The multivesicular body (MVB) sorting pathway is required for a number of biological processes, including downregulation of cell-surface proteins and protein sorting into the vacuolar lumen. The function of this pathway requires endosomal sorting complexes required for transport (ESCRT) composed of class E vacuolar protein sorting (Vps) proteins in Saccharomyces cerevisiae, many of which are conserved in Schizosaccharomyces pombe. Of these, sst4/vps27 (homologous to VPS27) and sst6 (similar to VPS23) have been identified as suppressors of sterility in ste12Δ (sst), although their functions have not been uncovered to date. In this report, these two sst genes are shown to be required for vacuolar sorting of carboxypeptidase Y (CPY) and an MVB marker, the ubiquitin-GFP-carboxypeptidase S (Ub-GFP-CPS) fusion protein, despite the lack of the ubiquitin E2 variant domain in Sst6p. Disruption mutants of a variety of other class E vps homologues also had defects in sorting of CPY and Ub-GFP-CPS. Sch. pombe has a mammalian AMSH homologue, sst2. Phenotypic analyses suggested that Sst2p is a class E Vps protein. Taken together, these results suggest that sorting into multivesicular bodies is dependent on class E Vps proteins, including Sst2p, in Sch. pombe.
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