Structural mechanism of the recovery stroke in the Myosin molecular motor.

Stefan Fischer*, Bjoern Windshuegel,  Daniel Horak,  Kenneth C. Holmes and Jeremy C. Smith.

Proceedings of the National  Academy of Sciences USA, vol. 102, 6873-6878 (2005)



During muscle contraction, after the force-generating power-stroke and ATP-induced unbinding from actin,  myosin swings back its lever-arm by ~60 degrees.  During this "recovery stroke" it must accomplish a crucial function, which is to couple its ATPase activation to the large rotation of the "converter" domain that carries the lever arm.  Here, the atomic details of this transition are determined by computing a minimum-energy path for the recovery-stroke transition, using the Conjugate Peak Refinement (CPR) method.  The path reveals how a series of structural changes along the "relay" helix amplify small changes near the ATP binding site into the large motion of the lever arm. This has lead to a comprehensive picture of this essential chemo-mechanical coupling.
The Lymn-Taylor contraction cycle.
The recovery-stroke is between states II and III:  the lever arm (in yellow) attached to the converter domain (in green) swings back to prepare for the next power-stroke.
The two end-state structures of the recovery-stroke.
The converter domain (in green) rotates by ~60 degrees to swing the lever-arm (in yellow).  It is 40Angstroem away from the ATP (in van der Waals spheres).  The converter-domain orientation must be coupled to the conformation of the relay helix (in cyan) near the ATP in such a way that ATPase activity is switched on only when the Lymn-Taylor cycle is in state III.

Movie 1.  Overall view (Fig1 B&C of paper):
Overall view showing the recovery stroke of Myosin.  The converter domain (green) and the lever-arm (yellow) are seen to rotate ~65 degrees relative to the main body of myosin (orange).  After the end of this movie, the actin fibril (not shown) comes from the right side and re-binds to the main body for the power stroke.  The movie frames are made from coordinate sets taken from the minimum energy path (MEP) computed by Conjugate Peak Refinement (CPR), as described in the paper.  Here, only the protein backbone (colored as in Figure 1B&C) and the bound ATP (in van der Waals) are shown.  File size 1Mb.
  Download the movie  (Low-res. version, 1Mb, as in PNAS suppl. materials)
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Movie 2.
Same as movie 1, but showing all the atoms used in the calculation.
  Download the movie  (Large file,  12Mb, as in PNAS suppl. materials)

Movie 3.  Hinging of the converter domain (see Fig.2-left of paper).
Motions described in Figure 2.  As movie 1, but different view (down the SH1-helix, in purple) showing the converter domain (green) and the lever-arm (yellow) rotating ~65 degrees relative to the main body of myosin (orange), around an axis close to the axis of the SH1-helix.  Shows the hinging point of the converter domain on the SH1-helix, and the linkage between the converter domain and the C-terminus of the relay helix (in cyan).
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  Download the movie  (High-res. version, 5Mb)

Movie 4.  Seesaw and local unwinding of the relay helix (see Fig.2-right of paper).
Motions described in Figure 2 (same view and coloring as in movie 1).  Shows how the closing of the Switch2 loop (orange) over the bound ATP (van der Waals) is coupled to a large translation of the C-terminus of the relay helix (cyan), which accompanies the rotation of the converter domain (of which a small piece is shown in green).  To accomodate this rotation, the C-terminal third of the relay helix undergoes an unwinding by 1/8th turn, breaking the helical H-bond at residue 486. As a result of the unwinding, the aromatic ring of Phe487 (orange) reorients and must thread between the relay helix and the relay loop (in white).
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  Download the movie  (High-res. version, 8.5Mb)

 The seesaw motion of the relay helix that is seen during the present CPR pathway is also present (with a smaller amplitude) in the spontaneous fluctuations of that helix during equilibrium Molecular Dynamics (MD) simulations of the pre-recovery state (State II). This was shown by performing Principal Component Analysis of the relay helix motions, thus providing strong evidence in favor of the proposed mechanics of the recovery-stroke.

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