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Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115
The key gluconeogenic enzyme, fructose1,6-bisphosphatase (FBPase), is induced when Saccharomyces cerevisiae are starved of glucose. FBPase is
targeted from the cytosol to the yeast vacuole for degradation when glucose-starved cells are replenished
with fresh glucose. Several vid mutants defective in the
glucose-induced degradation of FBPase in the vacuole
have been isolated. In some vid mutants, FBPase is
found in punctate structures in the cytoplasm. When
extracts from these cells are fractionated, a substantial amount of FBPase is sedimentable in the high speed
pellet, suggesting that FBPase is associated with intracellular structures in these vid mutants. In this paper we
investigated whether FBPase association with intracellular structures also existed in wild-type cells. We report the purification of novel FBPase-associated vesicles from wild-type cells to near homogeneity. Kinetic
studies indicate that FBPase association with these vesicles is stimulated by glucose and occurs only transiently, suggesting that these vesicles are intermediate
in the FBPase degradation pathway. Fractionation analysis demonstrates that these vesicles are distinct
from known organelles such as the vacuole, ER, Golgi,
mitochondria, peroxisomes, endosomes, COPI, or
COPII vesicles. Under EM, these vesicles are 30-40 nm
in diam. Proteinase K experiments indicate that the majority of FBPase is sequestered inside the vesicles. We
propose that FBPase is imported into these vesicles before entering the vacuole.
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