Figure 7.
Local propagation of myosin-induced tropomyosin movement on actin during the pre- to post-powerstroke transition. The figure shows transverse sections of tropomyosin coiled coils at potential sites of interaction with actin residue K328 on successive actin subunits. The sections displayed were taken from the initial and final trajectories of steered MD simulations in which two pre-powerstroke myosin heads were first docked onto two separate actin subunits of a 14-actin thin filament model in the C-state (PDB accession no. 7UTI). The two docked S1 heads were then steered to their post-powerstroke state and the effect on tropomyosin position along the entire filament was monitored. Sections from the beginning and the end of the simulation are shown superimposed for comparison. For the work illustrated, S1 heads were docked to actin subunits associated with tropomyosin pseudorepeats 3 and 6 on the filament’s helical strands; thus, two intervening actin subunits and subunits at the end of the model were S1-free as noted. In the sections shown, pre-powerstroke localized tropomyosin helices are colored yellow and post-powerstroke helices green. Side chains of actin residue Lys 328, known to form actin–tropomyosin salt bridges, are painted dark blue (pre-powerstroke) and pale blue (post-powerstroke) while those of the closest acidic residue on tropomyosin are shown in red. Myosin head residues are not shown to focus on potential actin–tropomyosin electrostatic contacts. Measurement of the distances between Lys 328 side chains of each successive actin and the side chains of interacting acidic tropomyosin residues is provided for each pseudorepeats along the thin filament; these respective distances are displayed below each corresponding graphics panel. In each case, the movement of tropomyosin on actin induced by myosin at pseudorepeats 3 and 6 propagates and causes local shifting of tropomyosin on neighboring S1-free actin subunits. Most strikingly, the post-powerstroke myosin-induced tropomyosin twist following the simulation is the same as reported experimentally by cryo-EM (Doran et al., 2020, 2023a, 2023b). The effect of S1 on tropomyosin pseudorepeat 2 was not measured, since the actin-tropomyosin interface at the end of the model is not well-defined (Yamada et al., 2020) and distance measurements are not meaningful. The same process was repeated in which a single pre-powerstroke myosin head was first docked onto the actin subunits associated with pseudorepeat 3 and the S1 head was then steered to its post-powerstroke state. The outcome was virtually identical to the illustrated results for double-labeled S1, namely, the local effect of S1-induced tropomyosin movement is propagated on all neighboring S1-free actin subunits. Corresponding quantitation of pre- and post-powerstroke actin–tropomyosin distances is indicated.

Local propagation of myosin-induced tropomyosin movement on actin during the pre- to post-powerstroke transition. The figure shows transverse sections of tropomyosin coiled coils at potential sites of interaction with actin residue K328 on successive actin subunits. The sections displayed were taken from the initial and final trajectories of steered MD simulations in which two pre-powerstroke myosin heads were first docked onto two separate actin subunits of a 14-actin thin filament model in the C-state (PDB accession no. 7UTI). The two docked S1 heads were then steered to their post-powerstroke state and the effect on tropomyosin position along the entire filament was monitored. Sections from the beginning and the end of the simulation are shown superimposed for comparison. For the work illustrated, S1 heads were docked to actin subunits associated with tropomyosin pseudorepeats 3 and 6 on the filament’s helical strands; thus, two intervening actin subunits and subunits at the end of the model were S1-free as noted. In the sections shown, pre-powerstroke localized tropomyosin helices are colored yellow and post-powerstroke helices green. Side chains of actin residue Lys 328, known to form actin–tropomyosin salt bridges, are painted dark blue (pre-powerstroke) and pale blue (post-powerstroke) while those of the closest acidic residue on tropomyosin are shown in red. Myosin head residues are not shown to focus on potential actin–tropomyosin electrostatic contacts. Measurement of the distances between Lys 328 side chains of each successive actin and the side chains of interacting acidic tropomyosin residues is provided for each pseudorepeats along the thin filament; these respective distances are displayed below each corresponding graphics panel. In each case, the movement of tropomyosin on actin induced by myosin at pseudorepeats 3 and 6 propagates and causes local shifting of tropomyosin on neighboring S1-free actin subunits. Most strikingly, the post-powerstroke myosin-induced tropomyosin twist following the simulation is the same as reported experimentally by cryo-EM (Doran et al., 2020, 2023a, 2023b). The effect of S1 on tropomyosin pseudorepeat 2 was not measured, since the actin-tropomyosin interface at the end of the model is not well-defined (Yamada et al., 2020) and distance measurements are not meaningful. The same process was repeated in which a single pre-powerstroke myosin head was first docked onto the actin subunits associated with pseudorepeat 3 and the S1 head was then steered to its post-powerstroke state. The outcome was virtually identical to the illustrated results for double-labeled S1, namely, the local effect of S1-induced tropomyosin movement is propagated on all neighboring S1-free actin subunits. Corresponding quantitation of pre- and post-powerstroke actin–tropomyosin distances is indicated.

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