Cryo-EM reconstructions of S1-decorated actin-tropomyosin filaments. (A–C) Isosurface rendering of the reconstructions containing wild-type S1 (A; blue) and 7G-mutant S1 (B; gold); in C, A and B are superposed, showing tropomyosin positions are different in the two reconstructions. The pointed ends of the filaments in the reconstructions are facing up, with six S1 and actin subunits and corresponding tropomyosin coiled coils shown. In A, tropomyosin is marked by a horizontal open arrow, the bottom-right S1 and actin subunits are indicated by oblique and vertical open arrows, respectively. Myosin loop-4 is most easily noted extending from the centrally located S1 density toward tropomyosin (white arrows, also see Fig. 3), while in A projecting densities representing side chains of myosin residues 369 and 368 at the loop-4 tip are apparent. Loop-4 in the glycine-substituted 7G-S1 (B), lacking these side chains, has a smooth contour. (D and E) Atomic models of F-actin, S1, and tropomyosin filaments were fitted to the front-facing filament strand of reconstructions shown in A and B; here, the reconstruction isosurface was made transparent in order to examine the atomic models; note the excellent model to map correspondences. (F) Overlay of D and E, again showing the distinctive positions for tropomyosin in the two reconstructions. Myosin loop-4 is indicated by an arrow, as above. (G and H) Tropomyosin extracted from the overlaying maps in C and F to highlight the different positions in the wild-type and mutant thin filaments. Scale bar, 5.0 nm. (I) Transverse section of superposed tropomyosin chains in H magnified for clarity and again showing the azimuthal displacement between the two reconstructions.