Quality Control of Plasma Membrane Proteins: A Dissertation

Publication Date

July 1999

Document Type

Doctoral Dissertation


Graduate School of Biomedical Sciences, Molecular Genetics & Microbiology


Endoplasmic Reticulum; Proteins; Academic Dissertations; Dissertations, UMMS


The temperature-sensitive α-factor receptor (Ste2-3p) and arginine permease (Can1tsp) were found to provide the model substrates for quality control of plasma membrane proteins in Saccharomyces cerevisiae. When the ste2-3 mutant cells were grown at 34°C, Ste2-3p failed to accumulate at the plasma membrane and was delivered to the vacuole for degradation without traversing the plasma membrane. Upon reaching the vacuole, cytoplasmic domains of both Ste2p and Ste2-3p appeared within the vacuolar lumen. Four stp mutants were identified to suppress temperature-sensitive defects in both Ste2-3p and Can1tsp. The stp22 and STP26 mutations also caused missorting of vacuolar protein carboxypeptidase Y, and a subset of vacuolar protein sorting mutants (vps) suppressed ste2-3 mutation. In the stp22 mutant, both Ste2p and Ste2-3p accumulated in the prevacuolar compartment (PVC) and on the plasma membrane. Three independent mutations that bypassed the phenotype of stp22Δ mutant were identified and mapped to the SNF8 locus, and they were found to affect a single amino acid residue (G209D). The mutant protein, Snf8bpp, but not Snf8p, was able to compensate for the lack of functional Stp22p and to restore PVC-to-vacuole trafficking. The order of function for some VPS genes involved in PVC-to-vacuole traffic (class E) was determined by using this special snf8bp allele. In addition, a PtdIns 4-kinase encoded by the PIK1 gene was found to be involved in Ste2-3p trafficking, possibly affecting the PVC function.


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