The effect of Ca(2+)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 m uscle myosin under conditions applied by Frado and Craig (1989) for demonst ration of the influence of Ca2+ on the structure of synthetic filaments of scallop striated muscle myosin. The results show that Ca2+ induces the tran sition of compact, ordered structure of filaments with a 14.5 nm axial repe at of the myosin heads close to the filament backbone (characteristic of th e relaxing conditions) to a disordered structure with randomly arranged myo sin 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 substan tial fraction of phosphorylated filaments in the relaxing solution is alrea dy disordered due to phosphorylation. Under rigor conditions some of the fi laments of unphosphorylated and phosphorylated myosin retain a certain degr ee 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 inhe rent property of smooth muscle myosin, like molluscan muscle myosin regulat ed exclusively by Ca2+ binding, and can play a modulatory role in smooth mu scle contraction.