Title : Quaternary organization of GPIb-IX complex and insights into Bernard-Soulier syndrome revealed by the structures of
GPIbβ and a
GPIbβ/ GPIX chimera
Abstract :
- Platelet GPIb-IX receptor complex has 3 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
- We have crystallized the GPIbβ ectodomain ( GPIbβ( E)) and determined the structure to show a single leucine-rich repeat leucine-rich repeat with N- and C-terminal disulphide-bonded capping regions
- The structure of a chimera of GPIbβ( E) and 3 loops (a,b,c) taken from the GPIX ectodomain sequence was also determined
- The chimera ( GPIbβ( Eabc)), but not GPIbβ( E), forms a tetramer in the crystal, showing a quaternary interface between GPIbβ and GPIX
- Central to this interface is residue Tyr106 from GPIbβ, 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 GPIbβ and GPIX in the full-length complex
- Eight GPIbβ 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 GPIbβ( E) but were unable to support GPIX surface expression
- The close structural proximity of these mutations to Tyr106 and the GPIbβ( E) interface with GPIX indicates they disrupt the quaternary organization of the GPIb-IX complex