β-Catenin associates with the actin-bundling protein fascin in a noncadherin complex

Abstract

Catenins were first characterized as linking the cytoplasmic domains of cadherin cell-cell adhesion molecules to the cortical actin cytoskeleton. In addition to their essential role in modulating cadherin adhesivety, catenins have more recently been indicated to participate in cell and developmental signaling pathways. β-Catenin, for example, associates directly with at least two receptor tyrosine kinases and transduces developmental signals within the Wnt pathway. Catenins also complex with the tumor suppresser protein adenomatous polyposis coli (APC), which appears to have a role in regulating cell proliferation. We have used the yeast two-hybrid method to reveal that fascin, a bundler of actin filaments, binds to β-catenin's central Armadillo repeat domain. Western blotting of immunoprecipitates from cell line and mouse and rat brain extracts indicate that this interaction exists in vive. Fascin and β-catenin's association was further substantiated in vitro using purified proteins isolated from recombinant bacterial and baculoviral sources. Immunoprecipitation analysis indicates that fascin additionally binds to plakoglobin, which is highly homologous to β-catenin but not to p120(cas), a newly described catenin which contains a more divergent Armadillo-repeat domain. Immunoprecipitation, in vitrocompetition, and domain-mapping experiments demonstrate that fascin and E-cadherin utilize a similar binding site within β-catenin, such that they form mutually exclusive complexes with β-catenin. Immunofluorescence microscopy reveals that fascin and β-catenin colocalize at cell-cell borders and dynamic cell leading edges of epithelial and endothelial cells. In addition to cell-cell borders, cadherins were unexpectedly observed to colocalize with fascin and β-catenin at cell leading edges. It is conceivable that β-catenin participates in modulating cytoskeletal dynamics in association with the microfilament-bundling protein fascin, perhaps in a coordinate manner with its functions in cadherin and APC complexes.