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Published online 28 May 2001. doi:10.1083/jcb.153.5.947
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© The Rockefeller University Press, 0021-9525/2001//947 $5.00
The Journal of Cell Biology, Volume 153, Number 5, , 2001 947-956

Original Article

An Atomic Model of Actin Filaments Cross-Linked by Fimbrin and Its Implications for Bundle Assembly and Function



Niels Volkmanna, David DeRosierb, Paul Matsudairac, and Dorit Haneina

a The Burnham Institute, La Jolla, California 92037
b The Rosenstiel Basic Medical Sciences Research Center and The W.M. Keck Institute for Cellular Visualization, Brandeis University, Waltham, Massachusetts 02254
c Whitehead Institute for Biomedical Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142
The Burnham Institute, 10901 North Torrey Pines Rd., La Jolla, CA 92037.(858) 646-3196(858) 646-3134

dorit{at}burnham.org

Actin bundles have profound effects on cellular shape, division, adhesion, motility, and signaling. Fimbrin belongs to a large family of actin-bundling proteins and is involved in the formation of tightly ordered cross-linked bundles in the brush border microvilli and in the stereocilia of inner ear hair cells. Polymorphism in these three-dimensional (3D) bundles has prevented the detailed structural characterization required for in-depth understanding of their morphogenesis and function. Here, we describe the structural characterization of two-dimensional arrays of actin cross-linked with human T-fimbrin. Structural information obtained by electron microscopy, x-ray crystallography, and homology modeling allowed us to build the first molecular model for the complete actin–fimbrin cross-link. The restriction of the arrays to two dimensions allowed us to deduce the spatial relationship between the components, the mode of fimbrin cross-linking, and the flexibility within the cross-link. The atomic model of the fimbrin cross-link, the cross-linking rules deduced from the arrays, and the hexagonal packing of actin bundles in situ were all combined to generate an atomic model for 3D actin–fimbrin bundles. Furthermore, the assembly of the actin–fimbrin arrays suggests coupling between actin polymerization, fimbrin binding, and crossbridge formation, presumably achieved by a feedback between conformational changes and changes in affinity.

Key Words: calponin homology domain • electron microscopy • image analysis • homology modeling • two-dimensional arrays

© 2001 The Rockefeller University Press

Abbreviations used in this paper: 2D, two-dimensional; 3D, three-dimensional; ABD, actin-binding domain; ABD1, NH2-terminal ABD of fimbrin; ABD2, COOH-terminal ABD of fimbrin; CH, calponin homology.


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