BYR4, A DOSAGE-DEPENDENT REGULATOR OF CYTOKINESIS IN S-POMBE, INTERACTS WITH A POSSIBLE SMALL GTPASE PATHWAY INCLUDING SPG1 AND CDC16

Coordination between karyokinesis and cytokinesis in the cell division cycle is fundamental to a precise transmission of duplicated genome i nto dividing daughter cells. byr4, a previously isolated essential gen e, affects the mitotic cell cycle and cytokinesis in S. pombe. Phenoty pic analyses of the null alleles and the overexpression of byr4 sugges t that byr4 is a dosage-Introduction dependent coordinator of karyokin esis and cytokinesis (Song et al., 1996). In this study, the functiona l mechanisms of byr4 were investigated using a byr4 mutant that exhibi ts byr4 overexpression phenotypes in thiamine deficient media. Genetic suppression analyses of this byr4 mutant,vith other cytokinesis regul atory genes in S. pombe, cdc16, cdc7, cdc15, cdc14 and plo1, show that byr4 overexpression phenotypes are suppressed by the overexpression o f cdc16 and cdc7, but not by plo1, cdc14, and cdc15. Also, the basal e xpression of byr4 and cdc7 suppresses the temperature-sensitive cdc16 mutation. However, the basal expression of either byr4 or cdc16 does n ot suppress the temperature-sensitive cdc7 mutation. The results of th ese suppression tests suggest that byr4 genetically interacts with cdc 16 and cdc7: byr4 functions at the same level with or downstream of cd c16 and upstream of cdc7. In the present study, we also show that Byr4 interacts with Cdc16 and Spg1 in the yeast two-hybrid assays. Recent reports suggest a possible small GTPase pathway to regulate the timing of cytokinesis where Cdc16 functions as a GAP (GTPase activating prot ein), Spg1 as a GTPase, and Cdc7 as a downstream effector. Combined ge netic and two-hybrid analyses of this study strongly suggest that Byr4 directly interacts with this possible small GTPase pathway including Cdc16, Spg1, and Cdc7 to regulate cytokinesis in S. pombe.