Pore formation on proliferating yeast Saccharomyces cerevisiae cell buds by HM-1 killer toxin.

The cytocidal effect of HM-1 produced by Hansenula mrakii on yeast Saccharomyces cerevisiae cells was studied. The HM-1 strongly inhibited the growth of S. cerevisiae cells at a low concentration (IC50: 2.1 x 10(-8) M) by reducing the number of viable cells. The killer action of HM-1 was most efficient when cells were actively proliferating. Cells in a resting state were resistant, but they became HM-1-sensitive after about 90 min of culturing at 30 degrees C, concomitantly with the increment of budding index. In association with the reduction of viable cell number, ultraviolet light-absorbing cellular components were discharged from sensitive cells. HM-1 molecules appear to bind to susceptible cells rather loosely since cells incubated with HM-1 were able to proliferate after having been washed. By phase-contrast light microscopy and scanning electron microscopy, discharge of cell material was observed at the budding portions of HM-1-treated cells. Addition of sorbitol to make the culture medium isotonic partially reduced the cell death induced by HM-1. These results suggest that HM-1 acts on the budding region of proliferating yeast cells, resulting in pore formation, leakage of cell material and eventual cell death.