Isolated actin exists in two forms: G-actin, a more or less globular molecule of molecular weight about 42000, and F-actin, a fibrous protein which is a polymer of G-actin. Neither form has any ATPase activity, but they will both combine with myosin.

F-actin consists of two chains of monomers connected together in the form of a double helix, as is shown in Fig. 10.15^. The thin filaments in intact muscle also contain tropomyosin and troponin, probably arranged as in Fig. 10.15c.

Fig. 10.15. Models of the structure of the thick and thin filaments. a, the myosin filament; b, F-actin; c, the thin filament. Based on Offer (1974) after various authors.

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