Calcium-induced structural changes in synthetic myosin filaments of vertebrate striated muscles

Using negative staining, freeze-drying, and shadowing techniques in electro n microscopy we have for the first time demonstrated Ca-induced reversible structural transitions in the synthetic filaments of dephosphorylated colum n-purified rabbit skeletal and cardiac muscle myosins formed by dialysis ag ainst solutions containing 120 mM KCl, 1 mM MgCl2, 10 mM imidazole-HCl buff er (pH 7.0), and either 0.1 mM CaCl2 or 1 mM EGTA It has been revealed that the compact ordered structure of the filaments with myosin heads and subfr agments-2 (S2) disposed close to the filament backbone with an axial period icity of about 14.5 nm in the absence of Ca2+ transforms into a spread diso rdered structure due to the movement of the heads and S2 away from the fila ment surface in the presence of Ca2+. Increasing the pH from neutrality to pH 7.8 leads to a spread, disordered structure while decreasing the pH valu e to 6.5 returns the filaments to their compact, rather ordered state indep endent of the Ca2+ concentrations used. The fact that the reversible struct ural transitions in synthetic filaments of myosin are observed in the absen ce of actin and actin and myosin-associated proteins suggests that Ca2+-ind uced S2 movement is an intrinsic property of myosin itself. Ca2+-induced S2 mobility may reflect the existence of functionally significant communicati ons between the myosin head domains and the tails of myosin molecules in th ick filaments, and its disappearance can be an indicator of the impairment of these communications, for example, in acute ischemia and myocardial infa rction. (C) 1999 Academic Press.