Import into and degradation of cytosolic proteins by isolated yeast vacuoles

In eukaryotic cells, both lysosomal and nonlysosomal pathways are involved in degradation of cytosolic proteins. The physiological condition of the ce ll often determines the degradation pathway of a specific protein. In this article, we show that cytosolic proteins can be taken up and degraded by is olated Saccharomyces cerevisiae vacuoles. After starvation of the cells, pr otein uptake increases. Uptake and degradation are temperature dependent an d show biphasic kinetics. Vacuolar protein import is dependent on cytosolic heat shock proteins of the hsp70 family and on protease-sensitive componen t(s) on the outer surface of vacuoles. Degradation of the imported cytosoli c proteins depends on a functional vacuolar ATPase. We show that the cytoso lic isoform of yeast glyceraldehyde-3-phosphate dehydrogenase is degraded v ia this pathway. This import and degradation pathway is reminiscent of the protein transport pathway from the cytosol to lysosomes of mammalian cells.