| The precise regulation of vesicular-mediated protein transport is of critical importance to eukaryotic organisms. The vacuolar protein sorting (VPS) pathway of Saccharomyces cerevisiae is an excellent model system for the study of vesicular-mediated protein transport, and more specifically, the docking and fusion of transport vesicles with acceptor organelles. This pathway consists of several steps, including two docking and fusion events, one at the prevacuolar endosome and one at the vacuole. The factors that mediate docking and fusion at the endosome include members of conserved protein trafficking families, such as a t-SNARE, Pep 12, a Rab GTPase, Vps21, and an SM protein, Vps45, as well as unique factors, such as a phosphatidyl-inositol 3-kinase and the phosphatidyl-inositol 3-phosphate (PI3P)-binding protein, Vac 1. In genetic and biochemical studies, we have found that Vac 1 is an effector of GTP-bound Vps21 as well as interacting with Vps45. These results suggest a model wherein Vac 1 acts to integrate P13P and Vps21GTP signaling, and subsequently influence Pep12-Vps45 interactions to facilitate endosomal SNARE complex formation. Further studies have identified an essential role at this stage for another group of VPS proteins, the Class C Vps complex, in addition to its previously documented role at the vacuole in facilitating SNARE complex formation. This complex associates with Vac 1, and may tether incoming Golgi-derived vesicles to the endosome, as an initial recognition event in the docking and fusion reaction. Together, these studies have attempted to better characterize the molecular mechanisms which mediate the docking and fusion of Golgi-derived vesicles with the endosome. |
