Electrophoretic studies of muscle proteins. I. Complex formation between delta protein and F‐actin

Complex formation between delta protein and F‐actin has been demonstrated by electrophoretic technique. The high turbidity of F‐actin solutions has made it necessary to work at low concentrations of this protein (0.8 to 1.6 mg/ml). Delta protein concentrations were four to six times greater. At higher concentrations all F‐actin was bound to delta protein, on both limbs. The combination ratio was about 1:1 by weight. We call this complex “delta‐actin.” When the complex formed there was a slight fall in viscosity, indicating side‐by‐side union, but the turbidity greatly increased. The mobility of delta‐actin was always less than that of free F‐actin and sometimes also less than that of free delta protein. We earlier reported that delta protein is probably a polymer of tropomyosin. Its sedimentation constant (4.4 to 6.0 S) is higher than the of any other form of tropomyosin so far described. It may be the native molecule, its structure preserved by our relatively simple method of extraction and purification. The filaments of the I band may be composed of delta‐actin. Since delta protein also forms a complex with myosin the filaments of the A band may be composed of delta‐myosin. Delta protein may be a structural component which, in addition to other activities, may direct the building of both filament arrays and strengthen them.