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

S. Mesentean, S. Koppole, J.C. Smith,  S. Fischer*

J. Mol. Biol., vol. 367, p. 591-602 (2007)


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's 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 see-saw motion of the relay helix, followed by a piston/seesaw 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.
The two end-state conformations of the recovery stroke.
The converter domain rotates by ~65 degrees to swing the lever arm (in yellow).  This rotation is coupled to the closing of the Switch-2 loop over the 40 Angstrom distant ATP.  The closing of Switch-2 forms two key H-bonds: 1) Between Gly457 and ATP, and 2) between Phe458 and the Phosphate loop (a.k.a. P-loop), which turn on the ATPase activity of Myosin.
The coupling model  has two phases.
Phase-I:  Formation of the H-bond between Gly457 and ATP pulls the relay helix and causes its seesaw pivoting (see arrows).  The converter domain attached at the C-terminal-end of the relay helix reacts with a rotation of 25 degrees.
Phase-II:  The H-bond between Phe458 and the P-loop pulls along the "Wedge-loop" (residues 572-574), which makes tight hydrophobic interactions with Phe458 of the Switch-2 loop. The Wedge loop pushes the SH1-helix, which responds with a piston/seesaw motion. The converter domain, which is attached to the SH1-helix, reacts with a 40 degree rotation.
The 2 end-conformations of the recovery-stroke
2 phases of the coupling mechanism

The  coupling model is checked by analyzing the motions of the implicated elements (converter domain, relay and SH1 helices, wedge loop) during equilibrium molecular dynamics (MD) of the protein in the two crystallographic endstates of the recovery stroke.
The principal motions, i.e., the deformations of largest amplitude that occur during the MD, are identified by Principal Component Analysis (PCA).  The principal motions that contribute most to the recovery-stroke transition are shown in the following movies. The color-coding in all the movies is the same as in the figures shown above.  Each movie shows one complete cycle of oscillatory motion and is best viewed by setting the Movieplayer on "Auto-replay".

Movie 1.  Converter domain rotation:
Principal motion of the converter domain in the MD of the post-recovery conformation (Principal Component #1, PC1).  It consists in a partial rotation of the converter domain (by ~8 degrees) around an axis parallel to the SH1 helix. This rotation is accompanied by a translation of the C-terminus of the relay helix. The amplitude shown in this movie is the same as the amplitude observed in the MD at room temperature.  The motions of PC1 correspond to the expected rotation of the converter domain and swinging of the lever-arm.
  Download the movie  (1Mb)

Movie 2.  Seesaw motion of the relay helix:
Principal motion of the relay helix in the MD of the pre-recovery conformation (PC#2).  The atoms at one end of the helix swing in the direction opposite to the direction of the atoms at the other end, while the stationary point of the relay helix is located in the middle of the helix, where Phe652 (in yellow) is the pivoting point of the seesaw.
  Download the movie  (1Mb)

Movie 3.  Correlated wedge/piston motion of the wedge-loop/SH1-helix:
Principal motion of the SH1 and SH2 helices plus the Wedge-loop in the MD of the post-recovery conformation (PC#3).  The atoms of the wedge-loop move towards the corner between the SH1 and SH2 helices, whose atoms move together towards the converter domain, undergoing a piston-like motion.  This is the correlated motion that is predicted by the coupling model near the end of the recovery stroke.
  Download the movie  (1Mb)


The present results provide strong evidence in favor of the proposed mechanics of the recovery stroke and the related coupling model (see also the paper).

Go to Home of S. Fischer