Tropomyosin: a new asymmetric protein component of the muscle fibril

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Abstract
A new asymmetric, homogenous protein, termed tropomyosin, was isolated from skeletal and cardiac muscle. Though water-soluble in neutral solns. after isolation, it could be extracted from the tissue only by salt solns. and most easily from ethanol- and ether-dried tissue; it thus appears to be firmly attached to the structural components of the fibril. In dilute salt solns. tropomyosin crystallizes in large birefringent plates containing only 9 to 11% protein. In absence of salt, solns. of the protein are exceedingly viscous due to the aggregation of molecules by electrostatic interaction into large fibers. In rabbit skeletal muscle it occurs to the extent of 0.5 g. / 100 g. of wet muscle. The isoelectric point is near pH 5. Its solubility is little affected by heat treatment or by depolymerizing agents such as urea, although the ability to crystallize is lost. On the alkaline side of the isoelectric point, the protein may be dried in organic solvents without loss of solubility. Its amino-acid composition is of myosin type, and like myosin, it gives an X-ray diffraction pattern of alpha-type. The properties of tropomyosin suggest that it is a prototype of myosin, and the possibility exists that it is one of the units from which the myosin filament is elaborated.