Cdc42, a conserved regulator of cell polarity, is activated by two GEFs, Gef1 and Scd1, in fission yeast. Why the cell needs two GEFs is unclear, given that they are partially redundant and activate the same GTPase. Using the GEF localization pattern during cytokinesis as a paradigm, we report a novel interplay between Gef1 and Scd1 that spatially modulates Cdc42. We find that Gef1 promotes Scd1 localization to the division site during cytokinesis through recruitment of the scaffold protein Scd2, via a Cdc42 feedforward pathway. Similarly, during interphase Gef1 promotes Scd1 recruitment at the new end to enable the transition from monopolar to bipolar growth. Reciprocally, Scd1 restricts Gef1 localization to prevent ectopic Cdc42 activation during cytokinesis to promote cell separation, and to maintain cell shape during interphase. Our findings reveal an elegant regulatory pattern in which Gef1 primes Cdc42 activation at new sites to initiate Scd1-dependent polarized growth, while Scd1 restricts Gef1 to sites of polarization. We propose that crosstalk between GEFs is a conserved mechanism that orchestrates Cdc42 activation during complex cellular processes. This article has an associated First Person interview with the first author of the paper.

Cytokinesis consists of a series of coordinated multi-step events that partition a dividing cell. Accurate regulation of cytokinesis is essential for proliferation and genome integrity. In fission yeast, these coordinated events ensure that the actomyosin ring and septum start ingressing only after chromosome segregation. How cytokinetic events are coordinated is unclear. The GTPase Cdc42 is required for the delivery of certain cell wall-building enzymes while the GTPase Rho1 is required for activation of these enzymes. Here we show that Cdc42 prevents early Rho1 activation during cytokinesis. Using a Rho-probe, we report that Rho1 is activated in late anaphase, just before the onset of ring constriction, even though, the primary Rho1 activators Rgf1 and Rgf3 localize to the division site in early anaphase. We find that loss of Cdc42 activation enables precocious Rho1 activation in early anaphase. Furthermore, this inhibition of Rho1 activation is dependent on the downstream Cdc42 effector Pak1 kinase. Disrupting pak1 function resulted in early Rho1 activation accompanied by precocious septum deposition and ring constriction. We provide functional and genetic evidence which indicates that Pak1 regulates Rho1 activation likely via the regulation of its GEF Rgf1. Our work proposes a mechanism of regulation of Rho1 by active Cdc42 to coordinate timely septum formation and cytokinesis fidelity.

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During cytokinesis a series of coordinated events partition a dividing cell. Accurate regulation of cytokinesis is essential for proliferation and genome integrity. In fission yeast, these coordinated events ensure that the actomyosin ring and septum start ingressing only after chromosome segregation. How cytokinetic events are coordinated remains unclear. The GTPase Cdc42 promotes recruitment of certain cell wall-building enzymes while the GTPase Rho1 activates these enzymes. We show that Cdc42 prevents early Rho1 activation during cytokinesis. Using an active Rho-probe, we find that although the Rho1 activators Rgf1 and Rgf3 localize to the division site in early anaphase, Rho1 is not activated until late anaphase, just before the onset of ring constriction. We find that loss of Cdc42 activation enables precocious Rho1 activation in early anaphase. Furthermore, we provide functional and genetic evidence that Cdc42-dependent Rho1 inhibition is mediated by the Cdc42 target Pak1 kinase. Our work proposes a mechanism of Rho1 regulation by active Cdc42 to coordinate timely septum formation and cytokinesis fidelity.

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