Actin-based motility in cells is usually associated with either polymerization/depolymerization in the presence
of cross-linkers or contractility in the presence of myosin motors. Here, we focus on a third distinct mechanism involving actin
in motility, seen in the dynamics of an active actin spring that powers the acrosomal reaction of the horseshoe crab (Limulus
polyphemus) sperm. During this process, a 60-mm bent and twisted bundle of cross-linked actin uncoils and becomes straight
in a few seconds in the presence of Ca2þ. This straightening, which occurs at a constant velocity, allows the acrosome to forcefully penetrate the egg. Synthesizing ultrastructural information with the kinetics, energetics, and imaging of calcium binding
allows us to construct a dynamical theory for this mechanochemical engine consistent with our experimental observations. It
also illuminates the general mechanism by which energy may be stored in conformational changes and released cooperatively
in ordered macromolecular assemblies