Calcium-induced dimerization of troponin-C.

Calcium-dependent self-association of rabbit skeletal troponin-C was investigated by equilibrium ultracentrifugation. Troponin-C which was homogeneous by the criterion of polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate was shown to undergo reversible dimerization (K2,app = 1.7 X 10(3) M-1) even in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) (0.1-1.0 M KCl, 0.01 M imidazole, 0.005 M potassium phosphate, 0.4 mM EGTA (pH 7.0), 1.0 mM dithiothreitol). The existence of a monomer-dimer reaction under these conditions could be demonstrated as long as the effects of the Donnan equilibrium were taken into account. The value of the observed second virial coefficient was consistent with the value expected from troponin-C at this pH. The dimerization could be enhanced by the addition of calcium to 0.1 mM (K2,app = 1.2 X 10(4) M-1). A monomer molecular weight of 18,700 was estimated from a "two-species" plot of the data. The effect of calcium was completely reversible.