The binding of divalent cations to tobacco mosaic virus and to some isometric plant viruses

Doctoral Thesis


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University of Cape Town

The binding of divalent cations (particularly calcium and magnesium) to strains of tobacco mosaic virus (TMV) and their isolated proteins was investigated, using equilibrium dialysis and potentiometric titration, in an attempt to elucidate the role of divalent cations in virus stabilisation. It was found that dissociation of bound calcium ions from TMV is apparently a necessary but insufficient condition for in vitro virus disassembly. TMV and the closely related strain, Y-TAMV, possessed three groups per protein subunit which titrated near neutral pH and which showed significant metal ion binding. The tightest of the three calcium binding sites, which was absent on the RNA-free protein, had a computed pKH of 8.3 and pKCa of 5.2 and had a significantly higher affinity for Ca⁺² over Mg⁺² This group thus had some of the characteristics to be expected for a calcium-mediated switch controlling in vivo virus disassembly, and possibly controlled the in vitro alkaline degradation of TMV as well. Both the U2 and cowpea strains of TMV bound one additional metal ion per protein subunit relative to vulgare, this binding site being retained by the polymerised proteins. However, calcium ions stabilised the polymerised forms of the proteins of all four TMV strains at pH values where depolymerisation would normally have occurred. Both bromegrass mosaic virus and turnip crinkle virus bound calcium ions, which stabilised compact forms of these viruses. The phenomenon of cation binding is thus not limited to TMV. In the light of published evidence, it appears that most if not all plant viruses are able to bind divalent cations, which thus represent a hitherto disregarded stabilising element.

Bibliography: pages 202-212.