Letters to Nature

Nature 431, 104-107 (2 September 2004) | doi:10.1038/nature02881; Received 13 May 2004; Accepted 20 July 2004

Structure of the acrosomal bundle

Michael F. Schmid1, Michael B. Sherman1,3, Paul Matsudaira2 & Wah Chiu1

  1. National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
  2. Whitehead Institute, Department of Biology and Division of Biological Engineering MIT, Cambridge, Massachusetts 02142, USA
  3. Present address: Purdue University, Department of Biological Sciences, West Lafayette, Indiana 47907-139, USA

Correspondence to: Wah Chiu1 Email: wah@bcm.tmc.edu
The accession number is EMD-1088.

In the unactivated Limulus sperm, a 60-microm-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 polymerization1. In a 9.5-Å 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.

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