(197i) Single-Molecule Studies of Cadherin Binding Interactions in Biomimetic Lipid Bilayer Models

Cadherin transmembrane proteins are responsible for cell adhesion in a number of tissues and therefore, modulate a variety of biological processes, such as tissue morphogenesis, cell motility, force transduction, and macromolecular transport across tissue boundaries. Furthermore, deficiencies in cadherin-mediated adhesion are correlated with several diseases, such as a number of skin diseases and cancers. Cell-cell adhesion is facilitated via adherens junctions, consisting of adhesive, trans-interactions, and lateral, cis-interactions, but the molecular role and interplay of these interactions is a topic of discussion. Here we used single-molecule microscopy to show the drastic, double cooperativity, of cis- and trans-interactions and how this can facilitate large junction formation using a biomimetic lipid bilayer cell adhesion model. Notably, the addition of cis-binding capability resulted in a 25-fold increase in trans-binding lifetimes between epithelial-cadherin extracellular domains. The nature of this cooperativity appears to be primarily due to allostery, as opposed to avidity. It has been thought the primary role of cis-interactions is to provide molecular ordering within junctions, increasing their size, but junctions still may form in the absence of cis-interactions. Furthermore, cis-interactions have been predicted to cause an elevated cadherin surface density in the adhesive zone between cells, potentially increasing adhesive binding strength though cluster avidity. We have directly observed strong cooperativity between cis- and trans-interactions, providing vital insight into the molecular mechanism of junction formation, and therefore, cadherin-mediated cell adhesion.