Study of Actin and Its Interactions with Heavy Meromyosin and the Regulatory Proteins by the Pulse Fluorimetry in Polarized Light of a Fluorescent Probe Attached to an Actin Cysteine

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Abstract
The decay of anisotropy of the N-iodoacetyl-N′-(5-sulfo-1-naphthyl)-ethylenediamine fluorescence attached to cysteine-373 of actin can be characterized by two correlation times θ1 and θ2. θ1 has a value of several nanoseconds and is thought to represent some local protein motion. θ2 is of the order of several hundreds of nanoseconds. Its value increases with actin concentration. It represents an average of the G and F actin correlation times. When actin interacts with heavy meromyosin, θ2 increases and becomes infinite at a molar ratio of one heavy meromyosin molecule per four actin protomers. It is concluded that a definite complex is then formed between F actin and heavy meromyosin. In the same time, G actin concentration becomes equal to zero. Finally, when F actin forms a complex with the regulatory proteins tropomyosin and troponin, the value of θ2 is greater in the absence than in the presence of Ca2+. This result indicates that micromolar concentrations of Ca2+ induces a conformation change of the complex of F actin with the regulatory proteins.