Docking of atomic model of the IHM into the reconstruction. The 2-D crystal model of smooth muscle myosin (PDB ID 1I84; Wendt et al., 2001) was used in the fitting. Molecular dynamics flexible fitting was not used, due to the low resolution. (A–C) Front, back, and side views, respectively, docked with ribbon representation of the atomic model. (D–F) D–F correspond to A–C, but with space-filling model. The atomic model fits well into the reconstruction in all views, except for the narrow part of the lever arm in the blocked head (single α-helix, between the two light chains; red arrow) and part of the ELC in the free head (yellow arrow). The low density in the reconstruction at these points may be due to extension of the stain pool that accumulates in the hole at the center of the IHM (cf. Figs. 2 B and S1) into these low-density regions of protein, obscuring them. Unfilled densities mostly represent portions of the three segments of the tail, for which there is no atomic model. There is also some unfilled density in the blocked head (green arrow) that would be filled by repositioning the flexibly connected SH3 domain (not done with the rigid-body docking procedure; see Materials and methods). In a previous study, molecular dynamics flexible fitting of cryo-imaged tarantula thick filaments demonstrated this point (Yang et al., 2016). Note: C and F rotated in opposite direction from C and F in Fig. 3.