SEC3P IS INVOLVED IN SECRETION AND MORPHOGENESIS IN SACCHAROMYCES-CEREVISIAE

Two new temperature-sensitive alleles of SEC3, 1 of 10 late-acting SEC genes required for targeting or fusion of post-Golgi secretory vesicl es to the plasma membrane in Saccharomyces cerevisiae, were isolated i n a screen for temperature-sensitive secretory mutants that are synthe tically lethal with sec4-8. The new sec3 alleles affect early as well as late stages of secretion. Cloning and sequencing of the SEC3 gene r evealed that it is identical to profilin synthetic lethal 1 (PSL1). Th e SEC3 gene is not essential because cells depleted of Sec3p are viabl e although slow growing and temperature sensitive. All of the sec3 all eles genetically interact with a profilin mutation, pfy1-111. The SEC3 gene in high copy suppresses pfy1-111 and sec5-24 and causes syntheti c growth defects with ypt1, sec8-9, sec10-2, and sec15-1. Actin struct ure is only perturbed in conditions of chronic loss of Sec3p function, implying that Sec3p does not directly regulate actin. All alleles of sec3 cause bud site selection defects in homozygous diploids, as do se c4-8 and sec9-4. This suggests that SEC gene products are involved in determining the bud site and is consistent with a role for Sec3p in de termining the correct site of exocytosis.