Investigations on Turnip Yellow Mosaic. Virus interactions with aliphatic mercurials: II. Conformational changes in the protein

Abstract

The interaction of the sulfhydryl groups of Turnip Yellow Mosaic Virus with some aliphactic mercurials, which leads to the dissociation of the virus protein into subunits, was studied further by fluorometry and polarography. The fluorescence spectrum of TYMV shows only one peak in the ultraviolet region, lying at 320 nm, and one in the visible region, at 440 nm. Treatment with mercurials causes a marked increase in ultraviolet fluorescence, while the position of the maximum shifts to 335 nm. The time course has a sigmoidal shape and the increase in fluorescence lags behind the substitution of the SH-groups and the degradation of the protein shell into subunits. Whereas compounds with a longer carbon chain on the mercury lead to a faster breakdown of the quaternary structure, the increase in fluorescence takes place at similar rates with all four mercurials tested. It is concluded that the protein subunits, which are primarly released either in the free state or as aggregates, undergo secondary conformational changes, altering the environment of the aromatic amino acid residues. In Brdička's ammoniacal cobaltous solution, a polarogram of TYMV shows only the first peak of the “catalytic double wave.” Upon treatment with mercurials, the second wave appears. This change in polarographic behavior runs more or less parallel to the change in fluorescence and also lags behind the blocking of the SH-groups. The explanation offered is that the changes in protein conformation result in a greater ability to form a catalytically active complex with cobalt.