H protein, a minor protein of TMV virions, contains sequences of the viral coat protein.

H protein, a minor protein found associated with virions of tobacco mosaic virus (TMV) at an average of about one copy per virion and previously believed to be host-coded (Asselin and Zaitlin, 1978, Virology 91, 173-181), has been shown to contain sequences of the viral capsid protein. Two-dimensional tryptic peptide maps of 125I-labeled H protein (Mr 26,500) and coat protein (Mr 17,500) from TMV strains U1 and Dahlemense show that the respective H proteins contain most if not all of the labeled peptides of the coat proteins in addition to 2-3 unique peptides. The H proteins also contain unique antigenic determinants, as antibodies can be isolated which react strongly with the H protein but not with the coat protein of Dahlemense TMV. Finally, amino acid composition analysis of the U1-TMV H protein has shown the presence of methionine and histidine, amino acids not present in the coat protein of that strain. H protein appears to contain the same NH2 terminus as coat protein, as there is an H protein tryptic peptide that both comigrates in a two-dimensional system and produces the same acid cleavage product as the NH2-terminal tryptic peptide of coat protein. H protein also seems to have the same COOH terminus as coat protein, as cyanogen bromide digestion of Dahlemense-TMV coat protein and H protein indicates that each has a methionine about 12 amino acids from one terminus (known to be the COOH terminus of the coat protein). Thus, H protein is not structurally equivalent to coat protein with an addition on either its NH2 or COOH terminus. However, H protein does not appear to be a noncovalent aggregate of coat protein and some other protein. Rather, the model we favor for H protein structure is that of a branched fusion product between coat protein and another polypeptide of host or viral origin.