Figure 2.
MD-based steering of myosin from its pre-powerstroke to its target post-powerstroke configuration triggers tropomyosin C-state to M-state movement. The figure illustrates the initial and final structures of targeted MD simulations in which myosin S1 was steered over our C-state thin filament model (PDB accession no. 7UTI) to the M-state position found on S1-decorated actin (PDB accession no. 8EFH). Here, the effect of the myosin transition on tropomyosin pseudorepeat 3 was monitored. (A and B) Actin is illustrated in surface view with its pointed end facing up; Loop-4 of myosin is painted magenta, and the myosin HLH motif and CM loop are maroon. Side chains of charged residues on the tip of Loop-4 (Lys 367, blue, Arg 369, blue, Glu 370, red) are shown (best seen by magnifying the high-resolution graphics). Basic and acidic side chains of adjacent tropomyosin residues are painted light blue and red; their sequence numbers are noted in the margins (all side chains are displayed in ball and stick format). To recapitulate the effect of the pre- to post-powerstroke transition, our model of pre-powerstroke S1 (cyan) was first docked onto the central actin–tropomyosin segment as depicted in A (actin, pale blue/gray; tropomyosin, yellow). S1 was then steered toward its post-powerstroke position on actin (PDB accession no. 8EFH) resulting in the reconfiguration of the myosin head shown in B (navy blue), while tropomyosin (green) was free to reposition. (C and D) Orthogonal images of A and B tipped downward in the plane of view, again illustrating the side chain of positively charged Loop-4 Arg 369 (blue spheres) relative to the positively charged residues on tropomyosin pseudorepeat 3 (light blue spheres), thus likely to cause charge repulsion during the powerstroke transition. In C, the pre-powerstroke model highlights tropomyosin residues Lys 112, and Arg 105, 101, 91, and 90; in D, the post-powerstroke model, only Arg 90, and 91 are shown. The figures indicate that the powerstroke transition involves the charged tip of Loop-4 sweeping across equivalent charges on tropomyosin while the tropomyosin coiled-coil freely moves to its M-state position. The transition of pre-powerstroke myosin head binding to actin-tropomyosin and subsequent Loop-4 based myosin-induced movement of tropomyosin can be best seen in Video 1. Note that in B and D and in Video 1, post-powerstroke Loop-4 Arg 369 projects toward residues Arg 90 and Arg 91 on tropomyosin. Also note that by default the CM loop moves to its actin-binding sites during steered MD, while the HLH motif is largely unchanged.

MD-based steering of myosin from its pre-powerstroke to its target post-powerstroke configuration triggers tropomyosin C-state to M-state movement. The figure illustrates the initial and final structures of targeted MD simulations in which myosin S1 was steered over our C-state thin filament model (PDB accession no. 7UTI) to the M-state position found on S1-decorated actin (PDB accession no. 8EFH). Here, the effect of the myosin transition on tropomyosin pseudorepeat 3 was monitored. (A and B) Actin is illustrated in surface view with its pointed end facing up; Loop-4 of myosin is painted magenta, and the myosin HLH motif and CM loop are maroon. Side chains of charged residues on the tip of Loop-4 (Lys 367, blue, Arg 369, blue, Glu 370, red) are shown (best seen by magnifying the high-resolution graphics). Basic and acidic side chains of adjacent tropomyosin residues are painted light blue and red; their sequence numbers are noted in the margins (all side chains are displayed in ball and stick format). To recapitulate the effect of the pre- to post-powerstroke transition, our model of pre-powerstroke S1 (cyan) was first docked onto the central actin–tropomyosin segment as depicted in A (actin, pale blue/gray; tropomyosin, yellow). S1 was then steered toward its post-powerstroke position on actin (PDB accession no. 8EFH) resulting in the reconfiguration of the myosin head shown in B (navy blue), while tropomyosin (green) was free to reposition. (C and D) Orthogonal images of A and B tipped downward in the plane of view, again illustrating the side chain of positively charged Loop-4 Arg 369 (blue spheres) relative to the positively charged residues on tropomyosin pseudorepeat 3 (light blue spheres), thus likely to cause charge repulsion during the powerstroke transition. In C, the pre-powerstroke model highlights tropomyosin residues Lys 112, and Arg 105, 101, 91, and 90; in D, the post-powerstroke model, only Arg 90, and 91 are shown. The figures indicate that the powerstroke transition involves the charged tip of Loop-4 sweeping across equivalent charges on tropomyosin while the tropomyosin coiled-coil freely moves to its M-state position. The transition of pre-powerstroke myosin head binding to actin-tropomyosin and subsequent Loop-4 based myosin-induced movement of tropomyosin can be best seen in Video 1. Note that in B and D and in Video 1, post-powerstroke Loop-4 Arg 369 projects toward residues Arg 90 and Arg 91 on tropomyosin. Also note that by default the CM loop moves to its actin-binding sites during steered MD, while the HLH motif is largely unchanged.

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