Myosin loop-4–tropomyosin interaction. (A–F) Loop-4 structure in reconstructions of S1-decorated actin-tropomyosin filaments containing wild-type S1 (A–C) and 7G-mutant S1 (D–F). (A and D) Here, the reconstructions in Fig. 2 were tilted orthogonally with the filament pointed ends facing away from the viewer and transverse sections of the reconstructions then made at the level of loop-4. (B and E) Atomic models of F-actin, S1, and tropomyosin filaments fitted within the corresponding EM volumes; with C showing A and B superposed, and F the same for D and E. The vertical open arrows mark loop-4 and in A highlights projecting densities likely to represent side chains of wild-type myosin residues Arg 369 and Gln 368 approaching the tropomyosin coiled coil (right-angle arrow). The loop is discontinuous in the 7G-S1 (D) likely reflecting disorder in the region of the glycine substitution. Horizontal open arrow points to the neighboring actin surface. Scale bar, 5.0 Å.