The Class C Vps Complex Functions at Multiple Stages of the Vacuolar Transport Pathway

Abstract

The Class C Vps complex, consisting of Vps11, Vps16, Vps18, and Vps33, is required for SNARE‐mediated membrane fusion at the lysosome‐like yeast vacuole. However, Class C vps mutants display more severe and pleiotropic phenotypes than mutants specifically defective in endosome‐to‐vacuole transport, suggesting that there are additional functions for the Class C Vps complex. A SNARE double mutant which is defective for both Golgi‐to‐endosome and endosome‐to‐vacuole trafficking replicates many of the phenotypes observed in Class C vps mutants. We show that genetic interactions exist between Class C vps alleles and alleles of the Class D vps group, which are defective in the docking and fusion of Golgi‐derived vesicles at the endosome. Moreover, the Class D protein Vac1 was found to physically bind to the Class C Vps complex through a direct association with Vps11. Finally, using a random mutagenic screen, a temperature‐conditional allele which shares many of the phenotypes of mutants which are selectively defective in Golgi‐to‐endosome trafficking was isolated (vps11–3 ts). Collectively, these results indicate that the Class C Vps complex plays essential roles in the processes of membrane docking and fusion at both the Golgi‐to‐endosome and endosome‐to‐vacuole stages of transport.