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

Mutational analysis indicated a requirement for phenylalanine residues at positions 85 and 87 in the Golgi retention signal of DPAP A (RRESFQFNDI90). At position 85, all of the replacements analyzed inactivated the signal to a similar extent including the relatively conservative tyrosine replacement. However, at position 87 the large aliphatic and aromarie substitutions (Val and Tyr) were not as severe as the others (Ala, Cys, Gly), suggesting that more variability is allowed at this position. Simultaneous replacement of F85 or F87 with alanines did not increase the severity of the phenotype of the F85A-or F87A-A-ALP mutant proteins suggesting that the phenylalanines do not act in an independent, additive fashion for retention. Rather, these data suggest that they form a single structural motif recognized by the yeast Golgi retention machinery. Alanine scanning mutagenesis of the entire 10-amino acid region (81-90) indicated that no specific sequence information was required at positions 81-84, 86, 88, or 90; however an Asp residue at position 89 was required for efficient retention. Additional mutational analysis will be needed to determine whether there is a specific requirement for an Asp residue at position 89 or whether other residues will function at this position. At the least, these results demonstrate that in addition to a Phe-X-Phe motif, certain contextual sequences are also required.