Mechanisms of the force-dependent interactions between the cadherin-catenin complex and F-actin
Keywords: Engineering.
Chemical Engineering.

Mechanotransduction at cell--cell adhesions govern processes fundamental to cell biology. Its dysregulation is associated with cardiomyopathies, developmental disorders, and cancer metastasis. Intercellular linkages at the adherens junction must coordinate long-range organization to facilitate cell migration, drive dynamic processes to enable embryonic morphogenesis and wound healing, and reinforce adhesion under tension to maintain tissue integrity. However, the mechanisms by which cellular linkages sense and response to load is not fully understood. Molecular-level interactions at the adherens junctions underlie higher-order cellular processes. Cadherin proteins not only maintain cell-cell adhesion by linking neighboring cells via their extracellular domain, but also couple to the force-generating actin cytoskeleton via intracellular interactions with β-catenin and αE-catenin. The linkage between the cadherin-catenin complex and actin is described as a catch bond, where the interaction is strengthened by mechanical force. To better understand the molecular details of mechanotransduction, I (1) directly demonstrated the catch bond mechanism using single-molecule force spectroscopy and (2) proposed mechanisms describing how vinculin influences ensemble-level force dependent binding between cadherin-catenin complexes and F-actin via Monte Carlo simulation. I performed optical trapping experiments and directly demonstrated that that the catch bond property principally resides in the αE-catenin actin-binding domain (ABD). Consistent with a structural analysis, deleting the first helix of the five-helix ABD bundle enabled stable interactions with F-actin under minimal load that was well described by a single-state slip bond, even when αE-catenin is complexed with β-catenin and E-cadherin. Our data argued for a conserved catch bond mechanism for adhesion proteins with structurally similar ABDs. This work also led to a surprising observation that a stably bound ABD strengthens load-dependent binding interactions between a neighboring complex and F-actin, but the presence of the other αE-catenin domains weakens this effect. These results provided mechanistic insight to the cooperative binding of the cadherin--catenin complex to F-actin, which regulate dynamic cytoskeletal linkages in epithelial tissues. Additionally, I developed simulations to describe how multiple cadherin-complexes cooperate to reinforce cell-cell junctions in response to load. I found that the cooperative actin binding lifetimes for cadherin-catenin complexes are most easily explained by a model in which one component bears essentially all the load, with the rest acting as bystanders. Additionally, I describe actin-binding mechanisms for a quaternary complex comprising the cadherin-catenin complex and the vinculin head region, which cannot itself bind actin. These models agree with experimental observations that the binding lifetimes of this quaternary complex increased as additional complexes bound F-actin, but only when load was oriented toward the (-) end. Together, these findings add insight to how molecular interactions in the cadherin-catenin complex facilitate higher-order cytoskeletal organization