A Rap1 binding site and lipid-dependent helix in talin F1 domain cooperate in integrin activation

Rap1 GTPases bind effectors, such as RIAM, to enable talin1 to induce integrin activation. In addition, Rap1 binds directly to the talin1 F0 domain (F0); however, this interaction makes a negligible contribution to integrin activation in CHO cells or platelets. Here, we show that talin1 F1 domain contains a previously undetected Rap1 binding site of similar affinity to that in F0. A structure-guided point mutant (R118E) in F1, which blocks Rap1 binding, abolishes the capacity of Rap1 to potentiate talin1-induced integrin activation. The capacity of F1 to mediate Rap1-dependent integrin activation depends on a unique loop in F1 that transforms into an amphipathic helix upon binding to membrane lipids. Basic membrane-facing residues of this helix are critical as charge reversal mutations led to dramatic suppression of talin1-dependent activation. Thus, a novel Rap1 binding site and a lipid-dependent amphipathic helix in talin1 F1 cooperate to enable a direct Rap1-talin1 interaction to cause integrin activation.