The structure of tropomyosin

Abstract

In all muscles so far examined there is present an unusual protein to which no function has yet been assigned. It occurs side by side with actin and myosin in the myofibril, and because of its similarity to myosin was named tropomyosin (Bailey 1948). Though soluble in water, it is not extracted from coarsely minced muscle by aqueous media; its extraction is facilitated by drying the minced fibre in ethanol and ether and finally extracting with strong salt solution. From fish muscle, Hamoir (1951) has been able to extract directly a nucleotropomyosin complex, and the extraction procedure given above might be explained by assuming that tropomyosin occurs in situ in association both with nucleic acid and with lipid. Of this there is no proof. Tsao has recently shown that tropomyosin can be brought out of fresh muscle mince provided the fibres are thoroughly disintegrated. Compared with myosin, tropomyosin has a rather low molecular weight—53000 against 850000—but in its amino-acid composition is very much like myosin except for the greater amounts of lysine and glutamic acid, which make it the most highly charged protein known, excluding the protamines, which do not carry a mixed charge. No less than 45% of the total residues consist of non-amidized acid plus base groups. Some of these points are illustrated in table 5. Isoelectric point and salting-out range are similar to myosin, and though fairly soluble in water alone above pH 7, its inherent globulin properties can be shown at pH 6 when the addition of salt increases its solubility.