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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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.
The authors declare that they have no competing financial interests.
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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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