Cardiolipin interacts with beta-2-glycoprotein I and forms an open conformation-Mechanisms analyzed using hydrogen/deuterium exchange.

Beta-2-glycoprotein I (β(2) GPI) is the major antigen for the antiphospholipid antibodies in the antiphospholipid syndrome. The exposed epitope in domain I of β(2) GPI can be recognized by the anti-β(2) GPI antibody. Here, we prepared the anionic di-oleoyl-phosphatidylserine (DOPS) and cardiolipin (CL) liposomes to interact with the β(2) GPI. The conformational changes of β(2) GPI upon binding with the liposomes were analyzed using hydrogen/deuterium exchange mass spectrometry. The exchange level of sequences 21-27 significantly increased after β(2) GPI had interacted with DOPS. This change indicated a reduced interaction between domain I and domain V, inferring to a protrusion of the sequences 21-27 from the ring conformation. After β(2) GPI had interacted with CL for 30 min, the exchange levels in 4 of the 5 domains increased significantly. The deuteration levels of sequences 1-20, 21-27, 196-205, 273-279 and 297-306 increased, suggesting that these regions had become more exposed, and the domain I was no longer in contact with domain V. The increasing deuteration levels in sequences 70-86, 153-162, 191-198, 196-205 and 273-279 indicated β(2) GPI undergoing conformational changes to expose these inner regions, suggesting a structural transition. Overall, DOPS and CL induced minor conformational changes of β(2) GPI at sequences 21-27 and forms an intermediate conformation after 10 min of interaction. After a complete protein-lipid interaction, high negatively charged CL membrane induced a major conformation transition of β(2) GPI.