The Extracellular Architecture of Adherens Junctions Revealed by Crystal Structures of Type I Cadherins

Abstract

Summary Adherens junctions, which play a central role in intercellular adhesion, comprise clusters of type I classical cadherins that bind via extracellular domains extended from opposing cell surfaces. We show that a molecular layer seen in crystal structures of E- and N-cadherin ectodomains reported here and in the C-cadherin structure corresponds to the extracellular architecture of adherens junctions. In all three ectodomain crystals, cadherins dimerize through a trans adhesive interface and are connected by a second, cis, interface. Assemblies formed by E-cadherin ectodomains coated on liposomes also appear to adopt this structure. Fluorescent imaging of junctions formed from wild-type and mutant E-cadherins in cultured cells confirm conclusions derived from structural evidence. Mutations that interfere with the trans interface ablate adhesion, whereas cis interface mutations disrupt stable junction formation. Our observations are consistent with a model for junction assembly involving strong trans and weak cis interactions localized in the ectodomain.

Crystal structures of Drosophila N-cadherin ectodomain regions reveal a widely used class of Ca 2+ -free interdomain linkers

“…The structures of DN-cadherin ectodomain fragments we report here reveal a V-shaped architecture, in clear contrast to the elongated curved structure of vertebrate classical cadherins (8,9). The "V" shape of DN-cadherin EC1'-EC4' is imparted by a kink between domains EC2' and EC3' that are connected by a unique Ca 2þ -free linker, and is thematically similar to the U-shaped structure of Ig1-Ig4 fragments from several members of the immunoglobulin cell adhesion molecule (IgCAM) family, including hemolin, axonin, Dscam, and neurofascin (18,(29)(30)(31).…”

“…The overall architecture of DN-cadherin EC1'-EC4' fragment does not resemble any of the previously determined structures of vertebrate cadherin ectodomains, which adopt an elongated curved structure (8,9) (Fig. 1).…”

“…By contrast, DN-cadherin forms a V-shaped structure, as the Ca 2þ -bound EC1'-EC2' and EC3'-EC4' interdomain regions orient domains EC1' and EC4' such that they do not make any intramolecular interdomain contacts. In vertebrate classical cadherins and T-cadherin, calcium-binding interdomain linkers are critical structural elements that determine interdomain orientation and facilitate strand swapping and X-dimer formation (8,9,21). The unique Ca 2þ -free linker seen between the EC2' and EC3' domains in all three structures presented here provides new clues about the ectodomain architecture of DN-cadherin and other related molecules.…”

“…The best characterized cadherins are vertebrate classical cadherins, a family of proteins which share similar domain structures, each consisting of an ectodomain with five tandem EC domains, a single transmembrane region, and a conserved cytoplasmic tail (7). The connections between each set of successive EC domains are rigidified by the stereotyped binding of three Ca 2þ ions (8,9). Classical cadherins have been shown to function in intercellular adhesion by binding through their ectodomains, which are in turn linked to the actin cytoskeleton through associations of the cytoplasmic domains with catenin adaptor proteins (reviewed in ref.…”

“…To gain insight into the structural basis for DN-cadherin adhesive functions, we sought to obtain structures of DN-cadherin ectodomain regions. We initially focused on ectodomain fragments containing the four predicted N-terminal EC domains in the mature protein, partly because prior structural and functional studies on vertebrate classical-and T-cadherins showed that cell adhesion is mediated through the N-terminal EC domains (8,9,(21)(22)(23). We determined crystal structures of two DN-cadherin ectodomain fragments: one comprising domains EC1'-EC3' in two crystal forms (I and II), both to 2.5 Å resolution, and the other comprising domains EC1'-EC4' to 2.7 Å resolution.…”