Stable kinetochore-microtubule attachment is sufficient to satisfy the spindle assembly checkpoint
Date
2016
Authors
Tauchman, Eric Cary, author
DeLuca, Jennifer G., advisor
Chen, Chaoping, committee member
Ross, Eric, committee member
Wilusz, Carol, committee member
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Abstract
During mitosis, duplicated sister chromatids attach to microtubules emanating from opposing sides of the bipolar spindle through large protein complexes called kinetochores. The kinetochore proteins that bind spindle microtubules are exquisitely regulated to ensure correct segregation of genetic material at mitotic exit. Aurora B Kinase (ABK) phosphorylates Hec1, a protein that directly binds microtubules. This is critical for enabling the release of incorrect kinetochore-microtubule attachments. Hec1 has nine ABK phosphorylation sites on its tail domain allowing for precise control over binding affinity. We find that at least 7 of these sites are required for wild-type kinetochore-microtubule (K-MT) attachment stability as evaluated by inter-kinetochore distance measures and chromosome alignment capability. We further observe that several sites may have more influence on K-MT attachment stability than others. Hec1 mutations preventing phosphorylation increase kinetochore-microtubule attachment stability. In the absence of stable kinetochore–microtubule (K-MT) attachments, a cell surveillance mechanism known as the spindle assembly checkpoint (SAC) produces an inhibitory signal that prevents anaphase onset. Precisely how the inhibitory SAC signal is extinguished in response to microtubule attachment remains unresolved. To address this, we induced formation of hyper-stable kinetochore–microtubule attachments in human cells using a non-phosphorylatable Hec1mutant, a core component of the attachment machinery. This mutant reduced the ability of ABK to cause release of erroneous K-MT so we could test the hypothesis that stable K-MT attachments satisfy the SAC even if those attachments deviate from the canonical bipolar form. We find that stable attachments are sufficient to satisfy the SAC in the absence of sister kinetochore bi-orientation and strikingly in the absence of detectable microtubule pulling forces or tension. Furthermore, we find that SAC satisfaction occurs despite the absence of large changes in intra-kinetochore distance, suggesting that substantial kinetochore stretching is not required for quenching the SAC signal. These results indicate a conformational change(s), within the kinetochore that occurs upon stable kinetochore-microtubule binding causes the eviction of SAC proteins. This advance in our understanding of SAC function offers insight into the mode of action and the variation in cellular response to mitotic arrest therapies often used in treatments of cancers.
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Subject
microtubule
kinetochore
spindle assembly checkpoint