Differential GAP requirement for Cdc42-GTP polarization during proliferation and sexual reproduction

The formation of a local zone of Cdc42 GTPase activity, which governs cell polarization in many cell types, requires not only local activation but also switch-off mechanisms. Here we identify Rga3, a paralog of Rga4, as a novel Cdc42 GTPase activating protein (GAP) in the fission yeast S. pombe. Contrary to Rga4, Rga3 localizes with Cdc42-GTP to sites of polarity. Rga3 is dispensable for cell polarization during mitotic growth, but limits the lifetime of unstable Cdc42-GTP patches that underlie cell pairing during sexual reproduction, masking a partly compensatory patch wandering motion. In consequence, cells lacking rga3 hyperpolarize and loose out in mating competition. Rga3 synergizes with the Cdc42 GAPs Rga4 and Rga6 to restrict Cdc42-GTP zone sizes during mitotic growth. Surprisingly, triple mutant cells, which are almost fully round, retain pheromone-dependent dynamic polarization of Cdc42-GTP, extend a polarized projection and mate. Thus, the requirement for Cdc42-GTP hydrolysis by GTPase activating proteins is distinct during polarization by intrinsic or extrinsic cues.