The structural coupling between ATPase activation and recovery-stroke in the Myosin II motor.

S. Koppole, J.C. Smith,  S. Fischer*

Structure, vol. 15, p.825-837 (2007)


Muscle contraction is driven by a cycle of conformational changes in the myosin II head. Before the Myosin motor head can perform the next power-stroke, it undergoes a large conformational transition in which the converter domain (which bears the lever arm) rotates by ~65 degrees. Simultaneously to this "recovery stroke", myosin activates its ATPase function by closing the Switch-2 loop over the bound ATP. This coupling between the motion of the converter domain and the motion of the 40Å distant Switch-2 loop is essential for the motor cycle since hydrolysis of the ATP is required for subsequent actin rebinding. The coupling mechanism is determined by finding a series of optimized intermediates between the crystallographic end-structures of the recovery stroke, yielding movies of the transition at atomic detail. This reveals a two-phase mechanism, in which the successive formation of  two hydrogen bonds by the Switch-2 loop  is correlated with the successive see-saw motions of the two helices that hold the converter domain: the relay helix and the SH1-helix. The converter domain responds to the relay see-saw by rotating 25 degrees, then to the SH1 see-saw by rotating a further 40 degrees.
Structures along the recovery-stroke pathway:
A) Pre-recovery state. The Switch-2 loop is fully open, the relay helix is straight. Arrows show the motions of Phase I.
B) Half-way along the transition pathway (converter domain rotated 25 degrees). The Switch-2 loop is half-closed (H-bond between Gly457 and ATP). The tilt of the relay helix relative to its orientation in (A) is indicated by the straight dotted line. Arrows show the motions of Phase II.
C) Post-recovery state (converted domain rotated 65 degrees). The Switch-2 loop is fully closed. The Wedge-loop has moved up against the SH1-helix. The relay helix is "kinked" (between the brown and the cyan coloring).

The closing of Switch-2 forms two key H-bonds: First between Gly457 and ATP (A->B), and then between Phe458 and the P-loop (B->C). This closing turns on the ATPase activity of Myosin.
The coupling model has two phases:

Phase-I.  Formation of the H-bond between Gly457 and ATP (dotted arrow) pulls the relay helix and causes its seesaw pivoting (small full 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 (dotted arrow) pulls along the "Wedge-loop" (straight full arrow), 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 (curved full arrow). The converter domain, which is attached to the SH1-helix, reacts with a 40 degree rotation.
The stages of the recovery-stroke
The 2 phases of the coupling mechanism


"lambda" measures the progress along the transition pathway from pre-recovery (0%) to post-recovery (100%).
The color-coding in all the movies is the same as in the figures shown above:  Converter domain in green, lever arm in yellow, SH1-helix in purple, SH2-helix in pink, Relay helix in cyan (C-terminal third in brown), Switch-2 loop in green, P-loop in orange, Wedge loop in red, Phe652 in yellow.

Movie 1.  Pathway of the whole recovery stroke (lambda = 0% - 100%).
Shows all structural elements involved.
  Download the movie  (1.8Mb)

Movie 2.  Two-step closing of the Switch-2 loop (lambda = 0% - 100%).
Shows first the formation of the hydrogen bond between Gly457 and the gamma-phosphate, then the formation of the hydrogen bond between Phe458 and Ser181.
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Movie 3.  Seesaw of the relay helix in Phase I.
Shows the motion forward and back (i.e., lambda = 0% - 20% - 0%, to be viewed in auto-replay mode).
  Download the movie  (0.6Mb)

Movie 4.  Phase II of the transition (lambda = 20% - 100%).
Shows the formation of the Phe458/Ser181 hydrogen bond, the wedging of the Wedge loop against the SH1/SH2 corner and the seesaw/piston motion of the SH1-helix.
  Download the movie  (1Mb)

Movie 5.  Seesaw of the SH1 helix in Phase II.
Shows the motion forward and back (i.e., lambda = 65% - 100% - 65%, view in autoreplay mode).
  Download the movie  (0.8Mb)

Principal component analysis during equilibrium molecular dynamics simulations revealed that the relevant principal motion of the relay helix during MD of the pre-recovery state is indeed the seesaw pivoting predicted in Phase I, while the relevant principal motion of the SH1-helix found during MD of the post-recovery state is the piston/seesaw motion described here for Phase II of the coupling mechanism.

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