Towards an atomic model of the thick filaments of muscle

The thick filaments of muscle and non-muscle cells are polymers of myosin molecules whose energy-transducing heads lie on the filament surface, where they interact with actin to generate force. A key structural question is how the myosin heads are arranged in the relaxed state, and how this arrangement changes on activation of contraction. We have fitted the atomic structure of the myosin head to the three-dimensional structure of myosin filaments of tarantula muscle determined by electron microscopy to produce a near-atomic model of the head arrangement. A good fit is obtained only when the two heads from a myosin molecule run along the helical tracks antiparallel to each other. Oppositely oriented heads from axially adjacent molecules in a helix interact with each other, with their nucleotide-binding pockets opposed. This arrangement, supported also by crosslinking evidence, suggests a simple mechanism for the stabilization of myosin head helices in relaxed muscle via the formation of intermolecular "dimers" of heads from axially adjacent myosin molecules.