Orientation of the IHM in relaxed thick filaments. Coloring scheme the same as Figure 1F. (A,C) Lethocerus thick filament with PDB 1I84 fit into the map [6]. Note how the free head is placed tangentially against the filament backbone and the blocked head extends out into solvent space. (B,D) Tarantula thick filament with PDB 3JBH [17]. The IHMs can be approximated as flat planes, indicated by the black disks. Arrows in (C) indicate the position of the proximal S2 which is bent towards the left. The IHM of the Lethocerus flight muscle is approximately perpendicular to the filament axis; whereas the tarantula IHM is approximately tangential to the backbone surface. The angle between the plane of the Lethocerus IHM and the plane of the tarantula IHM is 91.6°. The axes in (A,B) indicate the initial M-ward cross-section view and the two 90° rotations, indicated by the red arrows, needed to transform to the longitudinal view shown in (C,D). Scale bars = 10 nm. Adapted from [6].