ABSTRACT
Killer strains of Saccharomyces cerevisiae secrete into the growth medium a protein toxin that kills sensitive yeast strains. Cells transformed with a cDNA killer expression vector were pulse-labeled with Smethionine. Their intracellular proteins were precipitated with antitoxin antiserum. Three immunoreactive species were detected: a 42-kDa glycosylated protein and two 35- and 33-kDa unglycosylated ones. The α-β toxin is able to kill both whole cells and spheroplasts of sensitive strains. The α-subunit alone kills sensitive spheroplasts, but not whole cells. The kex 1 and kex2 mutations prevent secretion of active toxin but show normal expression of immunity. Protoxin is accumulated within the cell without being processed. The toxin recognizes and binds to a 1,6-3-βD-glucan yeast cell wall receptor. Sensitive cells are then killed by formation of cation-permeable pores in the cytoplasmic membrane.
