NMR structure of FLNa-Ig21 bound to αIIb CT in PIP2-enriched integrin-activating membrane environment. (A) (left) Superposition of 20 calculated complex structures of αIIb CT (cyan) and FLNa-Ig21 (green) with the lowest energies, showing a well-defined structure; (right) corresponding cartoon representation of the structure with the lowest energy. β-strands are labeled from A to G based on the canonical immunoglobulin topology. (B) Overlay of NMR solution structure of αIIb CT (magenta) in complex with FLNa-21 (green) and crystal structure of αIIb CT (yellow)–FLNa-21 (cyan) chimera, showing the structures are very similar (backbone root mean square deviation is 1.6 Å). (C) Selected region of HSQC of 0.05 mM ¹⁵N-labeled αIIb CT in the absence (black) and the presence of 0.25 mM (red) and 0.5 mM (green) IP3. IP3 significantly perturbed chemical shifts of N-terminal K989, F993, N996, and C-terminal D1003/D1004 of αIIb CT, consistent with an equilibrium shift from closed to an open conformation. (D) A model illustrating how negatively charged PIP2 embedded on the plasma membrane may open the closed conformation by interacting with the positively charged N–terminus while sterically and electrostatically repelling the negatively charged C–terminus. αIIb CT structure is modified from PDB 2MTP.