STRUCTURE OF SESBANIA MOSAIC-VIRUS AT 3 ANGSTROM RESOLUTION

Background: Sobemoviruses are a group of RNA plant viruses that have a narrow host range. They are characterized in vitro by their stability , high thermal inactivation point and longevity. The three-dimensional structure of only one virus belonging to this group, southern bean mo saic virus (SBMV), is known. Structural studies on sesbania mosaic vir us (SMV), which is closely related to SBMV, will provide details of th e molecular interactions that are likely to be important in the stabil ity and assembly of sobemoviruses. Results: We have determined the thr ee-dimensional structure of SMV at 3 Angstrom resolution. The polypept ide fold and quaternary organization are very similar to those of SBMV . The capsid consists of sixty icosahedral asymmetric units, each comp rising three copies of a chemically identical coat protein subunit, wh ich are designated as A, B and C and are in structurally different env ironments. Four cation-binding sites have been located in the icosahed ral asymmetric unit. Of these, the site at the quasi-threefold axis is not found in SBMV. Structural differences are observed in loops and r egions close to this cation-binding site. Preliminary studies on ethyl ene diamine tetra acetic acid (EDTA) treated crystals suggest asymmetr y in removal of the quasi-equivalent cations at the AB, BC, and AC sub unit interfaces. Conclusions: Despite the overall similarity between S MV and SBMV in the nature of the polypeptide fold, these viruses show a number of differences in intermolecular interactions. The polar inte ractions at the quasi-threefold axis are substantially less in SMV and positively charged residues on the RNA-facing side of the protein and in the N-terminal arm are not particularly well conserved. This sugge sts that protein-RNA interactions are likely to be different between t he two viruses.