An actin spring. (A) The actin bundle in a Limulus sperm is coiled (left) around the base of the nucleus and extends through a nuclear channel at activation (right) (17). The sperm head is 3 µm in diameter. (B) A portion of the coil (left) shows bends in the bundle and the superhelicity of the bundle. In contrast, the filaments in the extended bundle (right) are straight and unwound (53). (C) The coiled bundle stores energy in an overtwisted filament (left). When unlatched by calcium, the change in twist of the filament causes the bundle to extend.