Title: Calcium modulates conformational changes in F‐actin induced by smooth muscle heavy meromyosin
Abstract: The effect of Ca 2+ on conformational changes in rhodamine‐phalloidin‐labeled F‐actin induced by binding of smooth muscle heavy meromyosin (HMM) with either phosphorylated or dephosphorylated regulatory light chains (LC 20 ) was studied by polarized fluorimetry. LC 20 phosphorylation caused alterations in the F‐actin structure typical of the force‐producing (strong‐binding) state, while dephosphorylation of the chains led to alterations typical of the formation of non‐force‐producing (weak‐binding) state of the actomyosin complex. The presence of Ca 2+ enhanced the effect of LC 20 phosphorylation and weakened the effect of LC 20 dephosphorylation. These data suggest that Ca 2+ modulates actin‐myosin interaction in smooth muscle by promoting formation of the strong‐binding state.