Alpha-catenin can form asymmetric homodimeric complexes and/or heterodimeric complexes with beta-catenin

The cadherin-based transmembrane cell-cell adhesive complex is thought to be composed of a cadherin molecule, a beta-catenin, and an alpha-catenin, which connects the complex to the cytoskeleton. The precise stoichiometry of this complex remains uncertain. We have used a series of recombinant molecules and biophysical techniques to assess the multimeric state of human alpha- and beta-catenin in vitro and then visualized them by electron microscopy after rotary shadowing. Calculated solution molecular masses are 213 kDa for alpha-catenin, 73 kDa for beta-catenin, and 186 kDa for both. This suggests that alpha-catenin exists as a homodimer in solution, beta-catenin is a monomer, and when both are present, they form alpha/beta-catenin heterodimers. Co-precipitation and surface plasmon resonance assays localize the site of alpha-catenin dimerization to the NH2-terminal 228 amino acids. This region encompasses a high-affinity (Kd = 100 nM) binding site for beta-catenin that lies between residues 54 and 157. We anticipate that the oligomeric state of alpha-catenin and the relative stoichiometry of the components in the membrane adhesion complex will be dynamic and regulated by beta-catenin, cell adhesion, and probably other factors as well.