The J-elongated conformation of β2-glycoprotein I predominates in solution: implications for our understanding of antiphospholipid syndrome

Abstract

β2-glycoprotein I (β2GPI) is an abundant plasma protein displaying phospholipid-binding properties. Because it binds phospholipids, it is a target of antiphospholipid antibodies (aPLs) in antiphospholipid syndrome (APS), a life-threatening autoimmune thrombotic disease. Indeed, aPLs prefer membrane-bound β2GPI to that in solution. β2GPI exists in two almost equally populated redox states: oxidized, in which all the disulfide bonds are formed, and reduced, in which one or more disulfide bonds are broken. Furthermore, β2GPI can adopt multiple conformations (i.e. J-elongated, S-twisted, and O-circular). While strong evidence indicates that the J-form is the structure bound to aPLs, which conformation exists and predominates in solution remains controversial, and so is the conformational pathway leading to the bound state. Here, we report that human recombinant β2GPI purified under native conditions is oxidized. Moreover, under physiological pH and salt concentrations, this oxidized form adopts a J-elongated, flexible conformation, not circular nor twisted, in which the N-terminal domain I (DI) and the C-terminal domain V (DV) are exposed to the solvent. Consistent with this model, binding kinetics and mutagenesis experiments revealed that in solution the J-form interacts with negatively charged liposomes and with MBB2, a monoclonal anti-DI antibody that recapitulates most of the features of pathogenic aPLs. We conclude that the preferential binding of aPLs to phospholipid-bound β2GPI arises from the ability of its pre-existing J-form to accumulate on the membranes, thereby offering an ideal environment for aPL binding. We propose that targeting the J-form of β2GPI may provide a strategy to block pathogenic aPLs in APS.

Specific domain V reduction of beta-2-glycoprotein I induces protein flexibility and alters pathogenic antibody binding

“…This contrasts with the work of Ruben et al . 39 who show control β2GPI (unmodified) behaves similarly to the proven open expressed β2GPI.…”

“…Although it had been firmly established in the literature that circulating β2GPI was in the circular conformation 5 , a recent publication by Ruben et al . 39 has presented evidence to the contrary. Importantly, the work by Ruben et al uses a number of biophysical techniques which have not previously been applied to β2GPI including SAXS and FRET, whilst fomer studies focused on imaging by electron microscopy.…”

Anti-Phospholipid Antibodies in COVID-19 Are Different From Those Detectable in the Anti-Phospholipid Syndrome

“… Epitope specificity of anti-β 2 GPI antibodies in COVID-19 patients. (A) Three-dimensional structure of β 2 GPI solved at 2.4 Å by X-ray crystallography [PDI ID: 6V06 ( 27 )] displaying the positioning of the fragments used in this study. The N-terminal D1 is shown in red.…”

Anti-phospholipid antibodies in COVID-19 are different from those detectable in the anti-phospholipid syndrome

“…Epitope specificity of anti-β 2 GPI antibodies in COVID-19 patients. Panel A.Three-dimensional structure of β 2 GPI solved at 2.4 Å by X-ray crystallography (PDI ID: 6V06[32]) displaying the positioning of the fragments used in this study.The N-terminal D1 is shown in red. The C-terminal D4-5 fragment is shown in blue.…”