Gα13 Switch Region 2 Relieves Talin Autoinhibition to Activate αIIbβ3 Integrin*

James Schiemer, Andrew Bohm, Li Lin, Glenn Merrill-Skoloff, Robert Flaumenhaft, Jin-Sheng Huang, Guy C Le Breton, Athar H Chishti

Integrins function as bi-directional signaling transducers that regulate cell-cell and cell-matrix signals across the membrane. A key modulator of integrin activation is talin, a large cytoskeletal protein that exists in an autoinhibited state in quiescent cells. Talin is a large 235 kDa protein comprised of an N-terminal 45 kDa FERM domain, also known as the talin head domain (THD), and a series of helical bundles known as the rod domain. The talin head domain consists of 4 distinct lobes designated as F0-F3. Integrin binding and activation is mediated through the F3 region; a critically regulated domain in talin. Regulation of the F3 lobe is accomplished through autoinhibition via anti-parallel dimerization. In the anti-parallel dimerization model, the rod domain region of one talin molecule binds to the F3 lobe on an adjacent talin molecule, thus achieving the state of autoinhibition. Platelet functionality requires integrin activation for adherence and thrombus formation, thus regulation of talin presents a critical node where pharmacological intervention is possible. A major mechanism of integrin activation in platelets is through heterotrimeric G protein signaling regulating hemostasis and thrombosis. Here we provide evidence that Switch Region 2 of the ubiquitously expressed G protein (G13) directly interacts with talin, relieves its state of autoinhibition, and triggers integrin activation. Biochemical analysis of G13 shows SR2 binds directly to the F3 lobe of talins head domain and competes with the rod domain for binding. Intramolecular FRET analysis shows G13 can relieve autoinhibition in a cellular milieu. Finally, a myristoylated SR2 peptide shows demonstrable decrease in thrombosis in vivo. Altogether, we present a mechanistic basis for the regulation of talin through G13.