Conformational changes of F-actin in myosin-free ghost single fibre induced by either phosphorylated or dephosphorylated heavy meromyosin

The changes in F-actin conformation of myosin-free single ghost fibre induced by binding of phosphorylated or dephosphorylated heavy meromyosin have been studied by measuring polarized fluorescence of F-actin intrinsic tryptophan and of phalloidin-rhodamin bound to F-actin. The changes of polarization of both fluorescences were found to be dependent on low or high Ca²⁺ concenration and on the phosphorylated or dephosphorylated form of heavy meromyosin. Computer analysis of polarized fluorescence has shown that binding of phosphorylated heavy meromyosin with divalent ion binding sites saturated with Mg²⁺ (in the presence of 1 mM MgCl₂ and 1 mM EGTA) and dephosphorylated heavy meromyosin with divalent ion binding sites saturated with Ca²⁺ (in the presence of 1 mM MgCl₂ and 0.1 mM Ca²⁺) decreases the angles of emission and absorption dipoles and the angle between the F-actin axis and the fibre axis, thus suggesting that F-actin in ghost fibre becomes more flexible. On the other hand, the above-mentioned angles increase when phosphorylated heavy meromyosin at high and dephosphorylated heavy meromyosin at low Ca²⁺ concentration were bound to thin filaments, thus showing the decrease of F-actin flexibility under these conditions.