Effect of troponin‐tropomyosin complex and Ca2+ on conformational changes in F‐actin induced by myosin subfragment‐1

Abstract

The interaction of regulated and unregulated actin of myosin-free ghost single fibre with myosin subfragment-1 free of 5,5′-dithiobis(2-nitrobenzoic acid) light chains was investigated by polarized microphotometry. The anisotropy of intrinsic tryptophan fluorescence of regulated actin is Ca²⁺-dependent and has a maximal value at low (pCa ≥ 7) and a minimal value at high (pCa ≤ 6) concentrations of Ca²⁺. The interaction of myosin subfragment-1 with actin induces cooperative changes in actin structure, which manifest themselves in a decrease in the anisotropy of tryptophan fluorescence. The cooperativity of conformational changes in actin, induced by myosin subfragment-1, is high for regulated actin in the absence of Ca²⁺ and low for unregulated and regulated actin in the presence of Ca²⁺. The data obtained suggest that the decrease of the flexibility of actin filaments, induced by tropomyosin or by Ca-unsaturated troponin-tropomyosin complex, results in increased cooperativity of conformational changes of actin induced by myosin subfragment-1.