The αIIbβ3 TM helix dimer. (A) Structure of the αIIb (blue) β3 (red) TM helix dimer (PDB ID code 2K9J). Interacting residues in the OMC are in magenta, in the IMC clasp in orange, and the αIIb R995 to β3 D756 salt bridge is in green. (B) Snapshot (100 ns) from the αβ1-AT simulation (i.e., WT; see Table 1 for details) showing the two TM helices, the interacting residues of the OMC (magenta), and the phosphorus atoms of the POPC/POPG lipid bilayer (gray). (C) Helix crossing angle distributions for the αβ1-AT (blue), αβ2-AT (i.e., αIIbR995D; red), and αβ3-AT (i.e., αIIb F992A,F993A; green) simulations. A negative crossing angle corresponds to right-handed helix packing, a positive angle to left-handed packing.

Simulations of the αIIbβ3 TM helix dimer in complex with talin F2-F3. (A) Movements of the α and β TM domains in the αβ-F2F3-AT simulations. Three structures are shown: the closed structure at the start of the CG-MD simulations (αβ-F2F3c-AT), a representative structure from the open cluster after the CG-MD simulations (αβ-F2F3o-AT), and the final open structure from 100-ns AT-MD simulation. In these diagrams, only the Cα traces of the TM helices (α in blue; β in red) are shown, along with the phosphorus atoms of the nearby lipid headgroups (gray). The OMC residues and the aromatic residues of the IMC are shown in green. (B) Initial (blue) and final (t = 100 ns; red) configuration of the αβ-F2-F3 complex in the αβ-F2F3o-AT simulation. The αIIb TM helix from the initial and the final structures of the simulation are superimposed. (C) Snapshot corresponding to the final (t = 100 ns) configuration of the αβ-F2F3o-AT simulation showing that the interactions in both the IMC and the OMC are disrupted. The F2 subdomain of talin is omitted for clarity.

Tilting of the β3 TM helix in β3/talin simulations. (A) Initial (blue) and final (t = 70 ns; red) orientation of the F2-F3-β complex in one of the β-F2-F3 simulations. (B) The β3 TM helix tilt angle relative to the bilayer normal for β-F2F3-AT (black) and β-F3-AT (red) simulations.

(A) Proposed mechanism of integrin inside-out activation. The positively charged patch on the F2 and F3 subdomains of talin (1) interacts with the negatively charged headgroups on the membrane surface (2), orienting talin for binding to the β cytoplasmic tail (3). Upon talin binding (4), the αIIbR995-β3D723 salt bridge is disrupted because of the interactions of the αIIbR995 with the talin F3 acidic loop. Strengthening of interactions between the positively charged surface patch of F2 and the membrane surface rotates the talin F2 subdomain approximately 34° in a plane perpendicular to the bilayer normal (i.e., parallel to the lipid–water interface). This in turn rotates F3 (approximately 26°) and the β tail (approximately 30°) in the opposite direction and in the same plane. This, in combination with an approximately 15° increase in the tilt angle of the β helix relative to the bilayer normal, disrupts the interactions in both the IMC and the OMC of the αβ dimer (5). This is suggested to promote a switch of the integrin ectodomain to an active extended state. B and C illustrate the proposed scissors motion of the integrin α and β TM helices that occurs on activation by talin. B indicates four interhelix distances (d1 to d4) that characterize the scissors motion, and C shows the time evolution of d1 to d4 over the course of one of the three αβ-F2F3o-AT simulations.