A neutron scattering study of the structure of compact and swollen forms of southern bean mosaic virus.

Neutron small-angle scattering in H2O/D20 buffers has been used to investigate the radial distribution of protein and RNA in the compact and swollen forms of southern bean mosaic virus (SBMV). The architecture of the particle is described by a set of four concentric shells. Compact SBMV was found to consist of a tightly packed outer protein shell, with the RNA and about 15% of the protein at the interior of the particle. The protein penetrates as deeply as the RNA toward the center. This structure resembles that of tomato bushy stunt virus although the average distribution of the protein is much less bilobal. There is no major reorganisation of the architecture of the virion upon swelling induced by the removal of divalent ions at pH 8.5: the radii of all four shells increase by about 15 A, with a concomittant decrease of the packing density. Possible correlations between the mechanism of swelling and the architecture and stability of small isometric virions are discussed.