TnI-induced pivoting of tropomyosin. Transverse sections of C- and B-state thin filament structures shown in Fig. 1 viewed here over C-terminal TnI residues R192 (A–C) and R170 (E–G); these TnI residues localize in close proximity to tropomyosin E114 and E139. (A and E) C-state sections (B and F), B-state sections, (C and G), B- and C-state sections superposed (same color representation as in Fig. 1). (D and H) Corresponding dynamics of tropomyosin and TnI shown in superposed sections. (A–C) Sections highlight tropomyosin pivoting, and in B and C salt-bridge formation between tropomyosin E114 (red spheres) and TnI residue R192 (navy blue spheres). Note that in C, superposition of B- and C-state sections shows inner tropomyosin helices overlapping (short arrow), while TnI appears to have drawn the outer tropomyosin helix toward the B-state position (bold arrow). (E–G) Sections highlight tropomyosin pivoting and salt-bridge formation of tropomyosin E138 with actin residue K328 (blue on actin surface), and in F and G, a salt bridge between tropomyosin E139 with TnI residue R170 (navy blue spheres). Note that in G, the superposition of B- and C-state sections shows inner tropomyosin helices are closely apposed (short arrow), while TnI appears to attract tropomyosin E139 of the inner tropomyosin helix (oblique bold arrow) here, causing the outer helix to pivot toward the B-state position (horizontal bold arrow). (D and H) Representation of tropomyosin and TnI dynamics on actin observed during MD simulations viewed in sections in D corresponding to ones in A and B and in H to E and F. The data are from the last 12 ns of simulations and represent 109 frames of B-state and 77 frames of C-state structures superposed. (D) Representative positions of the charged side-chain atoms of tropomyosin residue E114 (magenta and gold points for low- and high-Ca²⁺ data, respectively) and TnI residue R192 (navy blue points) taken during MD simulations of B- and C-state thin filament models (magenta and gold points show the positions of negatively charged Glu OE1 and OE2 atoms on E114, respectively, and navy blue points for positively charged Arg atoms NH1, NH2, and NE on R192). (H) Analogously rendered positions of negatively charged side-chain atoms on tropomyosin residue E139 and positively charged atoms on TnI residue R170 during MD simulations again performed on B- and C-state filaments (same color scheme as above); points assumed by positively charged NZ atoms actin residue K328 in H also shown for reference (light blue). The red and cyan spheres in D and H show the respective COO⁻ and NH₃⁺ side-chain positions of tropomyosin and TnI at the beginning of the MD simulations; the blue sphere in Fig. 2 H shows the initial location of the terminal K328 NH₃⁺ at the onset of MD. The curved arrows indicate the direction of tropomyosin movement toward the actin outer domain (myosin steric-blocking position) associated with tropomyosin–TnI complex formation as in C and G.