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Published November 1, 1988 | public
Journal Article Open

Protein sorting in Saccharomyces cerevisiae: isolation of mutants defective in the delivery and processing of multiple vacuolar hydrolases


Using a selection for spontaneous mutants that mislocalize a vacuolar carboxypeptidase Y (CPY)-invertase fusion protein to the cell surface, we identified vacuolar protein targeting (vpt) mutants in 25 new vpt complementation groups. Additional alleles in each of the eight previously identified vpt complementation groups (vpt1 through vpt8) were also obtained. Representative alleles from each of the 33 vpt complementation groups (vpt1 through vpt33) were shown to exhibit defects in the sorting and processing of several native vacuolar proteins, including the soluble hydrolases CPY, proteinase A, and proteinase B. Of the 33 complementation groups, 19 were found to contain mutant alleles that led to extreme defects. In these mutants, CPY accumulated in its Golgi complex-modified precursor form which was secreted by the mutant cells. Normal protein secretion appeared to be unaffected in the vpt mutants. The lack of significant leakage of cytosolic markers from the vpt mutant cells indicated that the vacuolar protein-sorting defects associated with these mutants do not result from cell lysis. In addition, the observation that the precursor rather than the mature forms of CPY, proteinase A, proteinase B were secreted from the vpt mutants was consistent with the fact that mislocalization occurred at a stage after Golgi complex-specific modification, but before final vacuolar sorting of these enzymes. Vacuolar membrane protein sorting appeared to be unaffected in the majority of the vpt mutants. However, a subset of the vpt mutants (vpt11, vpt16, vpt18, and vpt33) was found to exhibit defects in the sorting of a vacuolar membrane marker enzyme, alpha-mannosidase. Up to 50% of the alpha-mannosidase enzyme activity was found to be mislocalized to the cell surface in these vpt mutants. Seven of the vpt complementation groups (vpt3, vpt11, vpt15, vpt16, vpt18, vpt29, and vpt33) contained alleles that led to a conditional lethal phenotype; the mutants were temperature sensitive for vegetative cell growth. This temperature-sensitive phenotype has been shown to be recessive and to cosegregate with the vacuolar protein-sorting defect in each case. Tetrad analysis showed that vpt3 mapped to the right arm of chromosome XV and that vpt15 mapped to the right arm of chromosome II. Intercrosses with other mutants that exhibited defects in vacuolar protein sorting or function (vpl, sec, pep, and end mutants) revealed several overlaps among these different sets of genes. Together, these data indicate that more than 50 gene products are involved, directly or indirectly, in the process of vacuolar protein sorting.

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Copyright © 1988 by the American Society for Microbiology. Received 24 June 1988/Accepted 16 August 1988. We thank John C.L. DeModena for excellent technical assistance and Cathy Elkins for patience in typing the manuscript. We are grateful to Elizabeth Jones and Charles Moehle for the generous gift of anti-PrB antisera and Howard Riezman, Joel Rothman, and Tom Stevens for communicating their results prior to publication. In addition, we thank David Bedwell, Paul Herman, and Tom Vida for critical reading of the manuscript and valuable discussions during the course of the work. This study was supported by Public Health Service grant GM-32703 from the National Institutes of Health (to S.D.E.). J.S.R. was supported by graduate fellowships from the Lucy Mason Clark fund and the Markey Charitable Trust fund, L.M.B. was supported by graduate fellowships from the National Science Foundation and General Electric Co. (Schenectady, N.Y.), D.J.K. was supported by a research fellowship from the Helen Hay Whitney Foundation, and S.D.E. is an NSF Presidential Young Investigator supported by grant DCB-8451633 from the National Science Foundation.


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