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

Abstract

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, alpha and beta, having 79 and 81 amino acids, respectively, both of which are necessary for its killer activity, The crystal structure of the alpha-subunit of KP6 (KP6 alpha) has been determined at 1.80-Angstrom resolution. KP6 alpha forms a single domain structure that has an overall shape of an ellipsoid with dimensions 40 Angstrom x 26 Angstrom x 21 Angstrom and belongs to the alpha/beta-sandwich family. The tertiary structure consists of a four-stranded antiparallel beta-sheet, a pair of antiparallel alpha-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 KP6 alpha is distinctly different in that the alpha/beta-sandwich motif has two right-handed beta alpha beta Split crossovers. Monomers of KP6 alpha 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.