Crystal structure of ust1lago mayd1s kp6 killer toxin a-subuntt: building block of A K+-selective pore?

The Ustilago maydls killer toxin KP6 is a virally encoded secreted protein that kills sensitive Ustilago cells. The toxin is unique in that a KP6 oligomer containing a- and β-subunits interacts wiih the cell membrane, apparently resulting in an efflux of K+ and death of the target cells. The determination of the X-ray structure of the toxin has been undertaken in order to investigate the structure-function relationship of the proposed ion channel. The a-subunit, a 79 amino acid long polypeptide, has been crystallized in the hexagonal space group P6322 with unit cell dimensions a=b=48.3A and c=124.2A, containing one subunit in the asymmetric unit. The structure has been determined from an electron-density map prepared by the isomorphous replacement method using a platinum heavy-atom and a seleno-methionine derivative. The tertiary structure is comprised of three a-helices and four βstrands. The central Ihree-finger β-sheet is similar to those observed in neurotoxins and cardiotoxins. However, the overall topology is distinctly different from those of other known toxin structures. The molecules are arranged in two crystallographic trimers in which monomers are linked by inter-moleculer salt bridges about the 32 symmetry axis, generating a pore lined with six phenylalanine rings, with an accessible diameter of 4.2A. The pore is similar to the proposedj>henyl-cage of a K+-selective ion channel. The molecule is refined at 1.8A resolution, together with one sulfate ion and 84 water molecules, to an R-value of 0.167.