Membrane protein sorting: biosynthesis, transport and processing of yeast vacuolar alkaline phosphatase

The Saccharomyces cerevisiae PHO8 gene product, repressible alkaline phosphatase (ALP), is a glycoprotein enzyme that is localized to the yeast vacuole (lysosome). Using antibodies raised against synthetic peptides corresponding to two distinct hydrophilic sequences in ALP, we have been able to examine the biosynthesis, sorting and processing of this protein. ALP is synthesized as an inactive precursor containing a C-terminal propeptide that is cleaved from the protein in a PEP4-dependent manner. The precursor and mature protein are anchored in the membrane by an N-terminal hydrophobic domain that also appears to function as an uncleaved internal signal sequence. ALP has the topology of a type-II integral membrane protein containing a short basic N-terminal cytoplasmic tail that is accessible to exogenous protease when associated both with the endoplasmic reticulum and the vacuole. Similar to the soluble vacuolar hydrolases carboxypeptidase Y (CPY) and proteinase A (PrA), ALP transits through the early stages of the secretory pathway prior to vacuolar delivery. Two observations indicate, however, that ALP is localized to the vacuole by a mechanism which is in part different from that used by CPY and PrA: (i) maturation of proALP, which is indicative of vacuolar delivery, is less sensitive than CPY and PrA to the defects exhibited by certain of the vacuolar protein sorting (vps) mutants; and (ii) maturation of proALP proceeds normally in the presence of a potent vacuolar ATPase inhibitor, bafilomycin A1, which is known to block vacuole acidification and leads to the mis-sorting and secretion of precursor forms of CPY and PrA. These results indicate that ALP will be a useful model protein for studies of membrane protein sorting in yeast.