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Description:
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To avoid missegregation of chromosomes during mitosis cells employ a surveillance
mechanism termed Spindle assembly checkpoint that senses the lack of
tension /attachment on the kinetochores and consequently blocks anaphase onset by
inhibiting an E3 ubiquitin ligase called anaphase -promoting complex . The roles of two kinases - BubR1 and Mps1 , implicated in spindle assembly checkpoint were investigated .
A checkpoint complex containing BubR1 and Bub3 has been purified from mitotic
human cells . BubR1 directly interacts with Cdc20 and inhibits the activity of APC in
vitro ,much more efficiently than Mad2 . Surprisingly , the kinase activity of BubR1 or
association with Bub3 is not required for the inhibition of APCCdc20 . Furthermore , BubR1
restores the mitotic arrest in Cdc20 -overexpressing cells treated with nocodazole . Mps1
is a dual specificity kinase that localizes to kinetochores in mitosis . Depletion of Mps1
by RNAi leads to the abrogation of spindle assembly checkpoint .
The kinetochore proteins involved in the recruitment of checkpoint proteins and the
generation of wait -anaphase signal have not been identified . Kinetochores also provide
the attachment sites for spindle microtubules and are required for the alignment of
chromosomes at the metaphase plate (chromosome congression ) . Components of the
conserved Ndc80 complex have been implicated in both these function . To better
understand the function of the Ndc80 complex , we have identified two novel subunits of
the human Ndc80 complex , termed human Spc25 (hSpc25 ) and human Spc24 (hSpc24 ) ,
using an immuno -affinity approach . Human Spc25 interacts with Hec1 (human Ndc80 )
throughout the cell cycle and localizes to kinetochores during mitosis . RNAi -mediated
depletion of hSpc25 in HeLa cells causes aberrant mitosis followed by cell death , a
phenotype similar to that of cells depleted for Hec1 . Loss of hSpc25 also causes multiple
spindle aberrations , including elongated , multipolar , and fractured spindles . In the
absence of hSpc25 , Mad1 and Hec1 fail to localize to kinetochores during mitosis
whereas the kinetochore localization of Bub1 and BubR1 is largely unaffected .
Interestingly , the kinetochore localization of Mad1 in cells with a compromised Ndc80
function is restored upon microtubule depolymerization . Thus , hSpc25 is an essential
kinetochore component that plays a significant role in proper execution of mitotic events . |