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

Truncated forms of Kexlp which lacked the cytoplasmic domain yet still remained membrane associated were not retained in the correct Golgi compartment, and even at subwildtype expression levels v~ere diverted to the vacuole. Similar mislocalization results have been found with DPAP A, where overexpression of the protein results in mislocalization to the vacuole, as do mutations within its cytoplasmically exposed domain (Roberts et al., 1992) . Differing results were obtained with Kex2p, where a mutation that deleted the cytoplasmic domain and part of the membrane-spanning domain resulted in a significant proportion of the Kex2p activity being mislocalized to the cell surface (Fuller et al., 1989) . The Kex2p activity study did not, however, address whether the truncated protein produced was membrane associated; if not, then the resulting soluble protein would be expected to be secreted to the cell surface. In addition, the strain used for such Kex2p activity studies contained wild-type activity levels of vacuolar hydrolases (PEP4; Jones, 1984) and, therefore, any Kex2p potentially mislocalized to the vacuole might be degraded and hence go undetected.