Cytokinetic events that promote actomyosin ring constriction and septum formation

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Proper cell division is critical for growth, development, and survival of an organism. Defects in cell division have been linked to developmental anomalies and various afflictions. The final stage of cell division is cytokinesis, where the two daughter cells are physically partitioned. Successful cytokinesis requires multiple steps that are precisely controlled both in time and space. The nature and molecular details of this precise regulation is poorly understood. This project aims at understanding how this complex multi-step process is precisely controlled to ensure successful cell separation. Cytokinesis is evolutionarily conserved from yeast to mammals and hence what is learned in yeast is directly applicable to mammals (such as humans). One of the best model systems to study cell division and cytokinesis is the fission yeast. Preliminary work using the fission yeast model system has, with great clarity, revealed the mechanistic details of the early stages of cytokinesis. Unraveling the molecular details in cytokinesis in the fission yeast will provide a paradigm to study similar processes in complex organisms and will lead to a better understanding of the etiology of various anomolies related to cytokinesis. Cytokinesis, the final step in cell division, is central to survival and development of all cells. Cytokinesis is a temporally organized multistep process that involves the physical separation of a cell into two. First, an actomyosin ring forms, which then undergoes constriction concurrent with septum ingression for membrane furrow formation. It is not clear how these events are temporally organized. In fission yeast, septum ingression is critical for membrane furrow formation. The ingressing septum provides the force required to overcome internal turgor pressure in the cell. It is not clear how actomyosin ring constriction and septum ingression occur concurrently. Membrane furrowing can occur without an actomyosin ring so what role does the actomyosin ring play in cytokinesis? Ring constriction and septum ingression occurs after a short waiting period, post ring assembly. What triggers the onset of ring constriction and septum formation? Preliminary data shows that post-ring assembly, the conserved GTPase Cdc42 is activated in a unique spatiotemporal manner. Cdc42 activation pattern during cytokinesis depends on the localization of its activators, Gef1 and Scd1. Gef1 promotes the recruitment of a septum synthesizing protein at the ring to allow timely onset of ring constriction and septum ingression, while Scd1 is required for normal septum formation. Gef1 also promotes uniform ring constriction and septum ingression. Based on this data the proposed central hypothesis is that spatiotemporal activation of Cdc42 enables the assembled actomyosin ring to act as a landmark and guide for proper ring constriction and septum formation. To test this, the following aims will be pursued; 1. To define how Cdc42 is spatiotemporally activated at the division site post ring assembly; 2. To determine how ring constriction and septum formation is promoted by spatiotemporal activation of Cdc42; and 3. To describe how the actomyosin ring acts as a landmark and guide for symmetric ring constriction and septum formation. This project will integrate live cell imaging, analysis of protein dynamics, genetics, and mathematical modeling. This project will provide a deeper understanding of how unique Cdc42 activation patterns are established, leading to distinct functions in cytokinesis and will provide insights into how signaling patterns organize complex multistep processes in the cell. This project will have widespread implications in the mechanistic understating of cytokinesis in most eukaryotes.