Thinking outside the cell: how cadherins drive adhesion

Abstract

Cadherins are a superfamily of cell surface glycoproteins whose ectodomains contain multiple repeats of β-sandwich extracellular cadherin (EC) domains that adopt a similar fold to immunoglobulin domains. The best characterized cadherins are the vertebrate 'classical' cadherins, which mediate adhesion via trans homodimerization between their membrane-distal EC1 domains that extend from apposed cells, and assemble intercellular adherens junctions through cis clustering. To form mature trans adhesive dimers, cadherin domains from apposed cells dimerize in a 'strand-swapped' conformation. This occurs in a two-step binding process involving a fast-binding intermediate called the 'X-dimer'. Trans dimers are less flexible than cadherin monomers, a factor that drives junction assembly following cell-cell contact by reducing the entropic cost associated with the formation of lateral cis oligomers. Cadherins outside the classical subfamily appear to have evolved distinct adhesive mechanisms that are only now beginning to be understood.

Figure 1

Overall architecture of classical cadherins. The extracellular domain of C-cadherin (pdb-ID: 1L3W) is depicted as a ribbon diagram (orange). Ca²⁺ ions (green spheres) are coordinated between consecutive domains, stabilizing an overall curved shape of the ectodomain, with an angle of close to 90° between domains EC1 and EC5. The structure of the stalk-region, transmembrane domain and parts of the intracellular domain are unknown and are shown as dotted lines. The cytoplasmic domain of cadherins binds to intracellular binding partners p120 (green barrels representing α-helices; pdb-ID: 3L6X) in the juxta-membrane region and β-catenin (blue barrels representing α-helices; pdb-ID: 1I7X) in the C-terminal region. β-catenin interacts with α-catenin, which in turn binds to actin filaments linking cadherins to the cytoskeleton. The depicted orientation, position and size of the intracellular binding partners relative to each other and to C-cadherin is purely schematic; the overall structural arrangement of the cytoplasmic side of adherens junctions is unknown.

Figure 2

Classical cadherins from adhesive dimers by exchange of the N-terminal β-strand. (a) A classical cadherin trans dimer is shown as ribbon diagram in two orthogonal orientations; one protomer is shown in blue, one in orange (from pdb-ID: 3Q2W). Membrane distal EC1 domains overlap and exchange N-terminal β-strands (expanded view). Note that substantial O- and N-linked glycosylation (magenta and green spheres, respectively) is found on extracellular domains EC2-4, but not on adhesive EC1 domains. Ca²⁺ ions are shown as green spheres. (b) The adhesive mechanism of classical cadherins is an example of 3D domain swapping, EC1 domains are shown for monomer and dimer (ribbon representation). The swapping element, residue Trp2 (side chain depicted as spheres) has an identical residue environment in the monomer (left panel) and ‘strand swapped’ dimer (right panel) [Adapted from [8]]. (c) Ribbon presentations of strand swapped EC1 domains of type I E-cadherin (pdb-ID: 2QVF), type II cadherin-11 (pdb-ID: 2A4E) and VE-cadherin (pdb-ID: 3PPE). Residues characteristic of adhesive interfaces of each subfamily are depicted as sticks. In type I cadherins, residue Trp2 in domain EC1 is swapped between binding partners. In type II cadherins, two Trp residues, Trp2 and Trp4, are exchanged, and, in addition, hydrophobic interactions occur between conserved residues Phe8, Ile10 and Tyr 13 giving rise to an extended interface. VE-cadherin exchanges Trp2 and Trp4 like type II cadherins, but the interface is limited to the apex of the domain, as in type I cadherins.

Figure 3

Strand swapped adhesive dimers of classical cadherins form through a non-swapped intermediate. E-cadherin monomers [orange and blue ribbon diagrams (left panel); only EC1-2 shown for clarity] associate via an ‘X-dimer’ interface in which N-terminal strands are not swapped but are closely apposed (middle panel). Swapping of strands leads to formation of mature strand swapped dimers (right panel). Assembly and disassembly of swapped dimers is likely to proceed via the same pathway.

Figure 4

Extracellular structure of adherens junctions formed through cis and trans ectodomain interactions. (a) Selected region of the N-cadherin EC1-5 crystal lattice (blue ribbon presentation; pdb-ID: 3Q2W) showing an array of N-cadherin molecules oriented as if emanating from the same cell membrane and connected by a cis interface formed between EC1 and EC2 domains of neighboring molecules. (b) Strand swapped trans dimers form together with cis interactions in the same crystal lattice. Trans interactions orient opposing cis arrays approximately perpendicularly such that each cis array (blue) forms trans interactions with multiple opposing cis arrays (orange). (c) The combination of cis and trans interactions enables cadherin ectodomains to form an ordered network that is thought to be the basis for the extracellular architecture of adherens junction. Adapted from [17].

Figure 5

Crystal structures of cadherin-23 and Drosophila N-cadherin reveal unique features of atypical cadherins. (a) Structures of mouse cadherin-23 EC1-2, which are involved in adhesive binding to protocadherin-15 (binding domain indicated by brackets in schematic) reveal successive EC domains (ribbon diagram , pdb-ID: 3MVS, 2WHV) with three Ca²⁺ ions (green spheres) coordinated in the linker region. Uniquely, a Ca²⁺ binding site was identified at the apex of EC1 (box), referred to as Ca²⁺ binding site 0. Structural determination of a complex of cadherin-23 with protocadherin-15 will help to identify the heterophilic binding interface. (b) Structures of DN-cadherin EC1-4, which is part of the adhesive interface for homodimerization (EC1-9, brackets in schematic), reveal four consecutive EC domains (ribbon diagram). Interestingly, Ca²⁺ coordination was found only between domains EC1-2 and EC3-4 and not between EC2-3 (pink arrow). This Ca²⁺-free linker introduces a ‘kink’ in the otherwise linear structure. Sequence analysis suggests a second occurrence of a Ca²⁺-free linker between EC7-8 in the ectodomain of DN-cadherin; these may contribute to folding of the 16 EC domains into a compact form within the intermembrane space of Drosophila adherens junctions.

Figure I

Schematic representation of members of the cadherin family, which all share a common structural motif: ‘the extracellular cadherin (EC) domain’. (a) Typical domain fold of an EC domain shown in ribbon representation (top panel from pdb-ID: 1L3W). Seven anti parallel β-strands (A-G) assemble two β-sheets as shown in the topology diagram (lower panel). Note that the A-strand is split into two halves: the A*- and A-strand connected by a loop, referred to as ‘hinge’. Three Ca²⁺ ions (green spheres) are coordinated between consecutive EC domains. (b) Schematic representation of overall domain organization of selected cadherin family members. All cadherins have two or more EC-domains in their extracellular domains (blue ovals, numbered from membrane distal to membrane proximal domain), which can also contain non-EC domains, such as EGF-repeats (green rectangles), Laminin A G domains (cyan diamonds) and flamingo boxes (pink oval). Some cadherins have, in addition to the signal peptide, a prodomain (grey ovals) that is removed by a furin protease on the cell surface. Asterisk: first EC domain of Drosophila E (DE) and N (DN) cadherin is predicted from sequence analysis [75].