Entropy-driven polymerization of ribgrass virus protein.

Holmes ribgrass virus (HRV), because of serological results, is regarded as a distantly related strain of tobacco mosaic virus (TMV). HRV protein differs substantially in amino acid sequence from TMV protein, especially in that it contains one histidine residue and three methionine residues, compared to none of either for TMV protein. Ultracentrifugation and hydrogen ion titration data on HRV protein, similar to those obtained previously for the early stage polymerization of TMV and E66 proteins, demonstrated some similarities and more distinct differences from those of the other two proteins. The major similarities are that the early polymerization of HRV protein is entropy driven and the first major polymerized product is a 20 S component, presumably a double disk or two-turn helix, as in the case of the other proteins. The major differences are that the unpolymerized HRV protein sediments at 3 S rather than at the 4 S for the others; it is presumably a dimer of the polypeptide chain. The enthalpy of polymerization per mole of A protein, delta H*, is 18,400 cal for HRV protein, compared to about 30,000 for TMV protein. One mol of H+ ion/mol HRV A protein, compared to 1.5 for TMV and E66 proteins, is bound during polymerization to the 20 S state. Contrasted with the other proteins, very little if any electrical work contribution was detected for the HRV protein. A major difference was found in hydrogen ion titration. Unpolymerized HRV protein binds hydrogen ions significantly in the unpolymerized A protein state, unlike the A proteins from the other two viruses.