The integrin α-subunit leg extends at a Ca²⁺-dependent epitope in the thigh/genu interface upon activation

Two activation-dependent Abs to the integrin α_L-subunit were used to study conformational rearrangement of α_Lβ ₂ on the cell surface. Activation lowered the concentration of Ca²⁺ required for maximal expression of each epitope. Each Ab requires the Ca²⁺-binding loop in the integrin genu and nearby species-specific residues in the thigh domain. Key thigh residues are shielded from Ab in the bent integrin conformation by the α-subunit calf-1 domain and the nearby bent β leg, suggesting that extension at the genu is required for epitope exposure. Activating stimuli and α/β I-like small molecule antagonists demonstrate that exposure of epitopes in the integrin α- and β-subunit legs is coordinate during integrin activation. A coordinating residue donated by the calf-1 domain is as important as Ca ²⁺ for mAb binding. Together with inspection of the Ca²⁺ structure, this result suggests that the genu/ calf-1 interface is maintained in integrin activation, and that extension occurs by a rearrangement at the thigh/genu interface.