Crystal structures of classical cadherins have revealed two dimeric configurations: in the first, Nterminal β-strands of EC1 domains "swap" between partner molecules. The second configuration (the "X-dimer"), also observed for T-cadherin, is mediated by residues near the EC1-2 calcium binding sites, and N-terminal β-strands of partner EC1 domains, though held adjacent, do not swap. Here we show that strand swapping mutants of type I and II classical cadherins form X-dimers. Mutant cadherins impaired for X-dimer formation show no binding in short timeframe surface plasmon resonance assays but in long timeframe experiments, have homophilic binding affinities close to wild-type. Further experiments show that exchange between monomers and dimers is slowed in these mutants. These results reconcile apparently disparate results from prior structural studies, and suggest that X-dimers are binding intermediates that facilitate the formation of strand swapped dimers.

We have determined the crystal structure at 2.4 A resolution of a ternary complex between the spliceosomal U2B"/U2A' protein complex and hairpin-loop IV of U2 small nuclear RNA. Unlike its close homologue the U1A protein, U2B" binds to its cognate RNA only in the presence of U2A', which contains leucine-rich repeats in its sequence. The concave surface of a parallel beta-sheet within the leucine-rich-repeat region of U2A' interacts with the ribonucleoprotein domain of U2B" on the surface opposite its RNA-binding surface. The basic carboxy-terminal region of U2A' interacts with the RNA stem. The crystal structure reveals how protein-protein interaction regulates RNA-binding specificity, and how replacing only a few key residues allows the U2B" and U1A proteins to discriminate between their cognate RNA hairpins by forming alternative networks of interactions.

Type I and II classical cadherins help to determine the adhesive specificities of animal cells. Crystal-structure determination of ectodomain regions from three type II cadherins reveals adhesive dimers formed by exchange of N-terminal beta strands between partner extracellular cadherin-1 (EC1) domains. These interfaces have two conserved tryptophan side chains that anchor each swapped strand, compared with one in type I cadherins, and include large hydrophobic regions unique to type II interfaces. The EC1 domains of type I and type II cadherins appear to encode cell adhesive specificity in vitro. Moreover, perturbation of motor neuron segregation with chimeric cadherins depends on EC1 domain identity, suggesting that this region, which includes the structurally defined adhesive interface, encodes type II cadherin functional specificity in vivo.

During spinal cord development, motor neurons with common targets and afferent inputs cluster into discrete nuclei, termed motor pools. Motor pools can be delineated by transcription factor expression, but cell surface proteins that distinguish motor pools in a systematic manner have not been identified. We show that the developmentally regulated expression of type II cadherins defines specific motor pools. Expression of one type II cadherin, MN-cadherin, regulates the segregation of motor pools that are normally distinguished by expression of this protein. Type II cadherins are also expressed by proprioceptive sensory neurons, raising the possibility that cadherins regulate additional steps in the development of sensory-motor circuits.