Drc1p/Cps1p, a 1,3-beta-glucan synthase subunit, is essential for divisionseptum assembly in Schizosaccharomyces pombe

Schizosaccharomyces pombe divides by medial fission through the use of an a ctomyosin-based contractile ring. A division septum is formed centripetally , concomitant with ring constriction. Although several genes essential for cytokinesis have been described previously, enzymes that participate in the assembly of the division septum have not been identified. Here we describe a temperature-sensitive mutation, drc1-191, that prevents division septum assembly and causes mutant cells to arrest with a stable actomyosin ring. U nlike the previously characterized cytokinesis mutants,,which undergo multi ple mitotic cycles, drc1-191 is the first cytokinesis mutant that arrests w ith two interphase nuclei. Interestingly, unlike drc1-191, drc1-null mutant s proceed through multiple mitotic cycles, leading to the formation of larg e cells with many nuclei. drc1 is allelic to cps1, which encodes a 1,3-beta -glucan synthase subunit. We conclude that Drc1p/Cps1p is not required for cell elongation and cell growth, but plays an essential role in assembly of the division septum. Furthermore, it appears that constriction of the acto myosin ring might: depend on assembly of the division septum. We discuss po ssible mechanisms that account for the differences in the phenotypes of the drc1-191 and the drc1-null mutants and also reflect the potential links be tween Drc1p and other cytokinesis regulators.