Modulation of myosin filament conformation by physiological levels of divalent cation

The effect of divalent cation, in particular Mg2+, on the properties of synthetic myosin filaments has been studied; and substantial changes in sedimentation and light scattering demonstrated to occur in the physiological range of free Mg2+. A pre-requisite for these studies has been the definition of a modified method for the preparation of myosin in highly monodisperse filament form, rigorously free from thin filament proteins. The sedimentation coefficient at infinite dilution shows a large increase (169 S to 193 S) in the range 0.2 mM to 3 mM in Mg2+. The anomalous frictional increment found for these filaments is thus substantially reduced. The concentration dependence (ks), however, shows a substantial decrease (470 ml/g to 334 ml/g) in the same range of Mg2+, and the calculated filament molecular weight is virtually unchanged. A change in the filament conformation is thus indicated. This is confirmed by an analysis of the turbidity of the filaments in the centrifuge cell, which shows a similar increase in response to the addition of Mg2+. These effects have been found to be independent of ionic strength (0.07 to 0.11), pH (7.0 to 7.6), the presence of MgATP or the presence of low levels of Ca2+ (approximately 100 microM). These effects studied indicate the action of Mg2+ through a low-affinity binding site (Kd approximately 1.5 X 10(-3) M). We consider that a significant change in crossbridge conformation can adequately explain these changes in physical and enzymic properties. A provisional model is proposed, in which the effect of Mg2+ is to bring the crossbridges into closer proximity to the filament shaft.