The Cdc42 GEF, Gef1, promotes uniform protein distribution along the actomyosin ring to enable concentric furrowing

During cytokinesis, fission yeast coordinates actomyosin ring constriction with septum ingression, resulting in concentric furrow formation. Mechanisms coordinating septum ingression with the actomyosin ring remain unclear. We report that cells lacking the Cdc42 activator Gef1, combined with an activated allele of the formin, Cdc12, display non-concentric furrowing. Although cells that furrow non-concentrically display normal actomyosin rings, the scaffold Cdc15 is unevenly distributed along the ring. This suggests that after ring assembly, uniform Cdc15 distribution along the ring drives proper furrow formation. We find that Cdc15 levels at the ring are reduced in the activated cdc12 mutant, or upon disruption of Arp2/3 complex-dependent endocytic patches. Furthermore, Cdc15 levels in endocytic patches increase in gef1 mutants. We hypothesize that assembled rings recruit Cdc15 from endocytic patches. Patches with higher Cdc15 levels and slower ring-association rate lead to uneven Cdc15 distribution. Based on this hypothesis we developed a mathematical model that captures experimentally observed Cdc15 distributions along the ring. We propose that, at the ring, Gef1 and endocytic events promote uniform Cdc15 distribution to enable proper septum ingression and concentric furrow formation.\n\nSummary StatementGef1 and endocytic events at the assembled actomyosin ring facilitate uniform Cdc15 distribution along the ring thus enabling concentric furrow formation.