A Cell-Free Assay Allows Reconstitution of Vps33p-Dependent Transport to the Yeast Vacuole/Lysosome

We report a cell-free system that measures transport-coupled maturation of carboxypeptidase Y (CPY). Yeast spheroplasts are lysed by extrusion through polycarbonate filters. After differential centrifugation, a 125,000-g pellet is enriched for radiolabeled proCPY and is used as "donor" membranes. A 15,000-g pellet, harvested from nonradiolabeled cells and enriched for vacuoles, is used as "acceptor" membranes. When these membranes are incubated together with ATP and cytosolic extracts, approximately 50% of the radiolabeled proCPY is processed to mature CPY. Maturation was inhibited by dilution of donor and acceptor membranes during incubation, showed a 15-min lag period, and was temperature sensitive. Efficient proCPY maturation was possible when donor membranes were from a yeast strain deleted for the PEP4 gene (which encodes the principal CPY processing enzyme, proteinase A) and acceptor membranes from a PEP4 yeast strain, indicating intercompartmental transfer. Cytosol made from a yeast strain deleted for the VPS33 gene was less efficient at driving transport. Moreover, antibodies against Vps33p (a Sec1 homologue) and Vam3p (a Q-SNARE) inhibited transport >90%. Cytosolic extracts from yeast cells overexpressing Vps33p restored transport to antibody-inhibited assays. This cell-free system has allowed the demonstration of reconstituted intercompartmental transport coupled to the function of a VPS gene product.