A novel pathway of protein targeting into the yeast vacuole for degradation has been identified in Saccharomyces cerevisiae. Fructose-1,6-bisphosphatase (FBPase), an important enzyme in the gluconeogenesis pathway, is imported from the cytosol to the vacuole for degradation when cells are transferred to media containing fresh glucose. Targeting and degradation of FBPase requires ubiquitin conjugation, endosomal formation, and peroxisomal assembly. Peroxisomes are important for the oxidation of fatty acids, alcohols, and amines in all organisms. Patients lacking peroxisomes (Zellweger's syndrome) display many abnormalities with a result of death within a few years after birth. In Saccharomyces cerevisiae, peroxisomes are delivered to the vacuole for degradation in response to glucose. Internalization of peroxisomes by the vacuole appears to participate in FBPase degradation since mutants lacking recognizable peroxisomes are defective in glucoseinduced degradation of FBPase. The findings that both FBPase and peroxisomes are targeted to the vacuole for degradation in response to glucose indicate that vacuolar protein degradation plays an important role in regulating cellular metabolism. To understand the pathway of vacuolar protein degradation at the molecular and the cellular level, we propose to: (1) study the role of peroxisomes in promoting the import and degradation of FBPase by cell fractionation and immunoelectron microscopy; (2) identify a putative receptor that mediates the import process by crosslinking and immunoprecipitation; (3) examine the importance of endosomal formation in FBPase degradation; (4) reconstitute an in vitro system to test directly the role of organelle assembly and ubiquitin conjugation in the import process.
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