Structure of the acrosomal bundle

Nature. 2004 Sep 2;431(7004):104-7. doi: 10.1038/nature02881.


In the unactivated Limulus sperm, a 60- micro m-long bundle of actin filaments crosslinked by the protein scruin is bent and twisted into a coil around the base of the nucleus. At fertilization, the bundle uncoils and fully extends in five seconds to support a finger of membrane known as the acrosomal process. This biological spring is powered by stored elastic energy and does not require the action of motor proteins or actin polymerization. In a 9.5-A electron cryomicroscopic structure of the extended bundle, we show that twist, tilt and rotation of actin-scruin subunits deviate widely from a 'standard' F-actin filament. This variability in structural organization allows filaments to pack into a highly ordered and rigid bundle in the extended state and suggests a mechanism for storing and releasing energy between coiled and extended states without disassembly.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Acrosome / chemistry*
  • Actins / chemistry*
  • Animals
  • Cryoelectron Microscopy
  • Elasticity
  • Horseshoe Crabs / cytology*
  • Male
  • Models, Molecular
  • Protein Structure, Quaternary


  • Actins
  • scruin protein, Horseshoe Crab