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During cytokinesis, animal and fungal cells form a membrane furrow via actomyosin ring constriction. Our understanding of actomyosin ring-driven cytokinesis stems extensively from the fission yeast model system. However, unlike animal cells, actomyosin ring constriction occurs simultaneously with septum formation in fungi. While the formation of an actomyosin ring is essential for cytokinesis in fission yeast, proper furrow formation also requires septum deposition. The molecular mechanisms of spatiotemporal coordination of septum deposition with actomyosin ring constriction are poorly understood. Although the role of the actomyosin ring as a mechanical structure driving furrow formation is better understood, its role as a spatiotemporal landmark for septum deposition is not widely discussed. Here we review and discuss the recent advances describing how the actomyosin ring spatiotemporally regulates membrane traffic to promote septum-driven cytokinesis in fission yeast. Finally, we explore emerging questions in cytokinesis, and discuss the role of extracellular matrix during cytokinesis in other organisms.

The conserved small Rho GTPase Cdc42 has recently reported to play a role in cytokinesis in fission yeast, Schizosaccharomyces pombe. While the role of Cdc42 in cell polarity has been widely studied, little is known about its role in the regulation of cytokinesis. Here we investigate specific roles of Cdc42 during cytokinesis in fission yeast. Cytokinesis in fission yeast involves the coupling of actomyosin ring constriction to division cell wall (septum) synthesis. Cdc42 activators have been shown to promote timely initiation of septum synthesis and proper septum morphology. However, direct evidence supporting a role for Cdc42 in these events are lacking. Using a thermosensitive cdc42 allele (cdc42-1625), we investigated the direct role of Cdc42 in cytokinesis. We show that Cdc42 is essential for recruitment of the major primary septum synthesizing enzyme Bgs1 during cytokinesis; and that disruption of Cdc42 activity, prevented the initiation of ring constriction. Further, we find that Cdc42 is required for proper localization of the septum digesting enzymes Eng1 and Agn1 at the division site. Thus, we demonstrate that Cdc42 is required during cytokinesis to enable primary septum synthesis, and proper cell separation.

During cytokinesis, fission yeast coordinates actomyosin ring constriction with septum ingression, resulting in concentric furrow formation by a poorly defined mechanism. We report that Schizosaccharomyces pombe cells lacking the Cdc42 activator Gef1, combined with an activated allele of the formin, Cdc12, display non-concentric furrowing. Non-concentrically furrowing cells display uneven distribution of the scaffold Cdc15 along the ring. This suggests that, after ring assembly, uniform Cdc15 distribution along the ring enables proper furrow formation. We find that, after assembly, Cdc15 is recruited to the ring in an Arp2/3 complex-dependent manner and is decreased in the activated cdc12 mutant. Cdc15 at cortical endocytic patches shows increased levels and extended lifetimes in gef1 and activated cdc12 mutants. We hypothesize endocytosis helps recruit Cdc15 to assembled rings; uneven Cdc15 distribution at the ring occurs when endocytic patches contain increased Cdc15 levels and the patch-association rate is slow. Based on this, 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 organization to enable proper septum ingression and concentric furrow formation.