Targeting of Aminopeptidase I to the Yeast Vacuole Is Mediated by Ssa1p, a Cytosolic Member of the 70-kDa Stress Protein Family

The two cytosolic members of the highly conserved 70-kDa stress protein family, Ssa1p and Ssa2p, were specifically retained by the prepro-NH2 extension of the vacuolar aminopeptidase I precursor (pAPI) conjugated to agarose (Sulfolink). A temperature-sensitive mutant strain a1tsa234 (ssa1tsssa2 ssa3 ssa4), when incubated at the restrictive temperature, was able to assemble the API precursor into dodecamers, but failed to pack pAPI into vesicles and to convert it into mature API (mAPI), a process that occurs in the vacuole. Altogether these results indicate that Ssa1p mediates the targeting of pAPI to the vacuole.