The Ustilago maydis virus-encoded toxin-Effect of KP6 on sensitive cells and spheroplasts

Cells of Ustilago maydis containing double-stranded RNA (dsRNA) viruses secrete a virus-coded toxin to which strains of the same species and other related species are sensitive. The KP6 toxin encoded by P6 viruses has been shown to consist of two polypeptides designated VP10 and VP12.5. Each of these polypeptides is secreted by two nonkiller mutants that exhibit complementary killing activity. In this study, the effects of KP6 and its individual polypeptides were tested on sensitive U. maydis cells and spheroplasts. The results indicated that KP6 can either stimulate or inhibit the growth of cells. Light microscopy indicated that there was a decrease in cell size after exposure to the toxin. Scanning electron microscopy (SEM) of treated cells showed major changes in morphology. Ultimately, the cells seemed to collapse and their contents appeared to leak out, but their rod shape was preserved. Individual polypeptides had no effect on cell growth or viability, but a mixture of VP10 and VP12.5 had the same effect as KP6. VP10 caused a slight change in morphology of the cell surface as observed by SEM but VP12.5 had no discernible effect. Spheroplasts of sensitive cells were not sensitive to the toxin. Once the spheroplasts began to regenerate, they became sensitive to the toxin and their colony-forming ability decreased as more spheroplasts regenerated in the presence of the toxin. These results suggest that the cell wall is essential for the toxic effect, perhaps due to its ability to adsorb the polypeptides and channel the complex to the target.