In vivo translocation of the cell wall acid phosphatase across the yeast endoplasmic reticulum membrane: are there multiple signals for the targeting process?

The repressible Saccharomyces cerevisiae acid phosphatase (APase) coded by the PHO5 gene is a cell wall protein that follows the yeast secretory pathway. We had previously described the in vivo fate of a multicopy plasmid-encoded modified protein, lacking 15 out of 17 signal peptide amino acids. This modified protein accumulates mainly within the cell as an inactive unglycosylated form. However 30% of this precursor is translocated, glycosylated and dispatched to the cell wall. We establish, in the present report, that this phenomenon did not result from an overproduction of the plasmid encoded protein, since it was also observed in a normal single copy situation. The secretion persisted after a deletion including the single hydrophobic segment present in the N-terminus of the mature protein. The entry of both wild type and mutant APase into the ER was inhibited in sec62 mutants suggesting that the SEC62 gene product would not be implicated in signal peptide recognition.