title: Beta2-glycoprotein I can exist in 2 conformations: implications for our understanding of the antiphospholipid syndrome.
abstract: The antiphospholipid syndrome is defined by the presence of antiphospholipid antibodies in blood of patients with thrombosis or fetal loss. There is ample evidence that beta(2)-glycoprotein I (beta(2)GPI) is the major antigen for antiphospholipid antibodies. The autoantibodies recognize beta(2)GPI when bound to anionic surfaces and not in solution. We showed that beta(2)GPI can exist in at least 2 different conformations: a circular plasma conformation and an "activated" open conformation. We also showed that the closed, circular conformation is maintained by interaction between the first and fifth domain of beta(2)GPI. By changing pH and salt concentration, we were able to convert the conformation of beta(2)GPI from the closed to the open conformation and back. In the activated open conformation, a cryptic epitope in the first domain becomes exposed that enables patient antibodies to bind and form an antibody-beta(2)GPI complex. We also demonstrate that the open conformation of beta(2)GPI prolonged the activated partial thromboplastin time when added to normal plasma, whereas the activated partial thromboplastin time is further prolonged by addition of anti-beta(2)GPI antibodies. The conformational change of beta(2)GPI, and the influence of the autoantibodies may have important consequences for our understanding of the antiphospholipid syndrome.
authors.affiliation: Department of Experimental Vascular Medicine, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Cetin
authors.initials: C
authors.lastname: Agar
authors.forename: Gwendolyn M A
authors.initials: GM
authors.lastname: van Os
authors.forename: Matthias
authors.initials: M
authors.lastname: Mörgelin
authors.forename: Richard R
authors.initials: RR
authors.lastname: Sprenger
authors.forename: J Arnoud
authors.initials: JA
authors.lastname: Marquart
authors.forename: Rolf T
authors.initials: RT
authors.lastname: Urbanus
authors.forename: Ronald H W M
authors.initials: RH
authors.lastname: Derksen
authors.forename: Joost C M
authors.initials: JC
authors.lastname: Meijers
authors.forename: Philip G
authors.initials: PG
authors.lastname: de Groot
mesh_terms: D017152:Antibodies, Antiphospholipid / Q000276:immunology* / Q000302:isolation & purification; D000925:Anticoagulants / Q000494:pharmacology; D016736:Antiphospholipid Syndrome / Q000378:metabolism* / Q000473:pathology; D002308:Cardiolipins / Q000378:metabolism; D006801:Humans; D010314:Partial Thromboplastin Time; D011487:Protein Conformation; D017510:Protein Folding; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D020349:Surface Plasmon Resonance; D053482:beta 2-Glycoprotein I / Q000737:chemistry* / Q000235:genetics
chemical_list: D017152:Antibodies, Antiphospholipid; D000925:Anticoagulants; D002308:Cardiolipins; D053482:beta 2-Glycoprotein I