PB0811 - Cryo-EM structure of a GPIb-IX quaternary complex and implications for platelets activation and Bernard-Soulier syndrome

Background: The glycoprotein (GP) Ib-IX complex is an abundant platelet receptor essential for platelet functions. Its congenital deficiency is associated with Bernard-Soulier syndrome (BSS), a bleeding disorder characterized by unusually large platelets and thrombocytopenia. Despite its importance, the spatial arrangement of its subunits on the platelet membrane remains elusive.

Aims: To elucidate the structure of GPIb-IX complex.

Methods: A GPIb-IX complex quaternary complex, mimicking the glycocalicin-removed form, was recombinantly prepared from HEK293 cell membrane fractions and analysed by single-particle cryo-electron microscopy.

Results: The structure was solved at 3.39A resolution as a homodimer and showed that three subunits, GPIbα, GPIbβ, and GPIX, assemble in the quaternary complex with a ratio of 1:2:1 with their transmembrane domains forming a 4-helix bundle. GPIbα links to two GPIbβ subunits through two membrane-proximal disulfide bonds, while GPIX, structurally homologous to GPIbβ, forms a heterotrimer with two GPIbβs in the extracellular domain through non-covalent interactions. Notably, GPIbβ adopts Y106 to interact with neighbouring GPIbβ or GPIX, but forming stronger stabilization interactions with Trp83 and Asp86 of GPIX. This insight illuminates the disruptive impact of mutations like GPIbβ-Y106S and GPIX-D86_P89delinsA, providing a clear understanding of how these mutations interfere with GPIb-IX complex assembly.

Conclusion(s): Overall, our structural study not only unveils the molecular basis underlying BSS but also sheds lights on the mechanism of GPIb-IX complex meditated platelets adhesion and activation.