STRUCTURE OF SESBANIA MOSAIC-VIRUS AT 3 ANGSTROM RESOLUTION

Sesbania mosaic virus (SMV) is an isometric, ss-RNA plant virus found infecting Sesbania grandiflora plants in fields near Tirupathi, South India. The virus particles, which sediment at 116 S at pH 5.5, swell u pon treatment with EDTA at pH 7.5 resulting in the reduction of the se dimentation coefficient to 108 S. SMV coat protein amino acid sequence was determined and found to have approximately 60% amino acid sequenc e identity with that of southern bean mosaic virus (SBMV). The amino t erminal 60 residue segment, which contains a number of positively char ged residues, is less well conserved between SMV and SBMV when compare d to the rest of the sequence. The 3D structure of SMV was determined at 3.0 Angstrom resolution by molecular replacement techniques using S BMV structure as the initial phasing model. The icosahedral asymmetric unit was found to contain four calcium ions occurring in inter subuni t interfaces and three protein subunits, designated A, B and C. The co nformation of the C subunit appears to be different from those of A an d B in several segments of the polypeptide, These observations coupled with structural studies on SMV partially depleted of calcium suggest a plausible mechanism for the initiation of the disassembly of the vir us capsid. (C) 1997 Elsevier Science B.V.