FCHo2, instead of talin, enables inside-out activation of integrin ɑvβ5 in curved adhesions.

Inside-out activation of integrins is crucial for transducing mechanical forces through the extracellular ligand-integrin-talin-F-actin axis. Extensive studies have shown that talin is the essential player in this process by binding to the intracellular tail of β integrins. Here, we show that, while talin binding is essential for inside-out integrin activation in focal adhesions, it is dispensable in curved adhesions - a distinct adhesion architecture that is exclusively mediated by integrin αvβ5 and selectively formed at curved membranes. Instead, a curvature-sensing protein FCHo2 binds to the HDRRE motif in the cytoplasmic tail of integrin β5 (ITGβ5) and inside-out activates integrin αvβ5 in curved adhesions. Intriguingly, FCHo2 does not bind to a similar motif in the homologous integrin β3 tail. Through truncations and mutations, we identified a pivotal tryptophan (W) in the β3 tail, which is conserved in all homologous integrin β isoforms except β5, where it is replaced by a tyrosine (Y766). This tyrosine substitution is crucial for integrin β5's unique capability in forming curved adhesions. A Y766W mutation abolishes integrin β5's capacity to form curved adhesions, without affecting its ability to form focal adhesions. Furthermore, our studies suggest that the phosphorylation state of Y766 regulates whether integrin ɑvβ5 forms curved adhesions or focal adhesions, providing a cellular mechanism governing different adhesion types. Overall, our work unveils distinct molecular interactions and regulatory mechanisms between curved adhesions and focal adhesions, and establishes a molecular basis for the formation of curved adhesions by integrin ɑvβ5.