Thermally activated tritium atoms were used to probe the surface topography of the coat protein of potato virus X (PVX) potexvirus. The accessibility profile of amino acid residues in the polypeptide chain was determined from data on the intramolecular distribution of tritium label in the PVX coat protein. Tryptic peptides T1 and T2, as well as parts of peptides T3 and T5, from the PVX particles were all located in the N-terminal region of the PVX coat protein and were accessible to tritium labelling, whereas the C-terminal region of the coat protein was practically inaccessible to it. Indirect ELISA and immunoblotting with two PVX-specific monoclonal antibodies confirmed that the N terminus of the coat protein (residues 1 to 56) was exposed on the virus surface, and furthermore that this region forms a highly immunogenic virus-specific antigenic region. The data obtained support the spatial model of PVX, in which the N-terminal amino acids of the coat protein are exposed at the particle surface, and the C-terminal region is buried in the particle. The spatial organization of the PVX coat proteins differs from the model proposed for other filamentous plant viruses such as potyviruses and tobamoviruses where both the N and C termini of the coat protein are located at the particles' surface.


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