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

To test this idea, we 35S-labeled pho8A yeast cells carrying A-ALP alone on a single copy plasmid, or A-ALP with a plasmid overexpressing Kex2p from the GAL/ promoter (Johnston and Davis, 1984) , and immunoprecipitated A-ALP after 120 min of chase (Fig. 9) . No processing was observed in the presence of normal Kex2p levels but when wild-type Kex2p was overproduced (,M5-fold) a significant fraction of the A-ALP protein underwent PEP4-dependent processing (29%). However, the same level of overproduction of retention-defective mutant versions of Kex2p (Wilcox et al., 1992) lacking a critical tyrosine residue (Y713A) or the entire cytoplasmic tail (C-tailA; removes residues 702-814) resulted in greatly diminished levels of A-ALP reaching the vacuole (8 and <3% processing, respectively). Thus, A-ALP was less efficiently retained in the Golgi when wild type but not mutant Kex2p was overproduced, suggesting that the cytoplasmic retention signals of the two proteins may be recognized by the same retention machinery.