The structure of cucumber mosaic virus: cryoelectron microscopy, X-ray crystallography, and sequence analysis

The three-dimensional structure of cucumber mosaic virus (CMV) was analyzed at 23 A resolution by cryoelectron microscopy and image reconstruction, demonstrating structural similarity to cowpea chlorotic mottle virus (CCMV), another member of the Bromoviridae family. The CMV structure was determined at 8 A resolution by X-ray crystallography with phases determined by single isomorphous replacement and refined by fivefold noncrystallographic symmetry averaging. The X-ray structure agreed with the electron microscopy reconstruction; the electron density is consistent with beta-barrel subunits arranged with T = 3 quasi-symmetry in an orientation similar to that observed in CCMV. Strong density surrounding the icosahedral threefold axes (quasi sixfold axes in the T = 3 particle) between 80 and 100 A from the particle center formed a cylinder of radius 11 A, similar to the density observed in the same region of CCMV. This density corresponds to the beta-annulus of CCMV, which differentiates hexamers from pentamers and determines the formation of the T = 3 particles. The CMV and CCMV amino acid sequences were aligned, providing information (based on the CCMV atomic model) about the probable distribution of residues in the three-dimensional structure of CMV.