SUBSTRUCTURE OF MYOSIN MOLECULE .I. SUBFRAGMENTS OF MYOSIN BY ENZYMIC DEGRADATION

The formation of light meromyosin and heavy meromyosin by trypsin first demonstrated that myosin had regions of high α-helix content which could be separated from the ATPase and actin-binding centers of the molecule. Here we report the action of papain on myosin. Under suitable experimental conditions, low concentrations of papain can split the enzymic globular regions of myosin from the structural α-helical rod, leaving the latter intact along its length. Further digestion cleaves the rod along two-thirds of its length to form light meromyosin and the rod portion of heavy meromyosin. The size and shape of these proteolytic fragments as deduced from hydrodynamic measurements are compared to the structure seen in electron micrographs of shadow-cast preparations. From these studies we conclude that the myosin molecule consists of two globular units, each about 70 Å in diameter, attached to a double-stranded α-helical rod about 1350 Å long. The effect of Ca- and Mg-actin on the ATPase activity of the cleaved isolated globules is similar to the effect of these cations and actin on the globules in the native or attached state existing in heavy meromyosin. The rod portion of myosin is insoluble at low ionic strength and is therefore responsible for filament formation under physiological conditions. However, only the light meromyosin end of the rod is insoluble in water at neutral pH; the remainder of the rod is soluble at all ionic strengths. There is no evidence for an extensive amorphous region connecting these two subfragments. The intact rod has an α-helical content consistent with a fully helical molecule, and no proline residues could be detected by amino acid analysis. © 1969.