Yeast killer toxin: site-directed mutations implicate the precursor protein as the immunity component.

Yeast killer toxin and a component giving immunity to it are both encoded by a gene specifying a single 35 kd precursor polypeptide. This precursor is composed of a leader peptide, the alpha and beta subunits of the secreted toxin, and a glycosylated gamma peptide separating the latter. The toxin subunits are proteolytically processed from the precursor during toxin secretion. Using site-directed mutagenesis, we have identified a region of the precursor gene necessary for expression of the immunity phenotype. This immunity-coding region extends through the C-terminal half of the alpha subunit into the N-terminal part of the gamma glycopeptide. Mutations in other parts of the gene allow full immunity but produce precursors that fail to be processed. The precursor can therefore confer immunity, and we propose that it does so in the wild type by competing with mature toxin for binding to a membrane receptor.