Salt bridges gate α-catenin activation at intercellular junctions

Cadherin complexes transduce force fluctuations at junctions to activate signals that reinforce stressed intercellular contacts. alpha-Catenin is an identified force transducer within cadherin complexes that is autoinhibited under low tension. Increased force triggers a conformational change that exposes a cryptic site for the actin-binding protein vinculin. This study tested predictions that salt bridges within the force-sensing core modulate alpha-catenin activation. Studies with a fluorescence resonance energy transfer (FRET)-based alpha-catenin conformation sensor demonstrated that each of the salt-bridge mutations R551A and D503N enhances alpha-catenin activation in live cells, but R551A has a greater impact. Under dynamic force loading at reannealing cell-cell junctions, the R551A mutant bound more vinculin than wild-type alpha-catenin. In vitro binding measurements quantified the impact of the R551A mutation on the free-energy difference between the active and autoinhibited alpha-catenin conformers. A 2-mu s constant-force, steered molecular dynamics simulation of the core force-sensing region suggested how the salt-bridge mutants alter the alpha-catenin conformation, and identified a novel load-bearing salt bridge. These results reveal key structural features that determine the force-transduction mechanism and the force sensitivity of this crucial nanomachine.