Fitting atomic models of cardiac thin filaments to cryo-EM reconstructions. (A and B) Atomic models of the high-Ca²⁺ C-state (A; PDB ID 7UTI) and the low-Ca²⁺ B-state (B; PDB ID 7UTL) thin filaments. The pointed ends of actin are shown facing up. Actin, tropomyosin, and troponin are presented in ribbon format (actin subunits in A are in green and in B are blue; tropomyosin chains in A are in gold and in B are magenta; TnI, navy blue; TnC, orange; TnT, yellow). (C–F) The C-state atomic model in A was fitted to the high-Ca²⁺ cardiac thin filament reconstructions generated by Yamada et al. (2020) in C (EMDB ID 0729) and by (Risi et al, 2021a) in D (EMDB ID 22965), while B-state atomic model in B was fitted to reconstructions of low-Ca²⁺ filaments produced by Yamada et al. (2020) in E (EMDB ID 0728) and by (Risi et al, 2021a) in F (EMDB ID 22964). Please note the excellent fitting of the atomic models to corresponding reconstructions. Also note that the individual helices forming the tropomyosin strand in E are better defined than in F, which was consistent with a marginally higher cross-correlation of the atomic model in B to the reconstruction volume in E.