Stored elastic energy powers the 60-μm extension of the Limulus polyphemus sperm actin bundle

During the 5 s of the acrosome reaction of Limulus polyphemus sperm, a 60-μm-long bundle of scruin-decorated actin filaments straightens from a coiled conformation and extends from the cell. To identify the motive force for this movement, we examined the possible sources of chemical and mechanical energy and show that the coil releases ∼10⁻¹³ J of stored mechanical strain energy, whereas chemical energy derived from calcium binding is ∼10⁻¹⁵ J. These measurements indicate that the coiled actin bundle extends by a spring-based mechanism, which is distinctly different from the better known polymerization or myosin-driven processes, and that calcium initiates but does not power the reaction.