Structural analyses of β₂-glycoprotein I: is there a circular conformation?

Area of Science:

• Biochemistry
• Immunology
• Structural Biology

Background:

• Antiphospholipid antibodies targeting beta-2-glycoprotein I (β2GPI) are implicated in antiphospholipid syndrome (APS) pathogenesis.
• The precise mechanism by which these antibodies recognize β2GPI remains incompletely understood.

Purpose of the Study:

• To determine the structure of β2GPI in the context of human plasma.
• To investigate how pathogenic anti-domain I (DI) antibodies interact with β2GPI.

Main Methods:

• Recombinant β2GPI production and purification.
• Structural and functional analyses using electron microscopy, size-exclusion chromatography, single-molecule Förster resonance energy transfer, and microfluidic diffusional sizing.
• Reconstitution of β2GPI in human plasma.

Main Results:

• β2GPI exists in an elongated conformation, irrespective of purification or buffer conditions.
• Single-molecule Förster resonance energy transfer confirmed the elongated structure and stability in plasma.
• Microfluidic diffusional sizing demonstrated moderate affinity binding of β2GPI to anti-DI antibodies.

Conclusions:

• Circulating β2GPI is elongated and readily binds anti-DI autoantibodies.
• Phospholipid binding enhances autoantibody recognition by increasing antigen concentration, not by conformational change.
• These findings provide critical insights into APS pathogenesis and inform therapeutic and diagnostic strategies.