The crystal structure of GPIbβ ectodomain (GPIbβ_E). (A) Sequence alignment of GPIbβ_E (blue)_, GPIX_E (purple), and GPIbβ_Eabc (blue and purple) with respective residue numbers for GPIbβ_E and GPIX marked on the top and bottom. Elements of secondary structure in GPIbβ_E are shown on top and colored as in panel B. Residues affected by BSS missense mutations are in red. Three stretches of the GPIX_E sequence that are included in GPIbβ_Eabc are shown in purple. (B) Two orientations are shown of a ribbon diagram of the GPIbβ_E structure viewed from the concave face (left), with β-strands labeled in blue, α-helices in red, 3₁₀ helices in purple, and loop regions in gray. Asn residues 40, 41, and 64 are shown as stick, and a single residue from the N-linked oligosaccharide attached to Asn41 is also shown in green. The diagram on the right is rotated 180 degrees with side chains in the inter-LRR cap cation-π interaction shown as stick. (C) Refined 1.25-Å electron density, calculated with 2Fo-Fc coefficients and contoured at 1.5 rms.