Comparative proteomic analysis of transition of saccharomyces cerevisiae from glucose-deficient medium to glucose-rich medium
Fructose-1,6-bisphosphatase (FBPase) is synthesized in yeast during glucose starvation but is rapidly degraded in the vacuole following the addition of glucose. FBPase trafficking to the vacuole involves two distinct steps, import into intermediate transport vesicles (Vid vesicles) and Vid vesicle trafficking to the vacuole. FBPase import into Vid vesicles requires the VID22 gene. However, VID22 affects FBPase import indirectly through a cytosolic factor. To identify the required cytosolic component, wild type cytosol was fractionated and screened for proteins that complement Deltavid22 mutant cytosol using an in vitro assay that reproduces FBPase import into Vid vesicles. Cyclophilin A (Cpr1p) was identified as a cytosolic protein that mediates Vid22p function in FBPase import. Mutants lacking Cpr1p were defective in FBPase import. Furthermore, the addition of purified Cpr1p restored FBPase import in both the Deltacpr1 and the Deltavid22 mutants. The cyclosporin A binding pocket is important for Cpr1p function, since cyclosporin A binding-deficient mutants failed to complement FBPase import in Deltacpr1 and Deltavid22 mutants. The levels of Cpr1p were reduced in the Deltavid22 mutants, implying that the expression of Cpr1p is regulated by Vid22p. Our results suggest that Cpr1p mediates Vid22p function and is directly involved in the import of FBPase into Vid vesicles.