| Over the past two decades, kinases and phosphatases have been shown to play leading roles in many cellular processes, particularly in the control of the cell division. My thesis work has characterized unknown functional relationships between two key mitotic kinases (Aurora A (AurA) and cyclin-dependent kinase 1 (CDK1)), and the protein phosphatase 2 (PP2A). These proteins serve indispensable roles in cell division, and their misregulation is known to give rise to diseases, including cancer. Activation of AurA involves the binding of multiple co-activators, autophosphorylation, and downregulation of the phosphatase PP1. However, the cell cycle field still lacks a systematic integration of how different factors function together to control AurA activity spatiotemporally. Moreover, there remains disagreement regarding the roles of AurA in mitotic initiation, and how alterations in its abundance and activity affect mitotic progression. My research uncovered interplay between AurA and phosphatase activities that is crucial for AurA activity itself, as well as mitotic entry timing. It reveals that it is PP2A, rather than PP1, that serves as the interphase AurA T-loop phosphatase during the Xenopus embryonic cell cycle. Moreover, overexpression of AurA not only delays mitotic entry, but also causes mitotic arrest—potentially through saturating M-phase promoting phosphatase(s)—due to a persistent high CDK1 activity and an elevated level of residual cyclin B. Further research revealed that the lack of net increase of cyclin B prior to mitotic entry is not due to a balance between its translation and degradation, but results from a CDK1-mediated negative-feedback loop directed towards cyclin B synthesis. This negative feedback functions during normal mitotic cycles, increasing efficiency and robustness of the CDK1-anaphase-promoting complex (APC) oscillator. Together, my thesis work describes for the first time the functional importance of translational repression and phosphatase activities in their regulation of the kinases that control mitosis. |
