Vid22p, a novel plasma membrane protein, is required for the fructose-1,6-bisphosphatase degradation pathway

Fructose-1,6-bisphosphatase (FBPase), an important enzyme in the gluconeogenic pathway in Saccharomyces cerevisiae, is expressed when cells are grown in media containing a poor carbon source. Following glucose replenishment, FBPase is targeted from the cytosol to intermediate Vid (vacuole import and degradation) vesicles and then to the vacuole for degradation. Recently, several vid mutants that are unable to degrade FBPase in response to glucose were identified. Here, we present VID22, a novel gene involved in FBPase degradation. VID22 encodes a glycosylated integral membrane protein that localizes to the plasma membrane. Newly synthesized Vid22p was found in the cytoplasm and then targeted to the plasma membrane independent of the classical secretory pathway. A null mutation of VID22 failed to degrade FBPase following a glucose shift and accumulated FBPase in the cytosol. Furthermore, the majority of FBPase remained in a proteinase K sensitive compartment in the Deltavid22 mutant, implying that VID22 is involved in FBPase transport from the cytosol to Vid vesicles. By contrast, starvation-induced autophagy and peroxisome degradation were not impaired in the Deltavid22 mutant. This strain also exhibited the proper processing of carboxypeptidase Y and aminopeptidase I in the vacuole. Therefore, Vid22p appears to play a specific role in the FBPase trafficking pathway.