Structure of Ustilago maydis killer toxin KP6 α-subunit. A multimeric assembly with a central pore

Ustilago maydis is a fungal pathogen of maize, some strains of which secrete killer toxins. The toxins are encoded by double-stranded RNA viruses in the cell cytoplasm. The U. maydis killer toxin KP6 contains two polypeptide chains, α and β, having 79 and 81 amino acids, respectively, both of which are necessary for its killer activity. The crystal structure of the α-subunit of KP6 (KP6α) has been determined at 1.80-Å resolution. KP6α forms a single domain structure that has an overall shape of an ellipsoid with dimensions 40 Å x 26 Å x 21 and belongs to the α/β- sandwich family. The tertiary structure consists of a four-stranded antiparallel β-sheet, a pair of antiparallel α-helices, a short strand along one edge of the sheet, and a short N-terminal helix. Although the fold is reminiscent of toxins of similar size, the topology of KPα is distinctly different in that the α/β-sandwich motif has two right-handed βαβ split crossovers. Monomers of KP6α assemble through crystallographic symmetries, forming a hexamer with a central pore lined by hydrophobic N-terminal helices. The central pore could play an important role in the mechanism of the killing action of the toxin.