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

The role of the transmembrane domain of DPAP A in its retention in the Golgi apparatus, and in delivery to the vacuolar membrane of retention-defective DPAP A, was investigated. We previously demonstrated that a deletion of residues 85-106 in the cytoplasmic domain of DPAP A resulted in its delivery to the vacuolar membrane (Roberts et al., 1992) indicating that this domain is necessary for Golgi retention. While these data supported the hypothesis that the vacuole was the default destination for membrane proteins of the yeast secretory pathway, another possibility was that the transmembrane domain contained a "cryptic" vacuolar targeting signal, which directed the mutant protein to the vacuole in the absence of Golgi retention. To distinguish between these models, the transmembrane domain of wild-type DPAP A and of A85-106-AAA (Roberts et al., 1992) were replaced with a 21 residue synthetic hydrophobic sequence, X = L(LALV)s, resulting in the A-X-A and A85-106-A-X-A proteins ( Fig. 1 A) . Repeats of the sequence, LALV, were previously shown to functionally replace the transmembrane domain of the E. coli coliphage fl gene III protein (Davis and Model, 1985) .