Optogenetics-based localization of talin to the plasma membrane promotes activation of β3 integrins

Interaction of talin with the cytoplasmic tails of integrin beta triggers integrin activation, leading to an increase of integrin affinity/avidity for extracellular ligands. In talin KO mice, loss of talin interaction with platelet integrin alpha IIb beta 3 causes a severe hemostatic defect, and loss of talin interaction with endothelial cell integrin alpha V beta 3 affects angiogenesis. In normal cells, talin is autoinhibited and localized in the cytoplasm. Here, we used an optogenetic platform to assess whether recruitment of fulllength talin to the plasma membrane was sufficient to induce integrin activation. A dimerization module (Arabidopsis cryptochrome 2 fused to the N terminus of talin; N-terminal of cryptochrome-interacting basic helix-loop-helix domain ended with a CAAX box protein [C: cysteine; A: aliphatic amino acid; X: any C-terminal amino acid]) responsive to 450 nm (blue) light was inserted into Chinese hamster ovary cells and endothelial cells also expressing alpha IIb beta 3 or alpha V beta 3, respectively. Thus, exposure of the cells to blue light caused a rapid and reversible recruitment of Arabidopsis cryptochrome 2-talin to the N-terminal of cryptochrome-interacting basic helix-loop-helix domain ended with a CAAX box protein [C: cysteine; A: aliphatic amino acid; X: any C-terminal amino acid]-decorated plasma membrane. This resulted in beta 3 integrin activation in both cell types, as well as increasing migration of the endothelial cells. However, membrane recruitment of talin was not sufficient for integrin activation, as membraneassociated Ras-related protein 1 (Rap1)-GTP was also required. Moreover, talin mutations that interfered with its direct binding to Rap1 abrogated beta 3 integrin activation. Altogether, these results define a role for the plasma membrane recruitment of talin in beta 3 integrin activation, and they suggest a nuanced sequence of events thereafter involving Rap1-GTP.