Cell Wall Receptor for Yeast Killer Toxin: Involvement of (1 → 6)-β- d -Glucan

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Abstract
The linear (1 → 6)-β-d-glucans pustulan and luteose were effective competitive inhibitors of killer toxin action. Affinity chromatography of killer toxin on a pustulan-Sepharose column showed that toxin bound directly to a (1 → 6)-β-linked polysaccharide. Other polysaccharides found in yeast cell walls, including (1 → 3)-β-d-glucan, mannan, chitin, and glycogen, were not effective as inhibitors of toxin. Fractionation of yeast cell walls was attempted to identify the toxin receptor in sensitive Saccharomyces cerevisiae. The receptor activity was retained among the insoluble glucans in alkali-washed cells; yeast mannan and alkali-soluble glucan had little receptor activity. A minor fraction of receptor activity was removed from alkali-washed cells by hot acetic acid extraction, a procedure which solubilized some (1 → 6)-β-d-glucan and glycogen. The major fraction (>70%) of receptor activity remained with the acid-insoluble (1 → 6)-β-and (1 → 3)-β-glucans. Zymolyase, an endo-(1 → 3)-β-d-glucanase, solubilized a substantial fraction of the receptor activity in the acid-insoluble glucans. The receptor activity in yeast cell walls was periodate and (1 → 6)-β-d-glucanase sensitive, but was resistant to (1 → 3)-β-d-glucanase and α-amylase. The acid-soluble glucan fractions of a sensitive strain and a krel-l receptor-defective toxin-resistant mutant were examined. The krel-l strain had a reduced amount (ca. 50%) of (1 → 6)-β-d-glucan compared with the sensitive parent strain. A sensitive revertant of the krel-l strain regained the parental level of glucan. These results implicate (1 → 6)-β-d-glucan as a component of the yeast cell wall receptor for killer toxin. Images