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

Limulus sperm contains a dynamic macromolecular structure that rapidly extends a 50 microm 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 re...

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Bibliographic Details
Published in:Journal of molecular biology 1999-11, Vol.294 (1), p.139-149
Main Authors: Sherman, M B, Jakana, J, Sun, S, Matsudaira, P, Chiu, W, Schmid, M F
Format: Article
Language:English
Subjects:
Acrosome - ultrastructure
Actins - ultrastructure
Animals
Calmodulin - ultrastructure
Cryoelectron Microscopy
Crystallography
Horseshoe Crabs - physiology
Image Processing, Computer-Assisted
Limulus
Male
Marine
Models, Molecular
Models, Structural
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Summary:Limulus sperm contains a dynamic macromolecular structure that rapidly extends a 50 microm 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.
ISSN:0022-2836
DOI:10.1006/jmbi.1999.3222