Abstract

When glucose-starved cells are replenished with glucose, the key gluconeogenic enzyme, fructose-1,6-bisphosphatase (FBPase), is selectively targeted from the cytosol to the yeast lysosome (vacuole) for degradation. The glucose-induced targeting of FBPase to the vacuole for degradation occurs in cells grown under a variety of metabolic conditions. Immunoelectron microscopic studies demonstrate that the uptake of FBPase by the vacuole is mediated in part by an autophagic process. FBPase can be found on the vacuolar membrane and also at the sites of membrane invaginations. Furthermore, FBPase is associated with different forms of vesicles, which are induced to accumulate inside the vacuole. We have identified peroxisomes as the organelles that are delivered to the vacuole for degradation when cells are replenished with glucose. Ultrastructural studies indicate that peroxisomes are engulfed by the vacuole by an autophagic process, leading to the destruction of whole organelles in the vacuole. Furthermore, the galactose transporter (Gal2p) is also delivered from the plasma membrane to the vacuole for degradation in response to glucose. Gal2p is delivered to the vacuole through the endocytic pathway, as mutants defective in receptor-mediated endocytosis fail to degrade Gal2p in response to glucose.