Insights into the kinetochore

Anaphase-promoting complex/cyclosome controls HEC1 stability

OBJECTIVE

Chromosome segregation during mitosis requires a physically large proteinaceous structure called the kinetochore to generate attachments between chromosomal DNA and spindle microtubules. It is essential for kinetochore components to be carefully regulated to guarantee successful cell division. Depletion, mutation or dysregulation of kinetochore proteins results in mitotic arrest and/or cell death. HEC1 (high expression in cancer) has been reported to be a kinetochore protein, depletion of which, by RNA interference, results in catastrophic mitotic exit.

MATERIALS AND METHODS AND RESULTS

To investigate how HEC1 protein is controlled post-translation, we analysed the role of anaphase-promoting complex/cyclosome (APC/C)-Cdh1 in degradation of HEC1 protein. In this study, we show that HEC1 is an unstable protein and can be targeted by endogenous ubiquitin-proteasome system in HEK293T cells. Results of RNA interference and in vivo ubiquitination assay indicated that HEC1 could be ubiquitinated and degraded by APC/C-hCdh1 E3 ligase. The evolutionally conserved D-box at the C-terminus functioned as the degron of HEC1, destruction of which resulted in resistance to degradation mediated by APC/C-Cdh1. Overexpression of non-degradable HEC1 (D-box destroyed) induced accumulation of cyclin B protein in vivo and triggered mitotic arrest.

CONCLUSION

APC/C-Cdh1 controls stability of HEC1, ensuring normal cell cycle progression.

Protein architecture of the human kinetochore microtubule attachment site

Chromosome segregation requires assembly of kinetochores on centromeric chromatin to mediate interactions with spindle microtubules and control cell-cycle progression. To elucidate the protein architecture of human kinetochores, we developed a two-color fluorescence light microscopy method that measures average label separation, Delta, at <5 nm accuracy. Delta analysis of 16 proteins representing core structural complexes spanning the centromeric chromatin-microtubule interface, when correlated with mechanical states of spindle-attached kinetochores, provided a nanometer-scale map of protein position and mechanical properties of protein linkages. Treatment with taxol, which suppresses microtubule dynamics and activates the spindle checkpoint, revealed a specific switch in kinetochore architecture. Cumulatively, Delta analysis revealed that compliant linkages are restricted to the proximity of chromatin, suggested a model for how the KMN (KNL1/Mis12 complex/Ndc80 complex) network provides microtubule attachment and generates pulling forces from depolymerization, and identified an intrakinetochore molecular switch that may function in controlling checkpoint activity.