A NOVEL SUPPRESSOR OF RAS1 IN FISSION YEAST, BYR4, IS A DOSAGE-DEPENDENT INHIBITOR OF CYTOKINESIS

A novel gene, designated byr4, was identified in Schizosaccharomyces p ombe that affects the mitotic cell cycle and shows genetic interaction s with the ras1 signaling pathways. Null alleles of byr4 cause cell cy cle arrest in late mitosis and permit multiple rounds of septation. Th e multiple septa typically divide two nuclei, but the nuclei frequentl y do not stain equally with 4',6-diamidino-2-phenylindole (DAPI), sugg esting that byr4 is required for proper karyokinesis. Overexpression o f byr4 inhibits cytokinesis, but cell cycle progression continues lead ing to multinucleate cells. When byr4 is overexpressed, the early step s in the cytokinesis pathway, including formation of the medial F-acti n ring, occur normally; however, the later steps in the pathway, inclu ding contraction of the F-actin ring, septation, and rearrangement of the medial F-actin following mitosis, rarely occur. byr4 shows two gen etic interactions with ras1. The inhibition of cytokinesis by byr4 ove rexpression was exacerbated by null alleles of ras1 and scd1, suggesti ng a link between pathways needed for cell polarity and cytokinesis. O verexpression of byr4 also partially bypasses the need for ras1 for sp orulation. The electrophoretic mobility of the byr4 protein varied in response to mutants that perturb cytokinesis and karyokinesis, suggest ing interactions between byr4 and these gene products. A more rapidly migrating byr4 protein was found in cells with mutations in cdc16, whi ch undergo repeated septation, and in cdc15, which fail to form a medi al F-actin ring in mitosis. A slower migrating byr4 protein was found in cells with a mutation in the beta-tubulin gene, which arrests cells at the metaphase-anaphase transition.