Role of the myosin light chains in binding to actin

Abstract

Myosin from the fast twitch muscles of rabbit is hexameric and comprises two polypeptide chains of approx. mol. wt 200 000 (heavy chains) and 4 mol (per mol myosin) of light chains: two identical phosphorylatable polypeptides of mol. wt 18 000 (the P-LC) and two polypeptides of molecular weight 22 000 and 16 000 (the so-called alkali light chains, Al and A2 respectively (see review [l] and references therein). The four light chain components reside in the globular ‘heads’ of the myosin molecule, the subfragment 1 (S-l) regions, which possess both the actin binding and ATPase activities of the myosin [ 1 ] . Densitometric and radiochemical methods have shown that there is an unequal distribution of the two alkali light chains which supports the hypothesis that myosin isoenzymes exist [2,3]. Myosin from chicken breast-muscle also contains three species of light chain and it is likely that this myosin is similar to that of the rabbit in overall structure and design. Chymotryptic digestion of insoluble myosin filaments from rabbit fast-twitch muscle produces S-l species without the P-LC and such fragments have been separated into two species by ion-exchange chromatography, each of which contains a single type of alkali light chain [4]. The precise role of the myosin light chains is still