The principal motions involved in the coupling mechanism of the recovery stroke of the myosin motor.

Muscle contraction is driven by a cycle of conformational changes in the myosin II head. After myosin binds ATP and releases from the actin fibril, myosin prepares for the next power stroke by rotating back the converter domain that carries the lever arm by 60 degrees . This recovery stroke is coupled to the activation of myosin ATPase by a mechanism that is essential for an efficient motor cycle. The mechanics of this coupling have been proposed to occur via two distinct and successive motions of the two helices that hold the converter domain: in a first phase a seesaw motion of the relay helix, followed by a piston-like motion of the SH1 helix in a second phase. To test this model, we have determined the principal motions of these structural elements during equilibrium molecular dynamics simulations of the crystallographic end states of the recovery-stroke by using principal component analysis. This reveals that the only principal motions of these two helices that make a large-amplitude contribution towards the conformational change of the recovery stroke are indeed the predicted seesaw and piston motions. Moreover, the results demonstrate that the seesaw motion of the relay helix dominates in the dynamics of the pre-recovery stroke structure, but not in the dynamics of the post-recovery stroke structure, and vice versa for the piston motion of the SH1 helix. This is consistent with the order of the proposed two-phase model for the coupling mechanism of the recovery stroke. Molecular movies of these principal motions are available at http://www.iwr.uni-heidelberg.de/groups/biocomp/fischer.