Zyxin and LPP differ in their sensing of strained actin filaments at tricellular junctions

The actomyosin cytoskeleton plays important roles in cell-cell adhesion by generating and responding to forces. Actin-binding proteins support actomyosin networks by reinforcing actin filaments, promoting actin remodeling, and transmitting forces to transmembrane adhesion proteins. One family of actin-binding proteins, LIM domain-containing proteins, is recruited to strained actin filaments. Here, we investigate two members of the LIM domain family, Zyxin and Lipoma-Preferred Partner (LPP), using the embryonic epithelium of Xenopus laevis , where actin-associated cell-cell junctions connect cells to promote tissue integrity and barrier function. Specifically, we compare Zyxin and LPP's response to increased tension at tricellular junctions (TCJs), sites of heightened mechanical strain within the tissue. Upon increased tension, Zyxin and LPP mechanoaccumulate to different extents relative to F-actin, suggesting distinct mechanisms. Our results demonstrate that Zyxin's and LPP's LIM domain-containing regions (LCRs) are sufficient for mechanoaccumulation as well as responsible for the difference in mechanoaccumulation, while their N-termini play a regulatory role in their mechanosensitive responses. Docking simulations show that a hydrophobic interaction between the LCR and the "crack site" that forms in F-actin filaments under high force may underlie the LCR's sensing of strained F-actin. Additionally, the docking simulations reveal that previously-identified conserved residues are present at the binding interface. Mutations of conserved residues in Zyxin or LPP reduce the extent of mechanoaccumulation at TCJs, supporting the docking prediction. Together, this study advances our understanding of Zyxin and LPP's sensing of strained actin at TCJs under mechanical challenge.

The actomyosin cytoskeleton provides structural support for intercellular connections. This is particularly important at high force levels when actin must be reinforced by actin-binding proteins. Two of those proteins, Zyxin and LPP, stabilize strained actomyosin networks. Using frog embryos, we compare the force-dependent recruitment of these proteins at cell-cell junctions where three cells meet. Despite their similar sequences, Zyxin and LPP respond differently to increased tension. We find that the LIM-domain-containing region (LCR) confers this force-sensing behavior for strained actin at cell-cell junctions. The rest of the protein regulates the extent to which the proteins accumulate. Computational modeling suggests that the accumulation difference between Zyxin and LPP may be due to variations in the LCR and strained actin-binding interface.