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Structure of the acrosomal bundle

Nature volume 431, pages 104–107 (2004)Cite this article

Abstract

In the unactivated Limulus sperm, a 60-µm-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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Figure 1: Acrosomal bundle reconstruction.
Figure 2: Averaged actin.
Figure 3: Actin filament distortion.
Figure 4: Averaged scruin.

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Acknowledgements

This research is supported by the NCRR and NIGMS of NIH. We thank M. Baker for assistance in the helixhunter and foldhunter searches, and M. Dougherty for advice on graphical display.

Author information

Author notes

  1. Michael B. Sherman

    Present address: Department of Biological Sciences, Purdue University, West Lafayette, Indiana, 47907-139, USA

Authors and Affiliations

  1. National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas, 77030, USA

    Michael F. Schmid, Michael B. Sherman & Wah Chiu

  2. Department of Biology and Division of Biological Engineering MIT, Whitehead Institute, Cambridge, Massachusetts, 02142, USA

    Paul Matsudaira

Authors
  1. Michael F. Schmid
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  2. Michael B. Sherman
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Corresponding author

Correspondence to Wah Chiu.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary figure 1

Bundle Packing (DOC 441 kb)

Supplementary figure 2

Actin coordinates expressed as density (DOC 426 kb)

Supplementary movie

Transformation between the F-actin structure and the actin found in the bundle (MP4 2455 kb)

Figure legends (DOC 19 kb)

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Schmid, M., Sherman, M., Matsudaira, P. et al. Structure of the acrosomal bundle. Nature 431, 104–107 (2004). https://doi.org/10.1038/nature02881

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