The Mac-1 I:GP1bαN complex. (A) A cartoon diagram of the docked complex of the crystal structures of human GPIbαN and the Mac-1 I-domain with the Mac-1 secondary structures colored blue/green and GPIbαN colored red/orange. Boxed is a close-up view of the interface where the Mg²⁺ ion bound to the Mac-1 MIDAS site is shown as a sphere and electrostatic interactions are shown as dashed purple lines. The GPIbαN C-terminal LRR capping region is colored orange and residue F192 from the LRRs is in gray. (B) A second view related by a 90° rotation. GPIbαN residue D222 coordinates the Mac-1 MIDAS Mg²⁺ ion and surrounding residues on the MIDAS surface (light blue). (C) Charged surface representation of the GPIbαN LRR capping region showing an elongated pocket formed by GPIbα residues Y215, W219, and F192 is flanked by a region of positive charge (GPIbα R218) and negative charge (GPIbα D222) that form complementary interactions with Mac-1 residues shown as sticks (light blue).