Sec4p is one of the founding members of the Rab family of small GTPases, which controls post- Golgi exocytosis in the budding yeast Saccharomyces cerevisiae. Studying Sec4p in the facile yeast genetic system has provided important insight into the molecular mechanisms of Rab proteins function and regulation. Sec4p is activated by its guanine nucleotide exchange factor Sec2p, which is in turn recruited by Ypt31p and Ypt32p, the upstream Rab proteins that regulate vesicle budding from the tans-Golgi network. The downstream effectors of Sec4p include the exocyst, which mediates vesicle tethering, and the lgl proteins Sro7/77p that regulate SNARE assembly and membrane fusion. In addition, activation of Sec4p is required for the engagement of secretory vesicles to actin cables for directional transport of cargos to the daughter cell. Thus, through the control of exocytosis machinery and interaction with actin cytoskeleton, Sec4p plays an important role in asymmetric cell growth in budding yeast.
Keywords: Sec4, Exocyst, Sec2/Rabin8, Exocytosis, Cell Polarity.