Proposed inhibition mechanism of phosphate release and actin binding in the two heads. (A) Phosphate release in the blocked head is inhibited by interaction of its converter domain (cyan within red ellipse) with segment 2, and in the free head by interaction of its converter domain (cyan within blue ellipse) with the blocked head. (B) Back view, rotated 180° from A. (C and D) Front and back views show actin-binding loop (red ellipse) in the blocked head sterically blocked by the free head, and actin-binding loop (green ellipse) in the free head blocked by interacting with the tail. (E and F) Front and back views showing steric blocking of Ser19 (red sphere) by segment 3 in the blocked-head RLC, but not in the free-head RLC (magenta sphere; see Video 4).