Reactivity and structural role of protein amino groups in tobacco mosaic virus

Abstract

The role of protein amino groups in the regeneration of virus-like rods from tobacco mosaic virus A protein in the absence of viral RNA has been investigated. It has been found that in the isolated A protein the ϵ-amino groups of the two lysine residues per polypeptide chain are available for reaction with S-ethyl trifluorothioacetate and with various imidoesters. After total trifluoroacetylation, the protein is incapable of self-assembly to protein rods; but removal of trifluoroacetyl groups leads to recovery of this ability. On the other hand, complete reaction with even comparatively bulky imidoesters does not inhibit rod formation, which suggests that positive charge on one or both of the two lysine residues is essential for this process to occur. The ϵ-amino groups in the isolated A protein were observed to show little, if any, difference in reactability towards S-ethyltrifluorothioacetate. On the other hand, reaction of the imidoester, methyl picolinimidate, with the intact virus was found to be limited almost completely to lysine residue 68, with little or no reaction at lysine residue 53. These results indicate that the lack of reaction at lysine residue 53 is a function of the intact virus structure, and suggest that this residue may be involved in an ion-pair within the virus surface.