Structural polymorphism of bromegrass mosaic virus: A neutron small angle scattering investigation

Abstract Neutron small angle scattering has been used to investigate the architecture of bromegrass mosaic virus (BMV) in solution as a function of pH, ionic strength, and nature of the polyvalent ions at room temperature: The radial extensions of RNA and protein were determined from measurements performed in buffers containing suitable amounts of heavy water (D 2 0) to match the scattering of either component. Our results confirm the polymorphism of BMV known from the variation of its overall size ( N. L. Incardona and P. Kaesberg, 1964 , Biophys. J. 4 , 11–21; M. Zulauf, 1977 , J. Mol. Biol. 114 , 259–266) and extend it to the RNA. Indeed, we found that although at pH 5.5 the overall size of compact BMV remains constant, measurable changes in the radial extension of RNA take place under the compacting action of high ionic strength or spermine. At neutral pH both the overall size of BMV and the extension of RNA vary in a similar way and depend strongly upon the environment: Swelling is maximal in the absence of polyvalent cations (spermine, Ca 2+ , Mg 2+ ), and the compacting action of at least Mg 2+ has been shown to be partially reversed by increasing the concentration of K + . The thickness of the protein shell does not change significantly upon swelling. Back-titration of swollen BMV is reversible only in the presence of divalent ions. The lack of reversibility extends to both RNA and overall sizes and is accompanied by an increase of the thickness of the capsid. The scattering curve of empty capsids obtained by self-assembly of dissociated BMV protein differs qualitatively from that of the capsid of the virions, implying not only that the size is intermediate between those of compact and swollen BMV but also that the clustering of the protein subunits is probably different at the periphery of the particle. Implications on the relative importance of protein-protein and protein-RNA interactions as well as on the action of ions on the stability of BMV are discussed, and mechanism is proposed for the lack of reversibility of BMV upon back-titration.