Septins mediate cell-cycle-dependent exclusion of Cdc42 GAP Rga4 from growth sites in fission yeast

The polarized growth promoter Cdc42 is inactivated at the growing cell ends during mitosis and reactivated after division. In fission yeast, the Cdc42 inactivator/GAP Rga4 localizes to the cell ends during mitosis and is restricted to the sides in interphase. We show that this cell-cycle-dependent Rga4 localization is septin-dependent. Septins form linear filaments along the sides during interphase and recede to form a medial ring during mitosis. In septin mutants, Rga4 is mobile, homogeneously distributed, and increasingly localizes to the cell ends and cytoplasm, similar to mitotic cells. Accordingly, septin mutants are monopolar with disrupted Cdc42 activation dynamics. This regulation appears to be indirect, via septin-dependent lipid organization on the cortex. We find that the septin cytoskeleton prevents excessive phosphatidylinositol 4,5-bisphosphate (PI(4,5)P ₂ ) and its upstream kinase Its3 at the plasma membrane. Limiting PI(4,5)P ₂ levels enhances Rga4 puncta at the cortex. Our data describe an unusual form of polarity regulation where the septin cytoskeleton restricts GAP localization via membrane lipid organization.