Stable and unstable cadherin dimers: mechanisms of formation and roles in cell adhesion

Numerous attempts to elucidate the strength of cadherin dimerization that mediates intercellular adhesion have produced controversial and inconclusive results. To clarify this issue, we compared E-cadherin dimerization on the surface of living cells with how the same process unfolds on agarose beads. In both cases, dimerization was monitored by the same site-specific cross-linking assay, greatly simplifying data interpretation. We showed that on the agarose surface under physiological conditions, E-cadherin produced a weak dimer that immediately dissociated after the depletion of calcium ions. However, either at pH 5 or in the presence of cadmium ions, E-cadherin produced a strong dimer that was unable to dissociate upon calcium depletion. Both types of dimers were W156-dependent. Remarkably, only the strong dimer was found on the surface of living cells. We also showed that the intracellular cadherin region, the clustering of which through catenins had been proposed as stabilizer of weak intercadherin interactions, was not needed, in fact, for cadherin junction assembly. Taken together, our data present convincing evidence that cadherin adhesion is based on high-affinity cadherin–cadherin interactions

Cadherin exits the junction by switching its adhesive bond

Intercellular traction forces or lateral alignment of cadherin molecules can influence adherens junction dynamics by altering the cadherin dimerization interface

Adhesive and Lateral E-Cadherin Dimers Are Mediated by the Same Interface

E-cadherin is a transmembrane protein that mediates Ca(2+)-dependent cell-cell adhesion. To study cadherin-cadherin interactions that may underlie the adhesive process, a recombinant E-cadherin lacking free sulfhydryl groups and its mutants with novel cysteines were expressed in epithelial A-431 cells. These cysteine mutants, designed according to various structural models of cadherin dimers, were constructed to reveal cadherin dimerization by the bifunctional sulfhydryl-specific cross-linker BM[PE0]3. Cross-linking experiments with the mutants containing a cysteine at strand B of their EC1 domains did show cadherin dimerization. By their properties these dimers correspond to those which have been characterized by coimmunoprecipitation assay. Under standard culture conditions the adhesive dimer is a dominant form. Calcium depletion dissociates adhesive dimers and promotes the formation of lateral dimers. Our data show that both dimers are mediated by the amino-terminal cadherin domain. Furthermore, the interfaces involved in both adhesive and lateral dimerization appear to be the same. The coexistence of the structurally identical adhesive and lateral dimers suggests some flexibility of the extracellular cadherin region

Spontaneous assembly and active disassembly balance adherens junction homeostasis

The homeostasis of adherens junctions was studied using E-cadherin and its two mutants tagged by the photoconvertible protein Dendra2 in epithelial A-431 cells and in CHO cells lacking endogenous cadherin. The first mutant contained point mutations of two elements, Lys738 and the dileucine motif that suppressed cadherin endocytosis. The second mutant contained, in addition, an extensive truncation that uncoupled the mutant from β-catenin and p120. Surprisingly, the intact cadherin and its truncated mutant were recruited into the junctions with identical kinetics. The full-size cadherin was actively removed from the junctions by a process that was unaffected by the inactivation of its endocytic elements. The cadherin’s apparent half-residence time in the junction was about 2 min. Cadherin clusters made of the truncated mutant exhibited much slower but ATP-independent junctional turnover. Taken together, our experiments showed that adherens junction homeostasis consists of three distinctive steps: cadherin spontaneous recruitment, its lateral catenin-dependent association, and its active release from the resulting clusters. The latter process, whose mechanism is not clear, may play an important role in various kinds of normal and abnormal morphogenesis

Dynamic Interplay between Adhesive and Lateral E-Cadherin Dimers

E-cadherin, an adhesive transmembrane protein of epithelial adherens junctions, forms two types of detergent-resistant dimers: adhesive dimers consisting of cadherin molecules derived from two neighboring cells and lateral dimers incorporating cadherins of the same cell. Both dimers depend on the integrity of the same residue, Trp(156). While the relative amounts of these complexes are not certain, we show here that in epithelial A-431 cells, adhesive dimers may be a prevalent form. Inactivation of the calcium-binding sites, located between successive cadherin ectodomains, drastically reduced the amount of adhesive dimers and concomitantly increased the amount of lateral dimers. A similar interdependence of adhesive and lateral dimers was observed in digitonin-permeabilized cells. In these cells, adhesive dimers immediately disassembled after lowering the Ca(2+) concentration below 0.1 mM. The disappearance of adhesive dimers was counterbalanced by an increase in Trp(156)-dependent lateral dimers. Increasing the calcium concentration to a normal level rapidly restored the original balance between adhesive and lateral dimers. We also present evidence that E-cadherin dimers in vivo have a short lifetime. These observations suggest that cadherin-mediated adhesion is based on the dynamic cycling of E-cadherin between monomeric and adhesive dimer states

Homophilic and Heterophilic Interactions of Type II Cadherins Identify Specificity Groups Underlying Cell-Adhesive Behavior

Summary: Type II cadherins are cell-cell adhesion proteins critical for tissue patterning and neuronal targeting but whose molecular binding code remains poorly understood. Here, we delineate binding preferences for type II cadherin cell-adhesive regions, revealing extensive heterophilic interactions between specific pairs, in addition to homophilic interactions. Three distinct specificity groups emerge from our analysis with members that share highly similar heterophilic binding patterns and favor binding to one another. Structures of adhesive fragments from each specificity group confirm near-identical dimer topology conserved throughout the family, allowing interface residues whose conservation corresponds to specificity preferences to be identified. We show that targeted mutation of these residues converts binding preferences between specificity groups in biophysical and co-culture assays. Our results provide a detailed understanding of the type II cadherin interaction map and a basis for defining their role in tissue patterning and for the emerging importance of their heterophilic interactions in neural connectivity. : Type II cadherins are a family of vertebrate cell adhesion proteins expressed primarily in the CNS. Brasch et al. measure binding between adhesive fragments, revealing homophilic and extensive selective heterophilic binding with specificities that define groups of similar cadherins. Structures reveal common adhesive dimers, with residues governing cell-adhesive specificity. Keywords: cell adhesion, crystal structure, hemophilic specificity, heterophilic specificity, neural patterning, synaptic targeting, cadheri