The structure of the head-tail junction of the myosin molecule.

An atomic model of the junction between the two heads and tail of a myosin molecule has been created by attaching a scallop regulatory domain to the end of each of the two alpha-helical strands of a model of the scallop alpha-helical coiled coil. The C-terminal alpha-helix of the heavy chain of each regulatory domain was superposed over the corresponding sequence in the coiled coil. In the structure created, the two heads lie alongside one another with their bases in contact but remarkably without steric clash. The principal interactions between the two heads are between the regulatory light chains and there are also head-tail interactions between each regulatory light chain and its heavy chain partner in the coiled coil. The invariant proline residues cause the heavy chains to flare to form the fork. The direction of the turn at the WQW sequence within the regulatory domain causes the long alpha-helix of the heavy chains within the head to continue the sense of the supercoil. With the bases of heads interacting, motion of the heads could still occur by a flexing of the coiled coil close to the heads and by a flexing and twisting of the long alpha-helices in the head. The model accounts for some of the conserved sequence features in myosins from different sources and provides a structural basis for understanding the head-head interactions in regulated myosin. Using the C alpha atoms of subfragment 1 we have also constructed a model with two complete heads. The clockwise curvature of the heads when the model is viewed end-on towards the tail accounts for the most common appearance of myosin molecules in electron micrographs. These models are predicated on the assumption that the entire heptad sequence of the heavy chains forms a coiled coil. Previous evidence from electron micrographs of myosin molecules that this was not the case can be explained by the foreshortening of the tail close to the heads.