Thin filament C- and B-state structures drawn from PDB 7UTI and 7UTL coordinates. (A) The C-state, high-Ca²⁺ filament. (B) The B-state, low-Ca²⁺ filament. (C) The high and low-Ca²⁺ filaments superposed. All panels display the solvent-excluded surface of actin subunits rendered to only show the front-facing strand of actin for clarity (actin subunits colored light blue, light grey). The pointed ends of actin are shown facing up. Troponin and tropomyosin are presented in ribbon format in A–C. Color scheme: TnI, navy blue; TnC, orange; TnT, yellow; tropomyosin in its high-Ca²⁺ position, gold; and at low-Ca²⁺, magenta. The troponin core domain, where TnI, TnC, and TnT converge, points toward the barbed end of the filament. Note: Left-facing arrows point to the navy blue TnI C-terminal domain extension on the outer actin domains in B; also note the apparent movement of the magenta low-Ca²⁺ tropomyosin strand toward TnI. (A–C) Salt bridges between Lys 328 on actin (blue spheres) and acidic residues on tropomyosin (red spheres) are highlighted in A and B by right-handed arrows; again note that a residue on inner tropomyosin α-helices contact K328 of each actin subunit shown (canonical salt-bridge contacts listed in Table 1; other actin–tropomyosin contacts are provided in Fig. S2). The TnI C-terminal domain, which is released from TnC at low-Ca²⁺, is seen to bind to tropomyosin by means of salt bridges (indicated by left-handed arrows in B as well as by red and navy blue spheres in B and C). Non-polar interactions of TnI with tropomyosin and actin are not highlighted here but are discussed in Lehman et al. (2021). (D) The two-component α-helical tracks of coiled-coil tropomyosin that are defined by the high- and low-Ca²⁺ tropomyosin trajectories in C are rendered as gold and magenta threads and superposed on actin. In C and D, sites marked by arrowed brackets show regions where one set of α-helical chains of the superposed high- and low-Ca²⁺ tropomyosin coiled coils coincide within a 2-Å cutoff, while the rest diverge. This is most easily seen in D by noting the overlaying threads. This positional commonality extends over contiguous tropomyosin residues 65–74, 103–109, 148–151, 188–192, 233–244, and 262–269, located in pseudorepeats 2–7; the tropomyosin head-to-tail junction is not depicted here, but is detailed in Pavadai et al. (2020a); Pavadai et al. (2020b); and Risi et al. (2022). (E) The superhelical tracts defined by high- and low-Ca²⁺ tropomyosin are rendered here as gold and magenta threads, respectively, and represent the average center of mass of low- and high-Ca²⁺ tropomyosin on actin (magenta and gold, respectively); note the distance between the magenta and gold threads, which are furthest apart over middle actin subunits shown, i.e., between tropomyosin pseudorepeats 3–5, and they are closest together between C-terminal tropomyosin residues 248 and 265 (indicated by broad arrowed brackets at the bottom of the panel). Actin residues 328 are highlighted as blue spheres in D and E for reference. (F–H) Magnified versions of B–D focused on regions in which the C-terminal domain of TnI interacts with actin and tropomyosin.