Structure of the acrosomal bundle

Michael F. Schmid, Michael B. Sherman, Paul Matsudaira, Wah Chiu

Research output: Contribution to journalLetterpeer-review

72 Scopus citations


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 polymerization. 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.

Original languageEnglish (US)
Pages (from-to)104-107
Number of pages4
Issue number7004
StatePublished - Sep 2 2004
Externally publishedYes

ASJC Scopus subject areas

  • General


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