The three-dimensional structure of the Limulus acrosomal process: A dynamic actin bundle

Michael B. Sherman, Joanita Jakana, Shujun Sun, Paul Matsudaira, Wah Chiu, Michael F. Schmid

Research output: Contribution to journalArticlepeer-review

26 Scopus citations


Limulus sperm contains a dynamic macromolecular structure that rapidly extends a 50 μm process called the true discharge. The core of this structure is a bundle of ordered filaments composed of a complex of actin, scruin and calmodulin. We determined its structure by electron crystallographic reconstruction. The three-dimensional map reveals an actin-scruin helix that is azimuthally modulated by the influence of the interactions of a filament with its neighbors. There are a variety of density connections with neighboring filaments involving scruin. Scruin commonly contacts one neighbor, but we observe up to three interfilament connections involving both domains of the 28 scruin molecules in the unit cell. Our structure indicates that promiscuous scruin-scruin contacts are the major determinants of bundle stability in the true discharge. It also suggests that rearrangements would be permitted, which can facilitate the transition from the coiled to the true discharge form.

Original languageEnglish (US)
Pages (from-to)139-149
Number of pages11
JournalJournal of Molecular Biology
Issue number1
StatePublished - Nov 19 1999
Externally publishedYes


  • Acrosomal bundles
  • Actin
  • Electron cryomicroscopy
  • Electron crystallography
  • Scruin

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology


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