Integrins are vital transmembrane receptors that mediate cell-cell and cell-extracellular matrix interactions and signaling. Talin is a 270 kDa protein and is considered a key regulator of integrin activity. The interaction between talin and integrin is commonly regarded as the final step of inside-out activation. In the cytosol, talin adopts an autoinhibited conformation, in which the C-terminal rod domain binds the N-terminal head domain, preventing the interactions of the head domain with the membrane surface and the integrin cytoplasmic domain. It has long been suggested that the presence of phosphatidylinositol 4,5-bisphosphate (PIP2) at focal adhesions plays a role in activating talin. However, a detailed picture and mechanism of PIP2 activation of autoinhibited talin remains elusive. Here, we use a fluorescence resonance energy transfer-based binding assay to measure the affinity of talin and lipid bilayers harboring anionic lipids. Results show that the R9 and R12R13 segments of the talin rod domain inhibit the binding of the talin head domain (THD) to anionic lipid bilayers. In contrast, we show that the binding of the THD to bilayers containing PIP2 is insensitive to the presence of the inhibitor domains, thereby directly implicating PIP2 as an effective activator of talin. Furthermore, we have mapped the activation to the interaction of PIP2 with the F2F3 domain of the talin head, showing that PIP2 plays a critical role in the regulation of the autoinhibited form of talin and stimulates recruitment of talin to the membrane, which is essential for integrin inside-out signaling.
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