Effect of chemicals on the early stages of tobacco mosaic virus protein polymerization.

Osmotic pressure and sedimentation velocity techniques have been used to investigate the effect of KSCN, thiourea, EDTA, acetamide, and sucrose on the 53,000 molecular weight A protein at pH 6.5-7.0. In the presence of all the compounds except thiocyanate, the number average molecular weight lies between 50,000 and 56,000 which corresponds to a trimer of three chemical subunits. In the presence of thiocyanate, the molecular weight decreases initially sharply with increasing concentration of thiocyanate to 0.1 M, then dissociation proceeds with less efficiency with increasing concentration of KSCN until a molecular weight close to a monomer (21,700) is obtained at 0.59 M KSCN. Sedimentation data agree with osmotic pressure data since, in the absence of thiocyanate, the measured values of S(20,w) indicate that the favored state is that of a trimer. In the presence of thiocyanate, however, S(20,w) decreases with thiocyanate concentration to S(20,w) = 1.9, which is the value reported for the monomer.