Fluorescence anisotropy of labeled F-actin: influence of divalent cations on the interaction between F-actin and myosin heads

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Abstract
The interaction between [rabbit] F-actin and soluble proteolytic fragments of myosin, heavy meromyosin and myosin subfragment 1 without ATP was studied by measuring the static anisotropy and the transient anisotropy decay of the fluorescent chromophore N-(iodoacetyl)-N''-(5-sulfo-1-naphthyl)ethylenediamine bound to F-actin. In the presence of Ca2+ ions, the mobility of the chromophore was strongly decreased by adding heavy meromyosin or myosin subfragment 1 and this conformation change of F-actin showed a strong cooperativity; a very small amount of myosin heads induced the maximum anisotropy change. In the presence of Mg2+ ions, the addition of a small amount of myosin subfragment 1 or of heavy meromyosin increased the mobility of labeled F-actin that reached a maximum at a molar ratio of .apprx. 1/25 or 1/50, respectively. With further addition of myosin heads, the mobility of the labeled actin decreased. F-actin undergoes a conformation change by interacting with myosin heads, which depends on the nature of the divalent cations present in the solution.