Interdependence of actomyosin ring contraction and septum formation in budding yeast (p. 293)

During cytokinesis in budding yeast, invagination of the plasma membrane is accompanied by formation of a chitin-based septum separating mother and daughter cells. This invagination is driven by constriction of an actomyosin ring at the bud neck; however, the relationship between septum synthesis and ring constriction is unclear. Enrico Cabib and co-workers have examined these two processes genetically. They show that mutations in Myo1p (a type II myosin responsible for contractile ring function) and Chs2p (chitin synthase II - an enzyme required for primary septum formation) produce similar phenotypes: plasma membrane invagination and primary septum formation do not occur, instead cells produce a remedial septum and undergo abnormal budding. Significantly, these defects are not more pronounced in myo1 chs2 double mutants. They are, by contrast, exacerbated in the presence of mutations in chitin synthase III. The authors conclude that actomyosin ring constriction and primary septum formation are interdependent aspects of the same process and that an alternative Ch3p-dependent salvage pathway forms a remedial septum if this process is compromised.