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

The similarity of the DPAP A Golgi retention signal to internalization signals raises the question of whether localization of DPAP A involves cycling between the plasma membrane and the Golgi. According to this model, DPAP A may be constitutively transported to the plasma membrane, endocytosed via interaction of its "retention signal" with clathrincoated pits, and returned to the Golgi. This type of model is consistent with the observation that disruption of the clathrin heavy chain gene (CHC1) resulted in mislocalization of a significant pool of Kex2p and DPAP A to the plasma membrane rather than the vacuole (Payne and Schekman, 1989; Seeger and Payne, 1992) . However, much data argues against this model. The localization of A-ALP (Nothwehr, S., and T. Stevens, unpublished data), Kexlp (Cooper and Bussey, 1992) , and Kex2p (Redding et al., 1992) are not affected by blocking secretory vesicle fusion with the plasma membrane (secl-ts mutation). Furthermore, this model would predict that removal of Golgi retention information would cause DPAP A to accumulate at the cell surface. We have found that neither wild-type nor retention-defective DPAP A accumulate at the cell surface as judged by the immunofluorescence experiments presented here and by enzymatic activity measurements (Roberts et al., 1992; Nothwehr, S., and T. Stevens, unpublished data) .