The H protein isolated from tobacco mosaic virus reassociates with virions reconstituted in vitro.

Virions of two strains of tobacco mosaic virus (U1 and Cc) have associated with them a small amount of a minor protein called H protein (A. Asselin and M. Zaitlin, 1978, Virology 91, 173-181), now known to be related to the viral coat protein (C.W. Collmer, V.M. Vogt, and M. Zaitlin, 1983, Virology 126, 429-448.). In the present study, a quantification technique involving disruption of virions followed by direct analysis of the component parts on SDS polyacrylamide gels was used to confirm an average of one molecule of H protein per virion for U1 TMV. H protein was separated from coat protein and purified by electrofocusing in a flatbed of granulated gel under stringent dissociating conditions. When assayed in the presence of urea, H protein has a pI of approximately 5.4, coat protein has a pI of approximately 4.9. Proteinase K-treated TMV RNA and H-protein-free TMV coat protein were reconstituted in vitro with or without H protein and the resulting virions were analyzed. A small amount of H protein reassociated with virions reconstituted in vitro (less than 10% of the amount found in native virions) and became resistant to degradation by trypsin, but such virions were no different from virions reconstituted without H protein in terms of yield of reconstituted particles or infectivity. In mixed reconstitution experiments with RNA and coat protein from strains U1 and Cc in all four possible combinations and with U1 H protein, the H protein always associated with the U1 coat protein. This demonstrated U1-H protein affinity for a specific coat protein rather than a specific RNA. It is unlikely that H protein functions in the early stages of viral infection, although the possibility of its having some other role in the life cycle of TMV remains.