chapter  13
Vacuolar Import and Degradation
Pages 11

Fructose-1,6-bisphosphatase (FBPase) is another key gluconeogenic enzyme that is inacti­ vated by glucose.1 A number of studies have attempted to identify the mechanisms of the catabolic inactivation of FBPase. Although there are many contributing factors, protein degra­ dation is the primary mechanism by which FBPase is inactivated. Protein degradation also plays a role in the inactivation of a number of other enzymes. For example, when cells are transferred to glucose, the plasma membrane galactose transporter is selectively delivered to the vacuole for degradation.6 Mutants defective in endocytosis block the degradation of the galac-

tose transporter, suggesting that it is delivered to the vacuole for degradation through the endocytic pathway. In a similar manner, the maltose transporter is delivered to the vacuole via endocytosis following a shift of cells to glucose.5 The inactivating enzyme for phosphoenolpyruvate kinase has been identified as proteinase В (Prbl),7 whereas the inactivating enzyme for uridine nucleosidase is proteinase A (Pep4).8 These proteinases are found in the vacuole and have broad substrate activities. However, whether phosphoenolpyruvate kinase or uridine nucle­ osidase are targeted to the vacuole for degradation has not been examined.