Structure of the cross-striated adductor muscle of the scallop

The structure of the cross-striated adductor muscle of the scallop has been studied by electron microscopy and X-ray diffraction using living relaxed, glycerol-extracted (rigor), fixed and dried muscles. The thick filaments are arranged in a hexagonal lattice whose size varies with sarcomere length so as to maintain a constant lattice volume. In the overlap region there are approximately 12 thin filaments about each thick filament and these are arranged in a partially disordered lattice similar to that found in other invertebrate muscles, giving a thin-to-thick filament ratio in this region of 6:1.The thin filaments, which contain actin and tropomyosin, are about 1 μm long and the actin subunits are arranged on a helix of pitch 2 × 38.5 nm. The thick filaments, which contain myosin and paramyosin, are about 1.76 μm long and have a backbone diameter of about 21 nm. We propose that these filaments have a core of paramyosin about 6 nm in diameter, around which the myosin molecules pack. In living relaxed muscle, the projecting myosin heads are symmetrically arranged. The data are consistent with a six-stranded helix, each strand having a pitch of 290 nm. The projections along the strands each correspond to the heads of one or two myosin molecules and occur at alternating intervals of 13 and 16 nm. In rigor muscle these projections move away from the backbone and attach to the thin filaments.In both living and dried muscle, alternate planes of thick filaments are staggered longitudinally relative to each other by about 7.2 nm. This gives rise to a body-centred orthorhombic lattice with a unit cell twice the volume of the basic filament lattice.