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

The reduced processing of K1 killer toxin by the soluble forms of Kexlp can be explained because of their secretion and consequent reduced concentration of Kexlp within the processing compartment. The membrane-associated truncated forms of Kexlp remain intracellular and, therefore, secretion cannot account for their reduced processing ability. The above results indicated that Kexlp-Hpa was membrane associated, had received glycosyl modifications in the Golgi apparatus, gave wild-type levels of total activity, and was retained intracellularly; yet processed the K1 killer toxin precursor to a lesser extent than Kexlp. Kexlp-Bcl and Kexlp-Hinc showed similar phenotypes to that of Kexlp-Hpa. A likely explanation for such observations was that these membrane-associated mutant forms of Kexlp were not retained within the correct Golgi compartment, but instead were mislocalized within the secretory pathway.