a, b, Network of ionic interactions clamping together the free-head motor, segment-1 and segment-3, shown in back view. a, Model fitted into density map with side chains of interacting residues shown (contour level 0.170). b, Surface view of model with acidic areas shaded red and basic areas shaded blue. Ring 1 and Ring 2 denote two rings of acidic character in segment-137. c-f, Comparison of blocked-head tail interactions between shutdown SmM and tarantula thick filament IHM (PDB id 3JBH); segment-2, blocked-head LCs and free head omitted for clarity. c, d Face view. e, f, Side view. g, Ionic interaction of loop O in the blocked-head motor with segment-3 (contour level 0.280); segment-2, blocked-head ELC and free-head omitted for clarity. h, Depiction of segment-1 crossing the free-head motor, with the motor shown in the same orientation as that of the blocked head in c-d.