STRUCTURE AND FUNCTION OF A VIRALLY ENCODED FUNGAL TOXIN FROM USTILAGO-MAYDIS - A FUNGAL AND MAMMALIAN CA2+ CHANNEL INHIBITOR

Background: The P4 strain of the corn smut fungus, Ustilago maydis, se cretes a fungal toxin, KP4, encoded by a fungal virus (UMV4) that pers istently infects its cells. UMV4, unlike most other (non-fungal) virus es, does not spread to uninfected-cells by release into the extracellu lar milieu during its normal life cycle and is thus dependent upon hos t survival for replication. In symbiosis with the host fungus, UMV4 en codes KP4 to kill other competitive strains of U. maydis, thereby prom oting both host and virus survival. KP4 belongs to a family of fungal toxins and determining its structure should lead to a better understan ding of the function and evolutionary origins of these toxins. Elucida tion of the mechanism of toxin action could lead to new anti-fungal ag ents against human pathogens. Results: We have determined the atomic s tructure of KP4 to 1.9 Angstrom resolution. KP4 belongs to the alpha/b eta-sandwich family, and has a unique topology comprising a five-stran ded antiparallel beta-sheet with two antiparallel alpha-helices lying at similar to 45 degrees to these strands. The structure has two left- handed beta alpha beta cross-overs and a basic protuberance extending from the beta-sheet. In vivo experiments demonstrated abrogation of to xin killing by Ca2+ and, to a lesser extent, Mg2+. These results led t o experiments demonstrating that the toxin specifically inhibits volta ge-gated Ca2+ channels in mammalian cells. Conclusions: Similarities, although somewhat limited, between KP4 and scorpion toxins led us to i nvestigate the possibility that the toxic effects of KP4 may be mediat ed by inhibition of cation channels. Our results suggest that certain properties of fungal Ca2+ channels are homologous to those in mammalia n cells. KP4 may, therefore, be a new tool for studying mammalian Ca2 channels and current mammalian Ca2+ channel inhibitors may be useful lead compounds for new anti-fungal agents.