PMID: 21908432

Authors:

McEwan PA, Yang W, Carr KH, Mo X, Zheng X, Li R, Emsley J

Title:

Quaternary organization of GPIb-IX complex and insights into Bernard-Soulier syndrome revealed by the structures of GPIbbeta and a GPIbbeta/GPIX chimera.

Journal:

Blood. 2011 Nov 10;118(19):5292-301. Epub 2011 Sep 8.

Abstract:

Platelet GPIb-IX receptor complex has 3 subunits GPIbalpha, GPIbbeta, and GPIX, which assemble with a ratio of 1:2:1. Dysfunction in surface expression of the complex leads to Bernard-Soulier syndrome. We have crystallized the GPIbbeta ectodomain (GPIbbeta(E)) and determined the structure to show a single leucine-rich repeat with N- and C-terminal disulphide-bonded capping regions. The structure of a chimera of GPIbbeta(E) and 3 loops (a,b,c) taken from the GPIX ectodomain sequence was also determined. The chimera (GPIbbeta(Eabc)), but not GPIbbeta(E), forms a tetramer in the crystal, showing a quaternary interface between GPIbbeta and GPIX. Central to this interface is residue Tyr106 from GPIbbeta, which inserts into a pocket generated by 2 loops (b,c) from GPIX. Mutagenesis studies confirmed this interface as a valid representation of interactions between GPIbbeta and GPIX in the full-length complex. Eight GPIbbeta missense mutations identified from patients with Bernard-Soulier syndrome were examined for changes to GPIb-IX complex surface expression. Two mutations, A108P and P74R, were found to maintain normal secretion/folding of GPIbbeta(E) but were unable to support GPIX surface expression. The close structural proximity of these mutations to Tyr106 and the GPIbbeta(E) interface with GPIX indicates they disrupt the quaternary organization of the GPIb-IX complex.