PhD Oral Exam - Daniel Beaudet, Biology
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Cytokinesis is the physical separation of a cell into two daughter cells. The core machinery is well-conserved across metazoans, yet the mechanisms that regulate cytokinesis can vary depending on the organism and cell type. Elucidating these mechanisms is important, because cytokinesis failure could lead to cell fate changes and aneuploidy, which are hallmarks of cancer. Cytokinesis occurs by the ingression of an actomyosin contractile ring that assembles between the segregating chromatids, and pinches in the plasma membrane. The mitotic spindle provides spatiotemporal signals that regulate the cytokinetic machinery. However, a number of studies have shown that spindle-independent pathways also regulate cytokinesis. The focus of my thesis is on the characterization of a Ran-GTP pathway that spatially regulates the contractile ring in response to chromatin position. During mitosis, there is a gradient of Ran-GTP with high levels around chromatin and lower levels toward the cortex. Ran-GTP competes importin(s) from the nuclear localization signal (NLS) of proteins, and we hypothesize that importin-binding to cortical proteins could regulate their function. In support of this hypothesis, we found that anillin, a key component of the contractile ring, contains an NLS within its C-terminus that is required for importin-binding and for its function during cytokinesis. Importin-binding facilitates anillin’s cortical recruitment, possibly by stabilizing an open conformation. Interestingly, the NLS is autoinhibited by the adjacent RhoA binding domain (RBD), and binding to active RhoA relieves this inhibition. I also found that feedback between the RBD and C2 domain, which contains the NLS, is required for optimal RhoA-binding and cortical recruitment. Thus, integrating a signal from chromatin could ensure that the contractile ring remains positioned between segregating chromatids. This mechanism could be crucial for the asymmetric division of polarized cells, or cells with aneuploidy.