Direct visualization of the myosin crossbridge helices on relaxed rabbit psoas thick filaments.

Thick filaments in relaxed, quick-frozen and freeze-etched psoas myofibrils display a prominent helical pattern of projections repeating at 43 +/- 1 nm. These helices are right-handed, and measurement of the pitch angle indicates that the thick filaments are three-stranded. Each half-turn of a helix is composed of three to five projections, 11 to 12 nm in diameter. These projections probably represent individual myosin crossbridges. This is the first direct visualization of the crossbridge helices in vertebrate striated muscle filaments whose three-dimensional structure is preserved without chemical fixation.