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Desmosomes are adhesive junctions composed of two desmosomal cadherins: desmocollin (Dsc) and desmoglein (Dsg). Previous studies demonstrate that E-cadherin (Ecad), an adhesive protein that interacts in both trans (between opposing cells) and cis (on the same cell surface) conformations, facilitates desmosome assembly via an unknown mechanism. Here we use structure-function analysis to resolve the mechanistic roles of Ecad in desmosome formation. Using AFM force measurements, we demonstrate that Ecad interacts with isoform 2 of Dsg via a conserved Leu-175 on the Ecad cis binding interface. Super-resolution imaging reveals that Ecad is enriched in nascent desmosomes, supporting a role for Ecad in early desmosome assembly. Finally, confocal imaging demonstrates that desmosome assembly is initiated at sites of Ecad mediated adhesion, and that Ecad-L175 is required for efficient Dsg2 and desmoplakin recruitment to intercellular contacts. We propose that Ecad trans interactions at nascent cell-cell contacts initiate the recruitment of Dsg through direct cis interactions with Ecad which facilitates desmosome assembly., Competing Interests: OS, MR, SS, AC, AK, CN, SS No competing interests declared, (© 2018, Shafraz et al.)

Classical cadherin transmembrane cell-cell adhesion proteins play essential roles in tissue morphogenesis and in mediating tissue integrity. Cadherin ectodomains from opposing cells interact to form load-bearing trans dimers that mechanically couple cells. Cell-cell adhesion is believed to be strengthened by cis clustering of cadherins on the same cell surface. This review summarizes biophysical studies of the structure, interaction kinetics and biomechanics of classical cadherin ectodomains. We first discuss the structure and equilibrium binding kinetics of classical cadherin trans and cis dimers. We then discuss how mechanical stimuli alters the kinetics of cadherin interaction and tunes adhesion. Finally, we highlight open questions on the role of mechanical forces in influencing cadherin structure, function and organization on the cell surface., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Classical cadherin cell-cell adhesion proteins play key morphogenetic roles during development and are essential for maintaining tissue integrity in multicellular organisms. Classical cadherins bind in two distinct conformations, X-dimer and strand-swap dimer; during cellular rearrangements, these adhesive states are exposed to mechanical stress. However, the molecular mechanisms by which cadherins resist tensile force and the pathway by which they convert between different conformations are unclear. Here, we use single molecule force measurements with an atomic force microscope (AFM) to show that E-cadherin, a prototypical classical cadherin, forms three types of adhesive bonds: catch bonds, which become longer lived in the presence of tensile force; slip bonds, which become shorter lived when pulled; and ideal bonds that are insensitive to mechanical stress. We show that X-dimers form catch bonds, whereas strand-swap dimers form slip bonds. Our data suggests that ideal bonds are formed as X-dimers convert to strand-swap binding. Catch, slip, and ideal bonds allow cadherins to withstand tensile force and tune the mechanical properties of adhesive junctions.

Cadherins are Ca(2+)-dependent cell-cell adhesion proteins with an extracellular region of five domains (EC1 to EC5). Adhesion is mediated by "strand swapping" of a conserved tryptophan residue in position 2 between EC1 domains of opposing cadherins, but the formation of this structure is not well understood. Using single-molecule fluorescence resonance energy transfer and single-molecule force measurements with the atomic force microscope, we demonstrate that cadherins initially interact via EC1 domains without swapping tryptophan-2 to form a weak Ca(2+) dependent initial encounter complex that has 25% of the bond strength of a strand-swapped dimer. We suggest that cadherin dimerization proceeds via an induced fit mechanism where the monomers first form a tryptophan-2 independent initial encounter complex and then undergo subsequent conformational changes to form the final strand-swapped dimer.