GpIbα-OS1 peptide complex crystal structure. (A) Ribbon representation of the complex with the regulatory (R) loop colored brown and the peptide colored cyan. Cysteine residues involved in disulphide bonds are highlighted in yellow. (B) Stick representations of GpIbα-OS1 interactions with hydrogen bonds colored as purple dotted lines. Peptide colored in cyan, GpIbα, colored in green with β-sheets highlighted as blue arrows. (C) A 1.8-Å electron density map is shown (yellow) with difference mFo-DFc co-efficients and contoured at 1 root mean square calculated with the OS1 peptide model omitted (program REFMAC). (D) Ligplot figure showing van der Waals and hydrogen bonds formed between GpIbα and OS1. Peptide depicted in purple, polar contacts printed in green. Side chains making hydrophobic interactions are shown as dashed circles. (E) Ribbon diagram of Gplbα (grey) showing only β-strands with OS1 (cyan) superposed onto the Gplbα-VWF-A1 complex with VWF-A1 illustrarted (light blue). The GpIbα R-loop is colored blue and orange for the VWF-A1 and OS1 complexes, respectively, with the Asp235-Lys572 salt bridge highlighted. (F) Binding footprints on the GpIbα surface shown in red for OS1 peptide (top) and VWF-A1 (bottom). (G) The R-loop conformational change from (E) is shown with Asp235 switching from forming a salt bridge to VWF-A1 to coordinating a main chain nitrogen of OS1 peptide. An α-helical form of the R-loop is stabilized by Asn9 from OS1 forming two helix capping hydrogen bonds.