Studies on the properties of tryptophan and tyrosine side chains in tobacco mosaic virus and tobacco mosaic virus protein

Abstract

Two of the three tryptophan residues in each tobacco mosaic virus (TMV) protein subunit were oxidised with N-bromosuccinimide at pH 7.0. Oxidation of these residues did not impair the ability of the subunits to polymerise into protein rods. On the other hand, in the virus or native protein rods the tryptophan residues were not oxidised by N-bromosuccinimide. Thus, in the protein subunit, one tryptophan residue is ‘buried’ and two are situated on the surface but in the virus or protein rod, these latter two residues are situated in regions of interaction between subunits. However, they are not necessarily directly involved in bonding interactions. Solvent perturbation studies showed that the equivalent of at least one tryptophan chromophore was exposed on the surface of the subunit. At least three of the four tyrosine residues in each subunit were titrated reversibly in the native protein with an apparent p K of 10.6. The hydroxyl groups of these residues are probably on the surface of the subunit. Solvent perturbation studies showed that the equivalent of at least two tyrosine chromophores were exposed on the surface of the subunit. In the virus the tyrosine residues titrated irreversibly over a higher range of pH with an apparent p K of 11.0. The titration was paralleled by the disaggregation of the virus particle into subunits. It is suggested that tyrosine residues may be directly involved in quaternary interactions between subunits.