Killer yeast win epic battle of toxins.

Normally peaceful, brewer's yeast cells get ornery once a certain virus infiltrates them. The virus reprograms the infiltrated yeast cells to secrete a toxin that opens pores in neighboring yeast and kills them. Vintners rely on this feat to help clear out undesirable yeast strains that stray into their barrels.

Yet the killer-yeast cells themselves somehow become immune to the toxin.

Now, Steve A.N. Goldstein and his colleagues at Yale University have discovered the molecular mechanism behind this survival. Goldstein showed last year that the toxin snaps open a potassium-controlling channel in the yeast membrane. The yeast cells die by potassium leakage.

The researchers report in the June 1 CELL that the same toxin, when inside the cell, closes the potassium channel and saves the cell. In effect, toxin on the inside plugs up holes made by toxin on the outside.

Evidence for this channel-closing role emerged when the group applied toxin to yeast membranes turned inside out. The team determined the state of the channel by measuring potassium flux with an electrode. In the presence of toxin outside, the inside-out channel snapped shut.

In another experiment, the researchers studded the membrane of a frog egg with the yeast's potassium channels. When they applied the toxin to the exterior of the frog egg, the channels opened. When the scientists applied the viral toxin to both sides of the egg membrane to mimic the killer-yeast cells' situation, the potassium channels stayed shut most of the time.

Goldstein speculates that the main role of these channels might be to give virus-infected yeast an edge. "It's really a beautiful symbiotic relationship," he says. "The yeast gives the virus a home, and the virus gives the yeast dominance."