Abstract Background: The P4 strain of the corn smut fungus, Ustilago maydis, secretes a fungal toxin, KP4, encoded by a fungal virus (UMV4) that persistently infects its cells. UMV4, unlike most other (non-fungal) viruses, does not spread to uninfected cells by release into the extracellular milieu during its normal life cycle and is thus dependent upon host survival for replication. In symbiosis with the host fungus, UMV4 encodes KP4 to kill other competitive strains of U. maydis, thereby promoting both host and virus survival. KP4 belongs to a family of fungal toxins and determining its structure should lead to a better understanding of the function and evolutionary origins of these toxins. Elucidation of the mechanism of toxin action could lead to new anti-fungal agents against human pathogens. Results We have determined the atomic structure of KP4 to 1.9 å resolution. KP4 belongs to the α/ β-sandwich family, and has a unique topology comprising a five-stranded antiparallel β-sheet with two antiparallel α-helices lying at ∼45° to these strands. The structure has two left-handed β α β cross-overs and a basic protuberance extending from the β-sheet. In vivo experiments demonstrated abrogation of toxin killing by Ca 2+ and, to a lesser extent, Mg 2+. These results led to experiments demonstrating that the toxin specifically inhibits voltage-gated Ca 2+ channels in mammalian cells. Conclusion Similarities, although somewhat limited, between KP4 and scorpion toxins led us to investigate the possibility that the toxic effects of KP4 may be mediated by inhibition of cation channels. Our results suggest that certain properties of fungal Ca 2+ channels are homologous to those in mammalian cells. KP4 may, therefore, be a new tool for studying mammalian Ca 2+ channels and current mammalian Ca 2+ channel inhibitors may be useful lead compounds for new anti-fungal agents.