Biochemical and structural definition of the 1-afadin- and actin-binding sites of α-catenin

Abstract

α-Catenin is an integral component of adherens junctions, where it links cadherins to the actin cytoskeleton. α-Catenin is also required for the colocalization of the nectin/afadin/ponsin adhesion system to adherens junctions, and it specifically associates with the nectin-binding protein afadin. A proteolytic fragment of α-catenin, residues 385-651, contains the afadin-binding site. The three-dimensional structure of this fragment comprises two side-by-side four-helix bundles, both of which are required for afadin binding. The α-catenin fragment 385-651 binds afadin more strongly than the full-length protein, suggesting that the full-length protein harbors a cryptic binding site for afadin. Comparison of the α-catenin 385-651 structure with the recently solved structure of the α-catenin M-fragment (Yang, J., Dokurno, P., Tonks, N. K., and Barford, D. (2001) EMBO J. 20, 3645-3656) reveals a surprising flexibility in the orientation of the two four-helix bundles. α-Catenin and the actin-binding protein vinculin share sequence and most likely structural similarity within their actin-binding domains. Despite this homology, actin binding requires additional sequences adjacent to this region.