Role of the γ component of preprotoxin in expression of the yeast K 1 killer phenotype

K 1 killer strains of Saccharomyces cerevisiae secrete a polypeptide toxin to which they are themselves immune. The α and β components of toxin comprise residues 45–147 and 234–316 of the 316‐residue K 1 preprotoxin. The intervening 86‐residue segment is called γ. A 26‐residue signal peptide is removed on entry into the endoplasmic reticulum. The Kex2 protease excises the toxin components from the 290‐residue glycosylated protoxin in a late Golgi compartment. Expression of a cDNA copy of the preprotoxin gene confers the complete K 1 killer phenotype on sensitive cells. We now show that expression of immunity requires that α component and the N‐terminal 31 residues of γ. An additional C‐terminal extension, either eight residues of γ or three of four unrelated peptides, is also required. Expression of preprotoxin terminating at the α C‐terminus, or lacking the γ N‐terminal half of γ causes profound but reversible growth inhibition. Inhibition is suppressed in cis by the same 31 residues of γ required for immunity to exocellular toxin in trans , but not by the presence of β. Both immunity and growth inhibition are alleviated by insertions in α that inactivate toxin. Inhibition is not suppressed by kex2, chc1 or kre1 mutations, by growth at higher pH or temperature, or by normal K 1 immunity. Inhibition, therefore, probably does not involve processing of the α toxin component at its N‐terminus or release from the cell and binding to glucan receptors. Some insertion and substitution mutations in γ severely reduce toxin secretion without affecting immunity. They are presumed to affect protoxin folding in the endoplasmic reticulum and translocation to the Golgi.