BYR4 AND CDC16 FORM A 2-COMPONENT GTPASE-ACTIVATING PROTEIN FOR THE SPG1 GTPASE THAT CONTROLS SEPTATION IN FISSION YEAST

Background: Spatial and temporal control of cytokinesis ensures the ac curate transmission of genetic material and the correct development of multicellular organisms. An excellent model system in which to study cytokinesis is Schizosaccharomyces pombe because there are similaritie s between cytokinesis in S. pombe and mammals and because genes involv ed in S. pombe cytokinesis have been characterized. In particular, for mation of the septum is positively regulated by the Spg1 GTPase and it s effector, the Cdc7 kinase. Septation is negatively regulated by Cdc1 6, a protein similar to GTPase-activating proteins (GAPs) for Ypt GTPa ses, and by Byr4, a protein of unknown biochemical function. This stud y investigates the relationship between Byr4, Cdc16, and Spg1. Results : Genetic interactions were observed between byr4, cdc16, and spg1 mut ants. Byrd; bound to Cdc16 and Spg1 in yeast two-hybrid assays and in coprecipitations in vitro and in yeast. Byr4 inhibited the dissociatio n and hydrolysis of GTP bound to Spg1, but when Byr4 and Cdc16 were co mbined together they displayed Spg1 GAP activity in vitro; Cdc16 alone had no detectable GAP activity. The binding of Byr4 to Spg1 and the B yr4-Cdc16 Spg1 GAP activity were specific because Byr4 and Cdc16 did n ot bind to or affect the GTPase activities of the seven known S. pombe Ypt family GTPases. Conclusions: Byr4 and Cdc16 form a two-component GAP for the Spg1 GTPase, Byr4 and Cdc16 appear to negatively regulate septation in S. pombe by modulating the nucleotide state of Spg1 possi bly in a spatially or temporally controlled manner.