Location of tyrosine residues in the disk of tobacco-mosaic-virus protein and comparison of the subunit packing with that of the virus

The products of iodination of the disk of tobacco mosaic virus coat protein have been investigated in order to locate the reactive amino acid residues in the three-dimensional structure. Reaction occurs mainly with tyrosine-139 and, to a lesser extent, tyrosine-2 and the positions of these modified residues have been determined by X-ray crystallography. Different extents of reaction are found in the two rings of the disk and also, on adding the high salt concentration needed for stabilisation of the crystal during reaction, some conformational changes in the polypeptide chain in the inner part of the disk. Comparison of the relative positions of residues 27 and 139 in the disk and virus shows that little distortion occurs in the outer part of the subunit during the transition between the disk and virus structures.