Membrane protein retention in the yeast Golgi apparatus: dipeptidyl aminopeptidase A is retained by a cytoplasmic signal containing aromatic residues

ALP is inactive until its propeptide is cleaved in a PEP4dependent manner in the vacuole (Klionsky and Emr, 1989) . Therefore, enzymatic activity can also be used as an assay for vacuolar delivery of the A-ALP proteins containing point mutations, since wild-type and mutant protein levels were essentially the same ( Fig. 6 ; data not shown). When the activity of wild-type A-ALP and the F85A,F87A-A-ALP mutant were assayed in a phoSA, PEP4 strain, the ALP activity for the mutant hybrid was sixfold greater than that of wildtype A-ALP after subtracting out the background activity of the cytoplasmic alkaline phosphatase, Phol3p (Kaneko et al., 1989) . This is in agreement with the localization and processing data above indicating that F85,F87-A-ALP is mislocalized to the vacuole, and also argues that F85A, F87A-A-ALP is correctly folded into an enzymatically active conformation.