Action of Yeast Killer Factor: a Resistant Mutant with Sensitive Spheroplasts

Yeast killer factor proteins bind to cells of both sensitive and killer-producing strains, although the latter are immune to killer action. Spheroplasts prepared from sensitive cells bind less than 1% of the killer bound to whole cells, but remain fully sensitive to killer. This finding and those obtained from binding studies of partially purified, radioactive killer protein suggest that most of the toxins remain bound to the yeast cell wall and do not function further in the killing process. A killer-resistant mutant R(18) was isolated from a sensitive strain. Whole cells of the mutant were unable to bind killer and were fully resistant. In contrast, spheroplasts of R(18) were fully sensitive to killer. These data suggest that the sites exposed to killer in spheroplasts are distinct from those on the cell wall. These wall sites appear to be necessary for killer action in whole cells.