Cell Cycle Control of Spindle Morphogenesis in Saccharomyces cerevisiae
Cell cycle, cyclin-dependent kinase, spindle polarity, cortical cues, microtubule capture, Saccharomyces cerevisiae
Spindle morphogenesis is regulated by cyclin-dependent kinases (CDKs) and monitored by checkpoints to enforce accurate chromosomal segregation and spatial coupling between the axis of the mitotic spindle and the division plane. These controls safeguard genetic integrity over cell divisions. Furthermore, regulated orientation of the mitotic spindle during development can produce asymmetric divisions with progeny cells differing in their developmental fate. The budding yeast S. cerevisiae is one of the best models for exploring cell cycle controls during an asymmetric division that gives rise to mother and daughter cells. In S. cerevisiae, the mitotic spindle becomes positioned near the bud neck and orients along the mother-bud axis prior to anaphase. This alignment ensures that the daughter cell receives one set of chromosomes during spindle elongation across the bud neck. Spindle assembly and orientation are controlled by the spindle pole bodies (SPBs), the functional counterparts of the centrosomes of animal cells. As part of our goal to dissect cell cycle control of spindle morphogenesis in S. cerevisiae we have focused on the events that determine SPB asymmetry or spindle polarity, i.e. the mechanisms directing one pole towards the daughter while retaining the second pole in the mother cell, bringing about orientation of the spindle. This entails two levels of control, currently under study in our lab:
a) Inherent SPB asymmetry: CDKs set a temporal asymmetry to limit astral microtubules to the old SPB (the one present from the preceding cell cycle) prior to spindle assembly. We wish to identify the CDK targets behind this control.
b) Bud6p-mediated cortical program for astral microtubule capture: SPBs are targeted to opposite cell cortex compartments differentially marked by factors promoting cortical capture of astral microtubules. The polarity determinant Bud6p sequentially accumulates at the bud and bud neck cortex. From the bud tip, it engages microtubules from the old SPB to contact the growing bud. Once at the bud neck, it stops microtubules of the new SPB from interacting with the bud. This programme commits the old SPB to enter the bud and confines the new SPB to the mother cell. We are exploring the molecular basis of this cortical control as well as the specific role of Bud6p in microtubule capture at the cell cortex.
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