The arrangement of myosin heads in relaxed crab muscle

To understand the mechanism of muscular contraction1,2 and its regulation by Ca2+ (ref. 3), it is important to know the arrangement and configuration of the myosin heads in the space between thin and thick filaments in relaxed muscle. This information is still lacking, mainly because the myosin reflections in the X-ray diffraction patterns are sampled by the myofilament lattice, which makes it difficult to deduce information about the configuration of the head. In crab muscle, however, unsampled myosin reflections can be obtained, and lattice spacing (between adjacent thick filaments) can be changed by osmotic pressure applied to chemically skinned single fibres. The lattice spacing affects the intensity profiles of the myosin reflections, indicating a change of configuration of the heads. The results presented here are consistent with a model in which the myosin heads are arranged in a helical manner with a 4-fold rotational symmetry around the thick filament axis. Each head is elongated, probably 16–18 nm long and 3–4 nm thick, and is, in living resting muscle, centred 13.5 nm from the mean position of the axis of the nearest thin filament, implying that the tip of the head is very close to the surface of the thin filaments