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

Nevertheless, the important observation of a role for clathrin in the retention of DPAP A (Seeger and Payne, 1992) combined with our identification of an aromatic residue Golgi retention signal in the cytoplasmic domain of DPAP A, strongly suggest that coated vesicle structures are somehow involved in its retention. The data are consistent with a retrieval model for Golgi membrane protein retention similar to that proposed for soluble ER proteins (Pelham et al., 1989 ). In this model, DPAP A protein may leave the Golgi via the default pathway for membrane proteins that eventually leads to the vacuole. Before reaching the vacuole, DPAP A may pass through a "prevacuolar compartment, wherein it interacts with clathrin-coated structures and is transported back to the late Golgi compartment via transport vesicles. In animal cells, the interaction of the cytoplasmiccoated pit signals with the clathrin coat is thought to be mediated by specific interactions with an "adaptor" complex (Pearse, 1988; Glickman et al., 1989; Beltzer and Spiess, 1991) and putative adaptor subunit homologues have been found in yeast (Kirchhausen, 1990) . By interaction with clathrin coat proteins, DPAP A in the prevacuolar compartment could be packaged into clathrin-coated vesicles bound for the Golgi. In the absence of the clathrin heavy chain, the retrieval system may become less specific and the vesicles may eventually fuse with the plasma membrane. Mutant DPAP A lacking a retrieval signal would not bind clathrin coat proteins and would be transported via the default pathway from the prevacuolar compartment to the vacuole (for additional discussion, see Seeger and Payne, 1992; Wilcox et al., 1992) . While such a role for clathrin-coated vesicles fits with all of the results in yeast, results from an in vitro assay (Draper et al., 1990) suggest that clathrin-coated vesicles are not involved in the recycling of the mannose 6-phosphate receptor from a prelysosomal compartment back to the TGN in animal cells.