Crystallographic studies of cowpea mosaic virus by electron microscopy and x-ray diffraction

Abstract

The particle packing in a hexagonal crystal form of cowpea mosaic virus (CpMV) was determined using electron micrographs of thin-sectioned crystals and single-crystal X-ray diffraction data. The unusual packing is characterized by large open channels along the hexad axes and along the a- and b-crystallographic axes. The unit cell contains six particles situated on crystallographic twofold axes. Each particle has four nearest neighbors at 284 A. A crystal form of southern bean mosaic virus (SBMV) has the same c-axis projection as the CpMV crystals. Previously reported rotation function results from these crystals are reinterpreted and a new particle orientation is proposed for the SBMV crystals. By comparing the CpMV and SBMV crystal forms and considering the relationship between particle and crystal symmetry reported for other crystalline viruses an orientation for the CpMV virions is proposed. This orientation dictales that neighboring particles have icosahedral twofold axes approximately aligned with vectors connecting the particles centers.