Yeast Kex1p is a Golgi-associated membrane protein: deletions in a cytoplasmic targeting domain result in mislocalization to the vacuolar membrane

Different mechanisms have been proposed for the retention of proteins within Golgi compartments. Interactions involving the cytoplasmic domain of Kexlp may result in the aggregation of the protein in the appropriate compartment, and thereby prevent it from entering transport vesicles that are exiting the Golgi compartment (Pfeffer and Rothman, 1987) . This retention via aggregation model is unlikely as it does not explain why aggregation would not occur earlier in the secretory pathway or why overproduction of a Kexlp results in mislocalization to the vacuole rather than greater aggregation in the Golgi compartment. A more likely explanation for the retention involves a receptor interacting with the cytoplasmically exposed domain of Kexlp ( Fig. 9 ). High level expression of Kexlp would saturate the receptor with the result that excess Kexlp is diverted to the vacuole. Removal of the cytoplasmic domain would abolish such a receptor-ligand interaction, also resulting in mislocalization to the vacuole. Retention of soluble ER proteins has been shown to involve a receptor-mediated retrieval of proteins from a pre-cis- Golgi compartment (Semenz~ et al., 1990; Lewis et al., 1990) . A cytoplasmicaUy based retrieval system analogous to the ER system can be envisaged in which Kexlp is retrieved from a trans compartment, possibly the plasma membrane, and returned to the correct Golgi compartment. To determine if Kexlp is retrieved from the plasma membrane an immunofluorescence experiment was performed by inducing expression of Kexlp in a secl strain, secl is a conditional mutation that, at the nonpermissive temperature, prevents the fusion of secretory vesicles with the plasma membrane (Novick et al., 1981) . The delivery of Kexlp to its Golgi location was not blocked by secl, and if KEX/expres-The Jouma! of Cell Biology, Volume 119, 1992 sion was constitutive before imposition of the secl block, the staining pattern of Kexlp was not altered even after 3 h at the restrictive temperature (data not shown). Therefore, if a receptor retrieval system exists, it is unlikely that Kexlp is retrieved from the plasma membrane as outlined in one of several proposals presented by Payne and Schekman (1989) . If a retrieval system is responsible for the retention of Kexlp, then the salvage compartment may instead be located between the Golgi compartment and the vacuole where failure to bind the receptor would result in delivery to the vacuole.