The stabilization of the particles of turnip rosette virus. III. Divalent cations.

Particles of turnip rosette virus (TRosV) contain 1-2 Ca2+ and about 1 Mg2+ ion per protein subunit. However, Ca2+ was much more efficient than other cations tested at stabilizing TRosV particles. Binding studies showed that at pH values of 5 and 6, TRosV particles bound 1 Ca2+ per protein subunit and, at pH 8.25, about 3 Ca2+; the Ca2+ was bound more firmly at lower pH values. Other cations competed poorly or not at all for the high affinity Ca2+ binding site. Preliminary results indicate that the cation stabilization of southern bean mosaic virus is similar to that of TRosV.