Fimbrin is a Cytoskeletal Protein That Crosslinks F-Actin in vitro

Fimbrin is a cytoskeletal protein associated with microfilaments in microvilli, microspikes, stereocilia, membrane ruffles, and cell-substratum attachment sites. Fimbrin purified from intestinal epithelial cell brush borders was found to be a monomeric protein of molecular weight 68,000. In a sedimentation assay, fimbrin bound to F-actin in a salt-dependent manner, with binding being optimal in 30 mM KCl and inhibited in > 100 mM KCl. In 50 mM KCl, which allows efficient polymerization of actin, the interaction was stabilized by the presence of polyethylene glycol. Under these conditions, binding was unaffected by the inclusion of up to 5 mM Ca²⁺ but was inhibited by >0.5 mM Mg²⁺. Electron microscopy revealed that fimbrin crosslinked F-actin into relatively straight bundles with shorter bundles being formed at high fimbrin-to-actin ratios. The results suggest that fimbrin crosslinks F-actin in such a way as to confer some rigidity on the bundle formed. This proposed function for fimbrin is consistent with its in vivo localization in straight, highly organized, microfilament bundles such as microvilli, microspikes, and stereocilia.