Vsl1p cooperates with Fsv1p for vacuolar protein transport and homotypic fusion in Schizosaccharomyces pombe

Members of the SNARE protein family participate in the docking-fusion step of several intracellular vesicular transport events. Saccharomyces cerevisiae Vam7p was identified as a SNARE protein that acts in vacuolar protein transport and membrane fusion. However, in Schizosaccharomyces pombe, there have been no reports regarding the counterpart of Vam7p. Here, we found that, although the SPCC594.06c gene has low similarity to Vam7p, the product of SPCC594.06c has a PX domain and SNARE motif like Vam7p, and thus we designated the gene Sch. pombe vsl1(+) (Vam7-like protein 1). The vsl1Δ cells showed no obvious defect in vacuolar protein transport. However, cells of the vsl1Δ mutant with a deletion of fsv1(+), which encodes another SNARE protein, displayed extreme defects in vacuolar protein transport and vacuolar morphology. Vsl1p was localized to the vacuolar membrane and prevacuolar compartment, and its PX domain was essential for proper localization. Expression of the fusion protein GFP-Vsl1p was able to suppress ZnCl2 sensitivity and the vacuolar protein sorting defect in the fsv1Δ cells. Moreover, GFP-Vsl1p was mislocalized in a pep12Δ mutant and in cells overexpressing fsv1(+). Importantly, overexpression of Sac. cerevisiae VAM7 could suppress the sensitivity to ZnCl2 of vsl1Δ cells and the vacuolar morphology defect of vsl1Δfsv1Δ cells in Sch. pombe. Taken together, these data suggest that Vsl1p and Fsv1p are required for vacuolar protein transport and membrane fusion, and they function cooperatively with Pep12p in the same membrane-trafficking step.