Crystal structure of the M-fragment of α-catenin: implications for modulation of cell adhesion

The cytoskeletal protein α-catenin, which shares structural similarity with vinculin, is required for cadherin-mediated cell adhesion, and functions to modulate cell adhesive strength and to link the cadherins to the actin-based cytoskeleton. Here we describe the crystal structure of a region of α-catenin (residues 377–633) termed the M-fragment. The M-fragment is composed of a tandem repeat of two antiparallel four-helix bundles of virtually identical architectures that are related in structure to the dimerization domain of α-catenin and the tail region of vinculin. These results suggest that α-catenin is composed of repeating antiparallel helical domains. The region of α-catenin previously defined as an adhesion modulation domain corresponds to the C-terminal four-helix bundle of the M-fragment, and in the crystal lattice these domains exist as dimers. Evidence for dimerization of the M-fragment of α-catenin in solution was detected by chemical cross-linking experiments. The tendency of the adhesion modulation domain to form dimers may explain its biological activity of promoting cell–cell adhesiveness by inducing lateral dimerization of the associated cadherin molecule.