Figure 1.
Myosin-induced movement of tropomyosin along actin filaments. (A–C) Top row: Cryo-EM-based models, taken from PDB accession nos. 7UTI and 8EFH, showing tropomyosin lying on the surface of actin subunits (pale blue, grey) and indicating the C-state to M-state tropomyosin translocation driven by myosin heads. Models of (A) C-state filaments, (B) M-state filaments, and (C) C- and M-state superimposed (C-state tropomyosin, yellow helices; M-state tropomyosin, green helices; myosin S1, navy blue ribbon); pointed ends of actin subunits are facing up. For simplicity, the models display only three neighboring actin subunits and a corresponding tropomyosin segment (in this case pseudorepeat 3). Panel A highlights tropomyosin residues Glu 96 and Glu 100 (red spheres) interacting with actin residue 328 (blue). Panel B is focused on myosin Loop-4 residue Arg 369 (blue spheres) approaching tropomyosin residue 91 (light blue spheres). Panels B and C are oriented to best visualize myosin Loop-4 (magenta); the HLH motif (red), and the cardiomyopathy (CM) loop (red) of myosin are somewhat hidden but are readily visualized in Videos 1, 2, and 3. Loop-4 and thus Arg 369 (blue spheres) can be regarded as landmarks at the end of the U50K domain of the myosin head. Loop-4 and the CM loop at the backside of the U50K domain form points of contact with actin. The HLH motif of myosin extends from the L50K part of the myosin head; it contacts actin between neighboring subunits. (D–F) Bottom row: Transverse sections through structures shown in A–C, sectioned at the level of actin–tropomyosin and Loop-4–tropomyosin interactions displayed in longitudinal view in the top row. In both A and D, the C-state model shows tropomyosin residues Glu 96 and Glu 100 (red spheres) interacting with oppositely charged actin residue Lys 328 (blue spheres). In B and E, the M-state model highlights Arg 369 on Loop-4 near to Arg 91 on tropomyosin (light blue spheres). (C and F) Structures in A and B as well as in D and E are superimposed. In F, only the charged residues on actin and tropomyosin are shown to indicate Loop-4 displacing tropomyosin from its binding site on actin during myosin attachment to actin. This triggers tropomyosin movement to the M-state (arrows). Although the images shown highlight the actin–tropomyosin organization at the level of tropomyosin pseudorepeat 3, the tropomyosin repositioning noted characterizes likely C-state/M-state transitions over all pseudorepeats examined.

Myosin-induced movement of tropomyosin along actin filaments. (A–C) Top row: Cryo-EM-based models, taken from PDB accession nos. 7UTI and 8EFH, showing tropomyosin lying on the surface of actin subunits (pale blue, grey) and indicating the C-state to M-state tropomyosin translocation driven by myosin heads. Models of (A) C-state filaments, (B) M-state filaments, and (C) C- and M-state superimposed (C-state tropomyosin, yellow helices; M-state tropomyosin, green helices; myosin S1, navy blue ribbon); pointed ends of actin subunits are facing up. For simplicity, the models display only three neighboring actin subunits and a corresponding tropomyosin segment (in this case pseudorepeat 3). Panel A highlights tropomyosin residues Glu 96 and Glu 100 (red spheres) interacting with actin residue 328 (blue). Panel B is focused on myosin Loop-4 residue Arg 369 (blue spheres) approaching tropomyosin residue 91 (light blue spheres). Panels B and C are oriented to best visualize myosin Loop-4 (magenta); the HLH motif (red), and the cardiomyopathy (CM) loop (red) of myosin are somewhat hidden but are readily visualized in Videos 1, 2, and 3. Loop-4 and thus Arg 369 (blue spheres) can be regarded as landmarks at the end of the U50K domain of the myosin head. Loop-4 and the CM loop at the backside of the U50K domain form points of contact with actin. The HLH motif of myosin extends from the L50K part of the myosin head; it contacts actin between neighboring subunits. (D–F) Bottom row: Transverse sections through structures shown in A–C, sectioned at the level of actin–tropomyosin and Loop-4–tropomyosin interactions displayed in longitudinal view in the top row. In both A and D, the C-state model shows tropomyosin residues Glu 96 and Glu 100 (red spheres) interacting with oppositely charged actin residue Lys 328 (blue spheres). In B and E, the M-state model highlights Arg 369 on Loop-4 near to Arg 91 on tropomyosin (light blue spheres). (C and F) Structures in A and B as well as in D and E are superimposed. In F, only the charged residues on actin and tropomyosin are shown to indicate Loop-4 displacing tropomyosin from its binding site on actin during myosin attachment to actin. This triggers tropomyosin movement to the M-state (arrows). Although the images shown highlight the actin–tropomyosin organization at the level of tropomyosin pseudorepeat 3, the tropomyosin repositioning noted characterizes likely C-state/M-state transitions over all pseudorepeats examined.

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