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

The observation that Kex2p overexpression reduces the efficiency of A-ALP retention in the Golgi (only when Kex2p has an intact Golgi retention signal) suggests that a common sorting apparatus, perhaps a receptor, can become saturated due to excess ligand. Comparison of the COOH-terminal cytoplasmic tail of Kex2p (type I) and the NH2-terminal tail of DPAP A (type II) reveals no obvious sequence similarity. However, a critical dement of the Kex2p Golgi retention signal has recently been shown to be a q~yr residue at position 713 of the cytoplasmic tail (Wilcox et al., 1992) . Interestingly, a phenylalanine residue is present at position 715 (YEF715). While the role of F715 in Kex2p localization has not been tested, comparison suggests that a general element for Golgi retention of yeast membrane proteins may be (Y/F-X-Y/F). The fact that DPAP A and Kex2p have different membrane orientations would not rule out identical retention mechanisms since clathrin-coated pit signals of membrane bound receptors are able to function equally well irrespective of their orientation with the membrane (Collawn et al., 1991; Jadot et al., 1992) . Although the above data are consistent with DPAP A and Kex2p having retention signals that are recognized by the same trans-acting factor, the preference for phenylalanine over tyrosine in the DPAP A signal would appear to be at odds with this interpretation. It is possible that there are separate components that recognize each signal, but that there are other more generic components that are shared in the retention of each protein. Alternatively, the dependence of the DPAP A signal on phenylalanine may be a result of the context of this signal.