Characterization of Class D VPS Proteins
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The vacuole of the yeast Saccharomyces cerevisiae is functionally similar to the mammalian lysosome. The components of the VPS (vacuolar protein sorting) system are responsible for proper delivery of vacuolar biosythetic enzymes. Efforts to dissect the genetics of this system have revealed several classes of mutants, each defective in one transport step in the VPS pathway. The Class D VPS proteins are thought to control anterograde traffic between the late Golgi and late endosome. Although most of these proteins have homologues of known function in other systems, two exceptions are the Vps3p and Vps8p proteins. Analysis of Vps3p reveals that it is associated with a highdensity structure, possibly a coated vesicle or a large protein complex. The Vps8p protein contains a C-terminal H2 RING finger motif, a domain often associated with E3 ubiquitin ligase activity. In vitro analysis reveals that a Vps8p fragment containing this domain has this activity. Deletion of the RING finger reveals that the endocytic marker Ste3p accumulates in an abnormally large late-endosome-derived structure, but that sorting of the soluble vacuolar cargo CPY is relatively unaffected. These results suggest a division of function within the Vps8p molecule.