Antiphospholipid syndrome (APS) is a systemic autoimmune disorder characterized clinically by vascular thrombosis and pregnancy morbidity and serologically by the presence of antiphospholipid antibodies (aPL). Despite their name, aPLs do not target phospholipids but recognize two circulating plasma proteins, prothrombin and β2-glycoportein-I (β2GPI), only after adsorption onto anionic surfaces such as phospholipids. Why this interaction requires the support of the membranes in vivo and a molecular mechanism for it remain unknown. As the idea of molecular plasticity starts to find structural evidence in the literature, a new mechanism has emerged in the field and predicts that protein binding to the phospholipids induces a conformational change leading to the exposure of antigenic epitopes. Recent results obtained with cryo-electron microscopy seem to support this model and have shown that β2GPI may exist in at least two conformations, open and closed. The closed-inactive conformation of the protein circulates in the plasma and is converted to the active-open form upon binding to the membranes, a transition that was found to be reversible through changing salt and pH. To gain insights on the structural properties of this molecule in solution, we collected small angle X-ray scattering (SAXS) data for the plasma purified and recombinant β2GPI under solution conditions that are reported to stabilize the open (pH 11.5, 1.15M NaCl) and closed (pH 3.4, 0.15M NaCl) conformations and also under physiological conditions (pH 7.4, 150 mM NaCl, 5 mM CaCl2). To our surprise, SAXS experiments failed to reveal major differences between the samples and the molecule seems to acquire similar elongated shapes. Indeed, after background subtraction and necessary corrections for inter-particle interactions the scattering profiles were superimposable up to a q-value of 0.15Å-1, and the radius of gyration (Rg), estimated via the Guinier approximation, was 41±1.9Å and 40.9±1.1Å for the closed and open conformations, respectively. Small but significant differences were observed in the higher-resolution region of the curve and were amplified in the Kratky plot, suggesting intrinsic flexibility of the molecule in solution. In conclusion, the lack of evidence of the closed form of β2GPI under these conditions suggests that binding of this protein to the autoantibodies requires alternative yet unidentified conformational changes, perhaps enabled by the plasticity of the linker domains.

Small angle X-ray scattering (SAXS) of β2-glycoprotein I suggests an alternative mechanism for antibody interaction in antiphospholipid syndrome

249th National Meeting (2015)

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