A specific domain in α-catenin mediates binding to β-catenin or plakoglobin

The E-cadherin-catenin adhesion complex has been the subject of many structural and functional studies because of its importance in development, normal tissue function and carcinogenesis. It is well established that the cytoplasmic domain of E-cadherin binds either β-catenin or plakoglobin, which both can assemble α-catenin into the complex. Recently we have identified an α-catenin binding site in β-catenin and plakoglobin and postulated, based on sequence analysis, that these protein-protein interactions are mediated by a hydrophobic interaction mechanism. Here we have now identified the reciprocal complementary binding site in α-catenin which mediates its interaction with β-catenin and plakoglobin. Using in vitro association assays with C-terminal truncations of α-catenin expressed as recombinant fusion proteins, we found that the Nterminal 146 amino acids are required for this interaction. We then identified a peptide of 27 amino acids within this sequence (amino acid positions 117-143) which is necessary and sufficient to bind β-catenin or plakoglobin. As shown by mutational analysis, hydrophobic amino acids within this binding site are important for the interaction. The results described here, together with our previous work, give strong support for the idea that these proteins associate by hydrophobic interactions of two α-helices.