Structure of the N terminus of cadherin 23 reveals a new adhesion mechanism for a subset of cadherin superfamily members

The cadherin superfamily encodes more than 100 receptors with diverse functions in tissue development and homeostasis. Classical cadherins mediate adhesion by binding interactions that depend on their N-terminal extracellular cadherin (EC) domains, which swap N-terminal beta-strands. Sequence alignments suggest that the strand-swap binding mode is not commonly used by functionally divergent cadherins. Here, we have determined the structure of the EC1-EC2 domains of cadherin 23 (CDH23), which binds to protocadherin 15 (PCDH15) to form tip links of mechanosensory hair cells. Unlike classical cadherins, the CDH23 N terminus contains polar amino acids that bind Ca2+. The N terminus of PCDH15 also contains polar amino acids. Mutations in polar amino acids within EC1 of CDH23 and PCDH15 abolish interaction between the two cadherins. PCDH21 and PCDH24 contain similarly charged N termini, suggesting that a subset of cadherins share a common interaction mechanism that differs from the strand-swap binding mode of classical cadherins.