α-catenin regulates cell junction fluidity by cooperative mechanosensing
-catenin is a crucial protein at cell junctions that provides connection between the actin cytoskeleton and the cell membrane. At adherens junctions (AJs), -catenin forms heterodimers with {beta}-catenin that are believed to resist force on F-actin. Outside AJs, -catenin forms homodimers that directly connect the cell membrane to the actin cytoskeleton, but their mechanosensitive properties are inherently unknown. Surprisingly, by using ultra-fast laser tweezers we found that a single -{beta}-catenin heterodimer does not resist force but instead slips along F-actin in the direction of force. Conversely, the action of 5 to 10 -{beta}-catenin heterodimers together with force applied toward F-actin pointed end engaged a molecular switch in -catenin, which unfolded and strongly bound F-actin as a cooperative catch bond. Similarly, an -catenin homodimer formed an asymmetric catch bond with F-actin triggered by protein unfolding under force. Our data suggest that -catenin clustering together with intracellular tension engage a fluid-to-solid phase transition at the membrane-cytoskeleton interface.