During the 5 s of the acrosome reaction of Limulus polyphemus sperm, a 60-m-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 1013 J of stored mechanical
D strain energy, whereas chemical energy derived from
calcium binding is 1015 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.