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

Several other observations are consistent with the vacuolar default model: (a) no single domain of DPAP B, a type II vacuolar membrane protein, is required for its vacuolar transport (Roberts et al., 1992) ; (b) Kexlp not retained in the Golgi apparatus due to removal of retention information or overproduction is delivered to the vacuolar membrane (Cooper and Bussey, 1992) ; and (c) a point mutation in the Kex2p cytoplasmic domain results in its vacuolar delivery (Wilcox et al., 1992) . In addition, mislocalization of mutant DPAP A to the vacuolar membrane does not occur via transport to the plasma membrane followed by subsequent uptake from the plasma membrane to the vacuole. This was demonstrated for the A22-AA-B protein (Roberts et al., 1992) and for F85A-A-ALP (Nothwehr, S., and T. Stevens, unpublished data) by the finding that these proteins were able to reach the vacuole in a secl-ts mutant strain (secl-ts mutant cells are conditionally blocked in secretory vesicle fusion with the plasma membrane; Novick et al., 1981; Salminen et al., 1987) at the restrictive temperature. The next step in testing the vacuolar default model is to determine whether discrete sorting signals are required for proper localization of a yeast plasma membrane protein, a project currently underway in our laboratory.