Amino acid sequence of southern bean mosaic virus coat protein and its relation to the three-dimensional structure of the virus.

The amino acid sequence of the coat protein of southern bean mosaic virus has been determined. Chemical techniques were supplemented by a 2.8-A-resolution electron-density map which helped in assigning absolute positions for each peptide. Four amino acids and the placement of one heptapeptide near the amino terminus of the protein were interpolated by comparison with the partially known southern bean mosaic virus RNA sequence. The subunit is heavily lined with basic residues on the side facing the RNA. Subunit-subunit interactions are hydrophobic and ionic. The calcium site on the quasi-threefold axis has three glutamic residues as ligands. There is a disulfide bond, between Cys 168 and 176, within a sequence which is inserted in southern bean mosaic virus relative to the tomato bushy stunt virus structure. This portion is a helix nestling between the "beta-barrel" subunit structure and the RNA interior.