Characterizing the Interaction of Desmosomal Cadherins at Single Molecule Level

Abstract

Desmosomes are cell-cell adhesion complexes that are present in tissues that resist mechanical stress. They are mainly composed of two adhesive proteins, which are members of the cadherin superfamily of cell adhesion proteins, desmocollin (Dsc) and desmoglein (Dsg). However, the role of these proteins in desmosomal adhesion is unclear. Here, we use the single molecule force spectroscopy with an Atomic Force Microscope (AFM-FS) to characterize the interactions of type-2 isoforms of desmocollin (Dsc2) and desmoglein (Dsg2). We show that Dsc2 forms Ca2+ dependent homophilic bonds by swapping a conserved Tryptophan (Trp) residue between opposing binding partners; mutating this Trp inhibits Ca2+ dependent homophilic binding. In contrast, Dsg2 forms Ca2+ independent heterophilic bonds with Dsc2 via a mechanism that does not involve Trp strand-swapping.Previous studies suggest that desmosome formation requires the presence of classical cadherins at the site of desmosome assembly. This suggests a cross-talk between desmosomal and classical cadherins at cell-adhesion contacts. We therefore used AFM-FS to test if Dsc2 and Dsg2 interact with E-cadherin, a classical cadherin present in the epithelium. Our data shows that while Dsc2 does not bind to E-cadherin in the presence of Ca2+, Dsg2 forms Ca2+ independent complexes with E-cadherin. Using cadherin mutants we show that the interactions between Dsg2 and E-cadherin occur via a previously uncharacterized binding interface that does not involve either Trp strand-swapping or X-dimer formation (two well established classical cadherin binding mechanisms).