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

A stringent test of the effect of the truncations of Kexlp was to determine the ability of the mutant proteins to carry out the Golgi-based proteolytic processing of K1 killer toxin in vivo (for review see Bussey, 1988) . Sc25k is a strain harboring the dsRNA virus that encodes K1 killer toxin, and produces a killing zone on plates seeded with a strain sensitive to the killer toxin (Bussey et al., 1983) . The pKX1-20 series of plasmids (centromeric based) were transformed into Sc25k-13 (kex/-A/), and the transformants analyzed for their ability to process K1 killer toxin and produce a killing zone (Fig. 6 A) . Sc25k-13 containing the vector plasmid produced no killing zone, whereas the introduction of a plasmidbased copy ofKEXI (pKX1-20WT) enabled the strain to produce active K1 killer toxin as shown by the appearance of a zone (Fig. 6 B) . This zone was smaller than that of Sc25k (KEXI) because of the KEX/promoter truncation in pKX1-20WT (discussed above). The truncated forms of Kexlp, both soluble and membrane associated, produced killer zones significantly smaller than that produced by pKX1-20WT (Fig. 6 A) . This functional complementation assay was also conducted in a different K1 killer strain (TC106ot-17) with similar results.