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.
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