α-catenin switches between a slip and an asymmetric catch bond with F-actin to cooperatively regulate cell junction fluidity

Abstract

Cell adhesions dynamically tune their mechanical properties during tissue development and homeostasis. Fluid connections required for cell mobility can switch to solid links to maintain the mechanical rigidity of epithelial layers 1,2 . Changes in the composition and clustering of adhesion molecules have been proposed to modulate cell junction fluidity, but the underlying mechanisms are unclear 3,4 . acatenin has been shown to play a fundamental role in different adhesion sites. At adherens cell-cell junctions (AJ), a-catenin localizes in cadherin-catenin complexes, where it provides a mechanical link between b-catenin and the actin cytoskeleton 5 . However, its function is controversial owing to the low affinity between actin and the a-b-catenin heterodimer 6 . Outside AJ, a-catenin binds itself to form homodimers that connect the cell membrane to the actin cytoskeleton to promote adhesion and migration, but its mechanosensitive properties are inherently unknown 7,8 . Here, using ultra-fast laser tweezers 9 we show that a single mammalian a-catenin molecule displays very different force-bearing properties depending on whether it is associated to b-catenin or not. We found that a single a-b-catenin heterodimer slips along an actin filament in the direction of force, while a single a-catenin homodimer forms a strong asymmetric catch-bond with actin, in which the bond lifetime increases, and the protein unfolds with force. Importantly, assemblies of multiple ab-catenin heterodimers show force-bearing and unfolding properties similar to the acatenin homodimer. Our results indicate that, outside AJ, single a-catenin homodimers act as a mechanical link with the actin cytoskeleton that resists force efficiently. Nonetheless, inside AJ, a-catenin's capability to hold cell-cell connections under physiological loads critically depends on the recruitment of multiple (5-10) complexes. Our data support a molecular model in which a-catenin clustering and intercellular tension engage a fluid-to-solid phase transition at the membranecytoskeleton interface.In any living cell, an array of mechanotransductor proteins responds to mechanical cues to trigger complex molecular signaling, driving cell morphology and gene expression profiles 10 . Among these proteins, a-catenin has been reported to act as a mechanosensor that regulates adherens junctions (AJ) in response to mechanical cues from neighboring cells in a tissue 5 . It is widely accepted that a-catenin in AJ binds b-catenin, which, in turn, connects to the cytoplasmatic portion of E-cadherin to constitute the cadherin-catenin complex (CCC). Since a-catenin also binds actin, a-catenin has been indicated as a major candidate for providing a link between the CCC and the actin cytoskeleton. This molecular link is required

Multi-level force-dependent allosteric enhancement of αE-catenin binding to F-actin by vinculin

“…Using an improved OT setup and determining the polarity of the filament by measuring the direction of its movement in a separate flow channel containing the pointed (-) end directed motor myosin VI [29], we demonstrated that vinculin itself shows catch bond behavior that is asymmetric: its lifetime bound to F-actin is longer when force is directed towards the pointed (-) end of the actin filament than toward the barbed (+) end [29]. Asymmetric catch bond behavior was also recently reported for aE-catenin alone [35]. We therefore re-measured the forcedependent association of the ternary complex comprising the E-cadherin cytoplasmic domain, b-catenin, and aE-catenin with F-actin (Fig.…”

“…Structural and biochemical studies [36, 42] found direct contacts between actin-bound αE- catenin ABDs, and suggested that these interactions enhance binding between αE-catenin and F-actin. Cooperative binding of αE-catenin to F-actin was also reported in solution [43] and in a biophysical study wherein a single β-catenin/αE-catenin heterodimer formed a short-lived slip bond, but a higher heterodimer surface density enabled the complex to form a directional catch bond with F-actin [35]. Studies from our laboratories likewise indicate that interactions between neighboring cadherin-catenin complexes facilitates F-actin binding under load [18, 30].…”

Cell Junction Mechanics beyond the Bounds of Adhesion and Tension

“…Actin recruitment was found to lead to a significant increase of E-cadherin density (typically between 150% to 200%; Engl et al, 2014;Gao et al 2018;Yonemura et al, 2010). Catenin clustering together with intracellular tension orchestrate a fluid-to-solid phase transition at the membranecytoskeleton interface (Arbore et al, 2020) demonstrating the ability of cells to regulate the rheological properties of their junctional cytoskeleton, independently of myosin activity.…”

Strain stiffening of Ndc80 complexes attached to microtubule plus ends

“…Speculating on other possible catch bond mechanisms, it is conceivable that by stretching the unstructured tail additional binding sites become available which are concealed in the entangled tail at low forces. Force-enhanced adhesion by unfolding is a common catch bond mechanism (46); examples are the von Willebrand factor (47, 48) and α -catenin bonding to F-actin (49) in the cytoskeleton. In conclusion, an intrinsic catch bond mechanism of Ndc80 complexes is compatible with current knowledge and worth considering as one of the ingredients that define the characteristics of Ndc80 strain stiffening.…”