PROTEIN TARGETING AND DEGRADATION IN THE YEAST VACUOLE

Protein degradation is an essential process in cells. Degradation of intracellular proteins increases when cells are starved of nutrients. Lysosomes are responsible for the enhanced protein degradation during starvation. To understand the degradation process that occurs in lysosomes, we studied the catabolite inactivation of fructose-1,6-bisphosphatase (FBPase) in Saccharomyces cerevisiae. Fructose-1,6-bisphosphatase, a key enzyme in the gluconeogenesis pathway, is induced when cells are starved of glucose and is degraded when cells are replenished with glucose. We have shown that catabolite inactivation of FBPase is mediated by a selective import of the enzyme into the vacuole (yeast lysosome) for degradation. Glucose-induced degradation of FBPase serves to regulate metabolism to prevent the energy futile cycle. In addition to FBPase, we have also demonstrated that peroxisomes, which are important in the oxidation of fatty acids, are delivered to the vacuole for degradation in response to glucose. Furthermore, the galactose transporter, which is induced when cells are grown in galactose, is internalized and delivered to the vacuole for degradation when cells are transferred to glucose.