The Refined Crystal Structure of Cowpea Mosaic Virus at 2.8 Å Resolution

Comoviruses are a group of plant viruses in the picornavirus superfamily. The type member of comoviruses, cowpea mosaic virus (CPMV), was crystallized in the cubic space group I23, a = 317 Å and the hexagonal space group P6122, a = 451 Å, c = 1038 Å. Structures of three closely similar nucleoprotein particles were determined in the cubic form. The roughly 300-Å capsid was similar to the picornavirus capsid displaying a pseudo T = 3 (P = 3) surface lattice. The three β-sandwich domains adopt two orientations, one with the long axis radial and the other two with the long axes tangential in reference to the capsid sphere. T = 3 viruses display one or the other of these two orientations. The CPMV capsid was permeable to cesium ions, leading to a disturbance of the β-annulus inside a channel-like structure, suggesting an ion channel. The hexagonal crystal form diffracted X rays to 3 Å resolution, despite the large unit cell. The large (∼200 Å) solvent channels in the lattice allow exchange of CPMV cognate Fab fragments. As an initial step in the structure determination of the CPMV/Fab complex, the P6122 crystal structure was solved by molecular replacement with the CPMV model determined in the cubic cell.