CELL MORPHOLOGY AND CELL DIVISION ARE PRECISELY CONTROLLED THOUGH DIFFERENTIAL REGULATION OF CDC42

The conserved Rho family GTPase Cdc42 is a master regulator of cell polarity in eukaryotes. In the fission yeast Schizosaccharomyces pombe, Cdc42 regulated polarized cell growth, cell shape, and cytokinesis. Cdc42 is activated by its two GEFs (Guanine nucleotide exchange factors), Gef1 and Scd1. Loss of either gef1 or scd1 gives rise to distinct cell shapes. How this occurs in unclear, given that their effects are both mediated through the same GTPase. Understanding Cdc42 regulation during polarized growth is complicated by the presence of two cell poles that compete for Cdc42 and its activators, by the overlapping localization pattern of its GEFs, by the oscillation of active Cdc42 between the two poles, and by the presence of positive and negative feedbacks. To overcome this limitation, the process of cytokinesis, in which there is no competition and the GEFs localize sequentially, was used as a paradigm to understand Cdc42 regulation, which could then be extended to test similar behaviors at the sites of polarized growth. This approach led to the discovery of a crosstalk between Gef1 and Scd1 that maintains cell shape and promotes bipolar growth. A similar approach led to the discovery that the F-BAR Cdc15 genetically interacts with Gef1, promoting its localization to both the division site to initiate ring constriction, and to the cell poles to initiate new end growth. Furthermore, the study of Cdc42 during cytokinesis uncovered the novel finding that the precise regulation of Cdc42 spatially restricts membrane trafficking to enable cell separation during cytokinesis.