Elucidating the Role of Cdc42 in the Regulation Cytokinesis Events

Cdc42 is activated by guanine nucleotide exchange factors (GEFs) and inactivated by GTPase activating proteins (GAPs) and GDP dissociation inhibitors (GDIs). GTPases function as molecular switches used in cells to turn on or off different signaling pathways. In animal cells, Cdc42 binds to actin nucleating factors, promotes nucleation of different actin networks, and drives the assembly of polarity protein complexes. While emerging work shows roles for Cdc42 in cytokinesis in germline cells, budding, and fission yeasts, where it is antagonistic with another Rho-GTPase Rho1, its role in this process remains elusive. The data presented in this work investigates the role of Cdc42 in cytokinesis. Cdc42 is activated at the cell division site during cytokinesis in a sequential manner by its activators GEFs Gef1 and Scd1, respectively. While Gef1 localizes to the division site to activate Cdc42 in the early stages of cytokinesis (Anaphase A), Scd1 activates Cdc42 later (Anaphase B). During cytokinesis, loss of gef1 delays the timing of ring constriction. An investigation into the role of Gef1 and Cdc42 in cytokinesis reveals that Cdc42 regulation promotes the proper progression of cytokinesis. The first chapter elucidates the role of Cdc42 in the localization of the essential septum building enzyme Bgs1. The second chapter describes how Cdc42 via Gef1 activation promotes uniform protein distribution along the ring-membrane interphase, and how this promotes concentric ring constriction and septum ingression. The third chapter identifies the role of Cdc42 activation in early anaphase where it blocks the premature activation of the Rho1 GTPase. These findings highlight the role of Cdc42 in the coordination of cytokinetic events. Furthermore, these data show how cells can utilize multifaceted networks such as Cdc42 regulation as one of many redundant mechanisms to ensure the integrity of essential multiple-step cellular processes.