Key result
The interaction between F-actin and myosin heads induces a conformation change in F-actin that depends on the presence of Ca2+ or Mg2+ ions.
The conformational change of F-actin upon interaction with myosin heads is dependent on the nature of the divalent cations (Ca2+ vs Mg2+) present.
Should not yet change practice in cardiac contractility; leaves open cation-specific effects on human sarcomere function.
The interaction between F-actin and soluble proteolytic fragments of myosin, heavy meromyosin and myosin subfragment 1 without ATP, has been 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; that is, a very small amount of myosin heads induced the maximum anisotropy change. On the other hand, 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 about 1/25 or 1/50, respectively. With further addition of myosin heads, the mobility of the labeled actin decreased. From these studies, one concludes that F-actin undergoes a conformation change by interacting with myosin heads, which depends on the nature of the divalent cations present in the solution.
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Miki et al. (1982) studied this question. Addition of heavy meromyosin or myosin subfragment 1 was evaluated on Fluorescence anisotropy of labeled F-actin. The interaction between F-actin and myosin heads induces a conformation change in F-actin that depends on the presence of Ca2+ or Mg2+ ions.
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