How the headpiece hinge angle is opened: New insights into the dynamics of integrin activation

How the integrin head transitions to the high- affinity conformation is debated. Although experiments link activation with the opening of the hinge angle between the βA and hybrid domains in the ligand- binding headpiece, this hinge is closed in the liganded α _vβ ₃ integrin crystal structure. We replaced the RGD peptide ligand of this structure with the 10th type III fibronectin module (FnIII ₁₀) and discovered through molecular dynamics (MD) equilibrations that when the conformational constraints of the leg domains are lifted, the βA/hybrid hinge opens spontaneously. Together with additional equilibrations on the same nanosecond timescale in which small structural variations impeded hinge-angle opening, these simulations allowed us to identify the allosteric pathway along which ligand-induced strain propagates via elastic distortions of the α1 helix to the βA/hybrid domain hinge. Finally, we show with steered MD how force accelerates hinge-angle opening along the same allosteric pathway. Together with available experimental data, these predictions provide a novel framework for understanding integrin activation.