Effect of calcium on synthetic myosin minifilaments

The effect of Ca2+ on the structural properties of myosin filaments has been examined by employing myosin minifilaments as a model system. Paired sedimentation studies of myosin minifilaments at pH 8.0 reveal only a minor increase in their sedimentation coefficient (0.7 +/- 0.3%) upon binding of Ca2+ (pCa = 4.5). This increase and the larger change observed at pH 7.0 (less than 3.7%) are attributed to protein aggregation. Light scattering measurements indicate that both Ca2+ and Mg2+ promote protein association in solutions of myosin minifilaments at pH 7.0 and 8.0 and in myosin filaments at pH 8.0. This association reaction is nonspecific and does not level off with increasing concentrations of divalent cations. Nevertheless, low concentrations of Ca2+ and Mg2+ have a rather limited effect on myosin polymerization. It is suggested that the binding of Ca2+ to the myosin light chains affects the association equilibrium in minifilaments and apparently does not alter other structural properties of these particles.