Structure of IgFLNa17 alone and in complex with the GPIbα peptide. (A) A family of 20 NMR structures of the IgFLNa17 in solution. (B) Asymmetric unit of the crystal. Two molecules of IgFLNa17 are colored green and blue (chains A and B, respectively). The GPIbα peptides are magenta (chain S) and gold (chain T). The N and C termini of all chains are shown. Note that the peptide T interacts with both FLN molecules. (C) Superimposition of the crystal structure (green) and the mean NMR structure (red). (D) Surface properties of the FLNa-GPIbα556-577 interaction. The accessible surface of IgFLNa17 has been colored according to surface potential and the peptide is shown on the top (gold and atom color). This representation shows the extended conformation of the peptide in the complex and the hydrophopic character of the interaction. (E) Main chain hydrogen bonding between the peptide (green) and IgFLNa17 strand C. (F) Details of the peptide interaction. An electron density map calculated with the final model without the peptide at the σ level of 1.4 shows that the side chains of the peptide are well defined. The G1897 and C1912 residues of FLNa that were mutated in subsequent studies are shown in magenta. Residue numbers for the peptide are shown in black and for FLNa in green.