Reversible swelling of SBMV is associated with reversible disordering

The structures of the compact and swollen southern bean mosaic virus (SBMV) particles have been compared by X-ray diffraction and proton magnetic resonance (PMR). Small-angle X-ray scattering showed that removal of divalent cations at alkaline pH causes the particle diameter to increase from 289Å in the native SBMV by 12% in solution and by 9% in microcrystals. The swelling is fully reversible upon re-addition of Ca²⁺ and Mg²⁺ ions, as shown by the X-ray patterns at 6Å resolution and by the 270MHz PMR spectra. Beyond 30Å resolution, X-ray patterns from the compact SBMV in solution and in microcrystals show fine fringes of ∼1/225Å^-1 width extending to 6Å resolution, whereas patterns from the swollen SBMV in solution and in microcrystals show only broader fringes of ∼1/90Å^-1 width, Model calculations demonstrate that the fine fringes from compact SBMV arise from regular packing of the protein subunits on the icosahedral surface lattice; the smearing of fine fringes in the swollen virus pattern can be simulated by uncorrelated displacements of pentamers and hexamers of protein subunits, with a standard deviation of 6Å from their mean locations. The PMR spectrum of compact SBMV is poorly resolved, whereas PMR spectrum of swollen SBMV shows sharp resonances in the methyl proton region. The line-narrowing for a fraction of the aliphatic protons upon swelling cannot be accounted for by rotational relaxation of the particle of 6×10⁶MW, but must be attributed to internal motion in small regions of the protein subunits.