The spherically averaged structures of cowpea mosaic virus components by X-ray solution scattering

The X-ray diffraction patterns of the four components of cowpea mosaic virus isolated from a cesium chloride gradient were measured, using film methods, to 30 A resolution. Diffraction patterns were analyzed by fitting computed two-shell spherical models to the observed data. The fitting procedure was applied to data to 80 A resolution to avoid the nonspherical contribution to the pattern observed at higher resolution. At pH 7.0 all four components displayed the same external spherically averaged radius of 140 +/-2 A. The lowest density component (top), which contains no RNA, was best modeled by an empty shell with an outer radius of 140 A and an inner radius of 101 +/- 3 A. The middle component, containing 27% RNA by weight, was modeled with a uniform electron density sphere. The bottom upper and bottom lower components, which are biologically identical but display different buoyant densities in cesium chloride solutions, were analyzed independently. The bottom upper component was best modeled with a 101 A inner (RNA containing) sphere of mean electron density 0.453e-/A(3) and a 101 to 140 A outer (protein containing) shell of electron density 0.410e-/A(3). The bottom lower component was fit with the same model except that the RNA containing region displayed a mean electron density of 0.459e-/A(3). The implications of the spherically averaged component structures for the protein structure, RNA and protein hydration, and cesium binding to RNA are discussed.