( AKT ) and Phosphoinositide dependent protein kinase-1 ( PDK ) to membrane via PH domain . AKT, in turn, negatively regulates GSK3 by phosphorylation thereby abrogate inhibitory effect GSK3 to eIF2B activity , .
Also EGFR induces MAPK cascade. It was shown, that MEK/ Mitogen-activated protein kinase ( ERK) pathway is required for activation of eIF2B. It was suggested that the active ERK activates phosphotase PP1. This leads to activation of eIF2B .