In budding yeast, contraction of the actomyosin ring and formation of the primary septum at cytokinesis depend on each other

Saccharomyces cerevisiae chs2 mutants are unable to synthesize primary septum chitin, and myo1 mutants cannot construct a functional contractile ring. The morphology of the two mutants, as observed by electron microscopy, is very similar. In both cases, neither an invagination of the plasma membrane, which normally results from contraction of the actomyosin ring, nor generation of a chitin disc, the primary septum, is observed. Rather, both mutants are able to complete cytokinesis by an abnormal process in which lateral walls thicken gradually and finally meet over an extended region, giving rise to a thick septum lacking the normal trilaminar structure and often enclosing lacunae. Defects in chs2 or myo1 strains were not aggravated in a double mutant, an indication that the corresponding proteins participate in a common process. In contrast, in a chs3 background the chs2 mutation is lethal and the myo1 defect is greatly worsened, suggesting that the synthesis of chitin catalyzed by chitin synthase III is necessary for the functionality of the remedial septa. Both chs2 and myo1 mutants show abnormalities in budding pattern and a decrease in the level of certain proteins associated with budding, such as Bud3p, Bud4p and Spa2p. The possible reasons for these phenotypes and for the interdependence between actomyosin ring contraction and primary septum formation are discussed.