The effect of Ca2+ on the structure of synthetic filaments of smooth muscle myosin

Using electron microscopy and negative staining we have studied the effect of Ca²⁺ on the structure of synthetic filaments of chicken gizzard smooth muscle myosin under conditions applied by Frado and Craig (1989) for demonstration of the influence of Ca²⁺ on the structure of synthetic filaments of scallop striated muscle myosin. The results show that Ca²⁺ induces the transition of compact, ordered structure of filaments with a 14.5 nm axial repeat of the myosin heads close to the filament backbone (characteristic of the relaxing conditions) to a disordered structure with randomly arranged myosin heads together with subfragments-2 (S-2) seen at a distance of up to 50 nm from the filament backbone. This order/disorder transition is much more pronounced in filaments formed of unphosphorylated myosin, since a substantial fraction of phosphorylated filaments in the relaxing solution is already disordered due to phosphorylation. Under rigor conditions some of the filaments of unphosphorylated and phosphorylated myosin retain a certain degree of order resembling those under relaxing conditions, while most of them have a substantially disordered appearance. The results indicate that Ca²⁺-induced movement of myosin heads away from the filament backbone is an inherent property of smooth muscle myosin, like molluscan muscle myosin regulated exclusively by Ca²⁺ binding, and can play a modulatory role in smooth muscle contraction.