X-ray and solution structures of human beta-2 glycoprotein I reveal a new mechanism of autoantibody recognition

Venous and arterial thromboses in patients suffering from the autoimmune disorder Antiphospholipid Syndrome (APS) are caused by the presence of antiphospholipid antibodies (aPL). Emerging evidence indicates that autoantibodies targeting the epitope R39-R43 in the N-terminal domain, Domain I (DI), of β-glycoprotein I (βGPI) are among the most pathogenic aPL in patients with APS. How such autoantibodies engage βGPI at the molecular level remains incompletely understood. Here, we have used X-ray crystallography, single-molecule FRET, and small-angle X-ray scattering to demonstrate that, in the free form, under physiological pH and salt concentrations, human recombinant βGPI adopts an elongated, flexible conformation in which DI is exposed to the solvent, thus available for autoantibody recognition. Consistent with this structural model, binding and mutagenesis studies revealed that the elongated form interacts with a pathogenic anti-DI antibody in solution, without the need of phospholipids. Furthermore, complex formation was affected neither by the neighboring domains, nor by the presence of the linkers, nor by the glycosylations. Since the pathogenic autoantibody requires residues R39 and R43 for optimal binding, these findings challenge longstanding postulates in the field envisioning βGPI adopting immunologic inert conformations featuring inaccessibility of the epitope R39-R43 in DI and support an alternative model whereby the preferential binding of anti-DI antibodies towards phospholipid-bound βGPI arises from the ability of the pre-existing elongated form to bind to the membranes and then oligomerize, processes that are likely to be supported by protein conformational changes. Interfering with these steps may limit the pathogenic effects of anti-DI antibodies in APS patients.