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-bundlin g 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 preve nted the detailed structural characterization required for indepth understa nding of their morphogenesis and function. Here, we describe the structural characterization of two-dimensional arrays of actin cross-linked with huma n T-fimbrin. Structural information obtained by electron microscopy, x-ray crystallography. and homology modeling allowed us to build the first molecu lar model for the complete actin-fimbrin cross-link. The restriction of the arrays to two dimensions allowed us to deduce the spatial relationship bet ween the components, the mode of fimbrin cross-linking, and the flexibility within the cross-link. The atomic model of the fimbrin cross-link, the cro ss-linking rules deduced from the arrays, and the hexagonal packing of acti n bundles in situ were all combined to generate an atomic model for 3D acti n-fimbrin bundles. Furthermore, the assembly of the actin-fimbrin arrays su ggests coupling between actin polymerization, fimbrin binding, and crossbri dge formation, presumably achieved by a feedback between conformational cha nges and changes in affinity.