The Septin Cytoskeleton excludes the Cdc42 GAP Rga4 from growing sites to promote bipolarity in fission yeast

The conserved GTPase Cdc42 regulates polarity in most eukaryotes and is spatiotemporally regulated to confine growth to discreet areas throughout the cell cycle. Cdc42 activity stops at the growing ends of fission yeast during mitosis and resumes after division when the ends remove the inactivator/GAP Rga4. In interphase, Rga4 appear punctate at the cell sides restricted from the growing ends. During mitosis, Rga4 appears homogeneous and localizes to the ends. We show that this cell-cycle-dependent Rga4 localization is septin cytoskeleton-dependent. Septins forms linear filaments along the sides during interphase. In mitosis, the septins recede from the sides and form a ring at the division site. In septin mutants, Rga4 is mobile, homogeneously distributed and localizes to the ends, regardless of cell-cycle stage. Accordingly, septin mutants show delayed Cdc42 reactivation after division and are monopolar with disrupted Cdc42 activation dynamics. Our data describe an unusual form of polarity regulation where a septin-dependent physical barrier restricts GAP localization for proper Cdc42 dynamics.