ABSTRACT
This pathway involves the selective transport of cytosolic FBPase into novel intracellular vesicles and subsequent fusion of these vesicles with the vacuole.5 This pathway may be unique to yeast. Alternatively, it may represent a variation of the selective lysosomal proteolytic pathway with targeting into a vesicle rather than directly into the lysosome/ vacuole. If this homology is correct, the targeting of FBPase into the intermediate vesicle should be stimulated by hsc73> the intermediate vesicle membrane should contain a receptor, and the lumen of the intermediate vesicles should contain a molecular chaperone to facilitate import. Recent results indicate that the import of FBPase into the intermediate vesicles requires ATP and cytosol (hsc73?) and is saturable (receptor?).6,7
W hy yeast would first target proteins to an intermediate vesicle rather than directly to the vacuole is not known. One possibility is that the roles of the yeast vacuole in nonlysosomal functions such as osmotic balance and nutrient storage might be compro mised by a gated protein import channel in the vacuolar membrane. Transporting protein substrates first into vesicles may be a simple solution to maintaining a completely sealed vacuolar membrane. Alternatively, the molecular chaperone in the vacuole of yeast may be susceptible to degradation, while hsc73 is resistant to lysosomal degradation in mammalian cells.8 Transporting the protein into an intermediate vesicle that does not contain proteases would bypass this potential problem.
