An atomic model of actin filaments cross-linked by fimbrin and its implications for bundle assembly and function.
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.
Volkmann
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The Burnham Institute, 10901 North Torrey Pines Rd., La Jolla, California 92037, USA.
DeRosier
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D
Matsudaira
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P
Hanein
D
D
D008841:Actin Cytoskeleton / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure*; D000199:Actins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure*; D001704:Biopolymers / Q000737:chemistry / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D018360:Crystallography, X-Ray; D005583:Fourier Analysis; D006801:Humans; D008562:Membrane Glycoproteins / Q000378:metabolism* / Q000648:ultrastructure; D008840:Microfilament Proteins; D008854:Microscopy, Electron; D008954:Models, Biological; D008958:Models, Molecular; D018583:Pliability; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D017385:Sequence Homology
D000199:Actins; D001704:Biopolymers; D008562:Membrane Glycoproteins; D008840:Microfilament Proteins; C026742:plastin; D002118:Calcium