Platelet glycoprotein (GP)Ib-IX receptor complex contains three subunits, GPIbα, GPIbβ and GPIX, which assemble with a ratio of 1:2:1. Dysfunction in surface expression of the complex leads to Bernard-Soulier syndrome (BSS). We have crystallized the GPIbβ ectodomain (GPIbβE) and determined the structure to reveal a single leucine-rich repeat with N- and C-terminal disulfide bonded capping regions. The central region of the structure can be divided into concave parallel β-sheet and convex loops. The crystal structure of a GPIbβE/GPIXE chimera that contains three non-continguous convex loops of GPIX and retains a GPIbβ-binding site of GPIX (Mo et al. J. Thromb. Haemost. 7:1533–40, 2009) was also determined. The chimera, but not GPIbβE, forms a homotetramer in the crystal, revealing a quaternary interface between GPIbβ and GPIX ectodomains. Central to this interface is residue Tyr106 from GPIbβ that inserts into a shallow and largely hydrophobic pocket generated by two convex loops from GPIX. Mutagenesis studies confirmed this interface as a valid representation of interactions between GPIbβ and GPIX in the full-length complex. Eight GPIbβ missense mutations identified from BSS patients were examined in transiently transfected Chinese hamster ovary cells for changes to the GPIb-IX complex surface expression. Six of the eight mutations lead to secretion defect and/or misfolding of GPIbβE. In contrast, the other two mutations, A108P and P74R, were found to maintain normal secretion and folding of GPIbβE but were unable to support GPIX surface expression. The close structural proximity of these mutations to Tyr106 and the GPIbβE interface with GPIX indicates that residues Ala108 and Pro74 in GPIbβ are located at the GPIbβE/GPIXE interfaces. Based on the tetrameric arrangement of the chimera structure, we propose a structural model for the GPIb-IX complex that embodies its organizing principles and helps to provide mechanistic insights on its assembly, function and regulation.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.