A coil of actin (red) springs open in front of the Limulus sperm nucleus (blue).

Although most cell biologists think of molecular motors as chemically driven machines, some of the fastest and most dramatic movements in nature may actually be powered by stored mechanical energy. On page 1183, Shin et al. present a detailed characterization of the forces driving acrosome extension in the sperm of the horseshoe crab Limulus polyphemus, and show that this process relies on mechanical energy stored in a molecular spring. Springs also underlie other phenomena such as bacteriophage infection.

To penetrate the jelly coat of an egg, Limulus sperm extends a bundle of actin filaments from a coiled position in the sperm head into a sturdy 60-mM-long rod called the acrosomal process. The reaction takes only five seconds. The authors calculated the amount of mechanical energy theoretically required to drive the movement from the energy stored in the structure and expended during extension. Neither ATP hydrolysis nor calcium binding provides enough energy during the reaction, but the potential mechanical energy in the coiled actin bundle is more than sufficient to drive acrosome extension. The data suggest that calcium binding triggers, but does not power, a progressive mechanical uncoiling reaction, extending the acrosomal process like a Jack-in-the-box toy. ▪