Electron microscopy and X-ray diffraction of a hexagonal net of light meromyosin

Light meromyosin, prepared by brief digestion of rabbit myosin, forms at low ionic strength tactoids with a 43 nm periodicity and open nets. These nets, when negatively stained, show strands intersecting at intervals of ~ 60 nm and at an angle of 120 ° to form hexagonal arrays (Huxley, 1963).By slow dialysis of light meromyosin from 0.35 to 0.1 m-KCl we have obtained large, highly ordered hexagonal nets, which we have subjected to structural analysis by electron microscopy of both negatively stained and sectioned material, and by X-ray diffraction. The net is a three-dimensional crystalline array whose overall shape is that of an oblate ellipsoid. Viewed down the short axis, a hexagonal appearance is seen. Analysis of other views of the net suggests that it has a simple layered structure, each layer consisting of a set of parallel strands of diameter about 10 nm. Each strand crosses over those in neighbouring layers at intervals of 64.4 nm and at an angle of 120 °, so that in the whole structure there is a 3-fold screw axis through each node of the net. A model for a strand is described in which light meromyosin molecules, ~ 100 nm in length, are arranged in an anti-parallel manner, each molecule having one end at a node of the lattice. If this end corresponds to the free end of the myosin tail, one of the interactions is similar to that found in type 1 segments of myosin rod (Harrison et al., 1971). The molecular packing within strands may be related to the packing of myosin tails in the bare zone of muscle thick filaments.