Modification of the coat protein charge and its effect on the stability of the U1 strain of tobacco mosaic virus at alkaline pH.
The coat protein of intact tobacco mosaic virus was selectively modified by three chemical treatments. These were amidination of lysine-68 by treatment with methylacetimidate or methylpicolinimidate, maleylation of lysine-68 by treatment with maleic anhydride, and nitration of tyrosine-139 by treatment with tetranitromethane. At pH values of 9.2 and 10.2 these modifications should cause changes of 0, -2, and -1, respectively, in the net electrostatic charge on each protein subunit. The slight gain in stability of the amidinated virus and the loss in stability of the maleylated or nitrated viruses at alkaline pH values could be correlated with these changes in net electrostatic charge. Lysine-68 and tyrosine-139 are situated on the outer "surface" of the virus rod, approximately 3.0 nm from the RNA, which is deeply buried in the protein at a radial distance of 4 nm from the viral axis. The strong influence of the RNA-protein interactions was demonstrated by the fact that native and chemically modified TMV showed the same pattern of 3'-coterminal intermediates during disassembly in alkali, which reflects the variation in RNA-protein interactions along the virus rod. Data on the (in)stability of naturally occurring strains of the virus at alkaline pH support the view that changes in charge distribution in the vicinity of juxtaposed "surface" residues 68 and 139 affect the overall stability of the virion.
Wilson
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TM
Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 IQW, England.
Perham
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RN