Results: Genetic interactions were observed between byr4, cdc16, and spg1 mutants. Byr4 bound to Cdc16 and Spg1 in yeast two-hybrid assays and in coprecipitations in vitro and in yeast. Byr4 inhibited the dissociation and hydrolysis of GTP bound to Spg1, but when Byr4 and Cdc16 were combined together they displayed Spg1 GAP activity in vitro; Cdc16 alone had no detectable GAP activity. The binding of Byr4 to Spg1 and the Byr4–Cdc16 Spg1 GAP activity were specific because Byr4 and Cdc16 did not 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 possibly in a spatially or temporally controlled manner.