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/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
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
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
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