Importin (karyopherin)-beta, Importin (karyopherin)-alpha, Tubulin (in microtubules), MAD1 (mitotic checkpoint), Importin (karyopherin)-beta, Mitotic cohesin complex, RCC1, Cyclin B, KNSL1, Ubiquitin, CDC20, Dynein 1, cytoplasmic, intermediate chains, Aurora-B, Tubulin alpha, ZW10, Aurora-A, CSE1L, TPX2, Nek2A, Importin (karyopherin)-alpha, HEC, MAD2a, Dynactin complex, Kid, DCTN2, Dynein 1, cytoplasmic, light chains, Ran, DNA, Securin, NUMA1, Dynein 1, cytoplasmic, heavy chain, Anaphase- promoting complex (APC), CDK1 (p34), APC/CDC20 complex, Separase
Spindle assembly and chromosome separation
The two critical steps for cell reproduction are to duplicate the contents of chromosomes and to segregate them into two daughter cells. The linkage between duplicated chromosomes is established during the S phase (cohesion), persists during their dramatic structural changes in prometaphase (condensation), and is finally dissolved at the metaphase-anaphase transition (separation) .
Microtubes are attached to the kinetochore complex during metaphase-anaphase via Dynein/ Dynactin system. Once microtubes attach, the kinetochore receptor (e.g. Highly expressed in cancer protein ( HEC )) binds to the Mitotic-arrest deficient 1, yeast, homolog-like 1 ( MAD1 )/ Mitotic-arrest deficient 2, yeast, homolog-like 1 ( MAD2a ) complex with reduced affinity . In addition, the cell cycle-regulated serine phosphorylation of HEC by NIMA (never in mitosis gene a)-related kinase 2 ( NEK2A ) is essential for faithful chromosome segregation . Zeste white 10 homolog ( Zw10 ) and MAD1/ MAD2a are simultaneously shed, as Dynein/ Dynactin pulls them off together, along the kinetochore microtubes. Removing the MAD1/ MAD2a complex shuts off the generation of MAD2a/ Cell division cycle 20 homolog ( CDC20 ), thus freeing CDC20 for activating the anaphase-promoting complex ( APC ) .
APC/CDC20 complex is a one of ubiquitin ligases, which plays a key role in the cell cycle. It ubiquitinates proteins, targeting these substrates for degradation by the 26S proteasome. APC/CDC20 complex is activated by Cyclin-dependent kinase 1 ( CDK1 )/ Cyclin B during metaphase-anaphase .
The APC/CDC20 complex -dependent degradation of pituitary tumor-transforming protein 1 ( Securin ) participate in cleavage of the cohesion complex, thereby allowing sister chromatid separation. The entry into anaphase is marked by the initiation of sister chromatid separation . Moreover, APC/CDC20 complex participate in cleavage other members sister chromatid cohesion and chromosome condensation (e.g. serine/threonine kinase 12 ( Aurora-B ) .
The spindle assembly is subjected to the regulatory controls of both the cell-cycle machinery and the Ras-related nuclear GTP binding protein ( Ran )-signaling pathway. CDK1/Cyclin B kinase phosphorylate the serines located in or near the nuclear localization signal (NLS) of human Regulator of chromosome condensation 1 ( RCC1 ), the nucleotide exchange factor for Ran. This phosphorylation is necessary for RCC1 to generate Ran-GTP on mitotic chromosomes in mammalian cells, which in turn is required for spindle assembly and chromosome segregation. Moreover, phosphorylation of the NLS of RCC1 is required to prevent the binding of Soluble receptor proteins Karyopherin alpha/beta complex to RCC1, thereby allowing RCC1 to couple Ran-GTP production to chromosome binding .
Ran-GTP, in turn, promotes the dissociation of Karyopherin alpha/beta complex from their cargo proteins and the export of Karyopherins from nucleus. Thus proteins are activated by dissociation of Karyopherins. Some from these proteins participate in spindle assembly. There are spindle assembly factors Microtubule-associated protein homolog Xenopus laevis TPX2  and Nuclear mitotic apparatus protein 1 ( NUMA1 ) , plus end-directed microtubule-based motor Kinesin-like DNA-binding protein ( Kid ) . In addition, Ran-GTP participates in activation plus end-directed microtubule-based motor Kinesin family member 11 ( KNSL1 ) .