The use of thermally activated tritium atoms for structural-biological investigations: the topography of the TMV protein-accessible surface of the virus

Thermally activated tritium atoms were used for studying the topography of the TMV protein-accessible surface of the virus. The accessibility profile of amino acid residues in a protein polypeptide chain was determined from data on the intramolecular distribution of a tritium label in the TMV protein. It was shown that tryptic peptides T3, T4, T12, the N-terminal region of peptide T1 and the proximal tryptic peptide T8 (located 20 to 25 A (1 A = 0.1 nm) from the viral axis) are accessible to tritium labelling. The fact of tritiation of the viral RNA was detected as well. This evidence was compared with the high-resolution X-ray analysis data for the TMV. A model is suggested to explain the exposure of the buried sites of the virus to thermally activated tritium atoms. The possibilities and limitations of this method in studying the surface topography of proteins in supramolecular systems as well as for location of protein antigenic regions are discussed.