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

Our previous observation that the cytoplasmic domain of DPAP A was sufficient to retain a vacuolar membrane protein (DPAP B) in the yeast Golgi (Roberts et al., 1992) , combined with the analysis of the transmembrane and cytoplasmic domains above, strongly indicate that all the information required for retention lies within the cytoplasmic domain. A hybrid membrane protein was constructed to characterize the retention signal within the liB-amino acid cytoplasmic domain in more detail. This A-ALP fusion protein consisted of the cytoplasmic domain of DPAP A fused to the transmembrane and luminal domains of alkaline phosphatase (ALP; Klionsky and Emr, 1990) , a type II integral membrane protein of the vacuole (Fig. 1 B) . ALP contains a •3-kD propeptide at its COOH terminus, which is removed in a manner dependent on protease A, the product of the PEP4 gene (Ammerer et al., 1986) , indicating that this modification occurs within the vacuole (Klionsky and Emr, 1989) . Therefore, this PEP4-dependent cleavage event can be used as an assay for vacuolar delivery of A-ALP. In addition, unlike DPAP A, A-ALP can be analyzed by immunofluorescence microscopy when expressed from the endogenous STEJ3 promotor on a single-copy, CEN plasmid, thereby avoiding the necessity to overproduce the protein.