PRIMARY STRUCTURE OF SESBANIA MOSAIC-VIRUS COAT PROTEIN - ITS IMPLICATIONS TO THE ASSEMBLY AND ARCHITECTURE OF THE VIRUS

Sesbania mosaic virus (SMV) is a plant virus that infects Sesbania gra ndiflora plants in Andhra Pradesh, India. The amino acid sequence of t he coat protein of SMV was determined using purified peptides generate d by cleavage with trypsin, chymotrypsin, V8 protease and clostripain. The 230 residues so far determined were compared to the corresponding residues of southern bean mosaic virus (SBMV), the type member of sob emoviruses. The overall identity between the sequences is 61.7%. The a mino terminal 64 residues, which constitute an independent domain (R-d omain) known to interact with RNA, are conserved to a lower extent (52 .5%). Comparison of the positively charged residues in this domain sug gests that the RNA-protein interactions are considerably weaker in SMV . The residues that constitute the major domain of the coat protein, t he surface domain (S-domain, residues 65-260), are better conserved (6 6.5%). The positively charged residues of this domain that face the nu cleic acid are well conserved. The longest conserved stretch of residu es (131-142) corresponds to the loop involved in intersubunit interact ions between subunits related by the quasi 3-fold symmetry. A unique c ation binding site located on the quasi 3-fold axis contributes to the stability of SMV. These differences are reflected in the increased st ability of the SMV coat protein and its ability to be reconstituted wi th RNA at pH 7.5. A major epitope was identified using monoclonal anti bodies to SMV in the segment 201-223 which contains an exposed helix i n the capsid structure. This region is highly conserved between SMV an d SBMV (70%) suggesting that it could represent the site of an importa nt function such as vector recognition.