The cycle of SEC4 function in vesicular transport.

Sec4 is a Ras-like GTP-binding protein required for exocytosis in yeast. Unlike Ras, it is the ability of Sec4 to cycle between the GTP- and GDP-bound forms rather than the absolute levels of the GTP-bound form that is critical for function. This cycle may be coupled to an observed cycle of Sec4 localization within the cell. Sec4 binds to secretory vesicles which then fuse with the plasma membrane in exocytosis. Sec4 can recycle from the plasma membrane through a soluble pool to rebind to a new round of vesicles. We have found an activity in yeast (Saccharomyces cerevisiae) comparable to that of the GDP dissociation inhibitor protein isolated from mammalian cells that releases GDP-bound Sec4 from membranes. DSS4-1, a dominant suppressor of the sec4-8 temperature-sensitive mutation, encodes a nucleotide exchange protein. The cycle of Sec4 may function to allow the assembly and subsequent disassembly of a set of proteins necessary for exocytosis. Candidates for members of this set of proteins are encoded by sec genes which show strong genetic interactions with sec4-8. Two of these (SEC8 and SEC15) encode large proteins which form a complex that is peripherally associated with the plasma membrane.