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

Abstract

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 Nterminal β-strands. Sequence alignments suggest that the strandswap 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 Ca 2+ . 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.T he organization of cells into tissues and organs depends on cadherin molecules that regulate such diverse processes as cell adhesion, synapse formation, and the development and function of sensory cells in the inner ear and retina (1-4). The defining feature of the cadherin superfamily is the extracellular cadherin (EC) domain, which occurs in varying repetitions in all cadherins. Based on sequence homology and domain structure, cadherins are divided into subfamilies including the classical cadherins, desmosomal cadherins, seven transmembrane cadherins, and protocadherins. Classical cadherins and desmosomal cadherins are the best-studied superfamily members and have well-documented roles in mediating cell adhesion and the formation of desmosomes, respectively (1-4). The function and adhesive properties of other cadherin superfamily members are less well studied.Crystallographic and biochemical studies have provided insights into the structure of cadherin extracellular domains and the mechanism that mediates adhesion between classical cadherins (5-13). The EC domain forms a protein module of the Ig-like fold consisting of seven β-strands that are arranged as two opposed β-sheets. The Nand C termini reside on opposite ends of the EC domain, facilitating the formation of tandem repeats. The connections between successive EC domains are rigidified by coordination of three Ca 2+ ions, which is mediated by amino acids that are conserved in all cadherins (5,9,14). Adhesion specificity resides in the EC1 domains. The crystallographic structures show that the adhesive binding interface is formed by "swapping" of the amino-terminal β-strands of opposite EC1 domains, whereby the strand of one monomer replaces the strand of the other. Critical for this interaction are the side chains of conserved Trp residues, which fit into hydrophobic pockets on the EC1 domain of the binding partner (5,8,10).Recent findings suggest that the formation of cadherin adhesion complexes is a multistep process, where cadherins ...

Large Protein Assemblies Formed by Multivalent Interactions between Cadherin23 and Harmonin Suggest a Stable Anchorage Structure at the Tip Link of Stereocilia

“…7). Each upper tip link is expected to have very limited amounts of cadherin23 homodimers, considering that the diameter of each tip link is about 8 -10 nm (4), and that the first and second cadherin repeats of cadherin23 are about 2.5 nm in width (6,38). We also assume here that cadherin23 in hair cell stereoclilia contains exon68 (18 -22).…”

“…The mechanoelectrical transduction channel is present at the tips of all but the tallest stereocilia (1), and gated by a filamentous ultrastructure, called tip link, to control stereocilia depolarization and electric signal propagation (2,3). Tip link connects the tip of one stereocilium to the side of the nearby taller stereocilium (4), and is formed by cadherin23 homodimer in trans association with protocadherin15 homodimer (5)(6)(7)(8). Both cadherin23 and protocadherin15 are usher syndrome I, single-transmembrane adhesion proteins with a long extracellular cadherin repeats followed by a short cytoplasmic tail.…”

Cadherins in Brain Morphogenesis and Wiring

“…The mechanism for the heterophilic interaction between Cdh23 and Pcdh15 is somewhat different from the strandswap binding of classical cadherins (84). The NH 2 terminus of Cdh23 contains polar amino acids that bind Ca 2ϩ , and this Ca 2ϩ binding is critical for the interaction of Cdh23 with Pcdh15.…”

Cadherins in the Auditory Sensory Organ

“…Molecular dynamics simulations and binding experiments have indicated that the bond formed by protocadherin-15 and cadherin-23 is mechanically strong enough to withstand the forces applied to the tip-link during hair bundle deflection (Sotomayor et al 2012). The three-dimensional (3D) structures of the cadherin-23 and protocadherin-15 EC1 domains are similar to those of other cadherins (three Ca 2þ -binding sites (1, 2, and 3) at the linker between EC1 and EC2), but with several unusual features, such as an elongated N-terminus stabilised at the tip by Ca 2þ -binding site 0 (Elledge et al 2010;Sotomayor et al 2010 , about 20-40 μM, in the endolymph (Bosher and Warren 1978).…”