The crystal structure of GPIbβ_Eabc and its comparison with GPIbβ_E. (A) A ribbon diagram of the GPIbβ_Eabc structure, showing the grafted GPIX convex loops (magenta) in front. Residues derived from GPIbβ_E are colored in blue. Side chains of several residues are shown in stick and labeled. GPIX-Ala29 and GPIX-Ser76 correspond to GPIbβ-Trp21 and GPIbβ-Arg71, respectively. (B) Superposition of GPIbβ_E (green) and GPIbβ_Eabc (blue/magenta) structures. (C) A close-up view of the conformational difference in the C-terminal cap region between GPIbβ_E (green) and GPIbβ_Eabc (blue) structures. The locations of several side chains in both structures are marked. (D) Topology diagrams of ligand-free (green) and ligand-bound structures in orange for GPIbα and blue for GPIbβ. The observed conformational change occurs in a topologically equivalent loop (boxed) and in each case involves unwinding of a helix for GPIbα (R-loop) and GPIbβ (loop containing residue Trp88) on engaging ligand; VWF-A1 or GPIX, respectively.