Competition of Annexin V and Anticardiolipin Antibodies for Binding to Phosphatidylserine Containing Membranes

Annexin V, an intracellular protein with a calcium-dependent high affinity for anionic phospholipid membranes, acts as an inhibitor of lipid-dependent reactions of the blood coagulation. Antiphospholipid antibodies found in the plasma of patients with antiphospholipid syndrome generally do not interact with phospholipid membranes directly, but recognize (plasma) proteins associated with lipid membranes, mostly prothrombin or β2-glycoprotein I (β2GPI). Previously, it has been proposed that antiphospholipid antibodies may cause thrombosis by displacing annexin V from procoagulant cell surfaces. We used ellipsometry to study the binding of annexin V and of complexes of β2GPI with patient-derived IgG antibodies to β2GPI, commonly referred to as anticardiolipin antibodies (ACA), to phospholipid bilayers composed of phosphatidylcholine (PC) and 20% phosphatidylserine (PS). More specifically, we investigated the competition of these proteins for the binding sites at these bilayers. We show that ACA−β2GPI complexes, adsorbed to PSPC bilayers, are displaced for more than 70% by annexin V and that annexin V binding is unaffected by the presence of ACA−β2GPI complexes. Conversely, annexin V preadsorbed to these bilayers completely prevents adsorption of ACA−β2GPI complexes, and none of the preadsorbed annexin V is displaced by ACA−β2GPI complexes. Using ellipsometry, we also studied the effect of ACA−β2GPI complexes on the interaction of annexin V with the membranes of ionophore-activated blood platelets as a more physiological relevant model of cell membranes. The experiments with blood platelets confirm the high-affinity binding of annexin V to these membranes and unequivocally show that annexin V binding is unaffected by the presence of ACA−β2GPI. In conclusion, our data unambiguously show that ACA−β2GPI complexes are unable to displace annexin V from procoagulant membranes to any significant extent, whereas annexin V does displace the majority of preadsorbed ACA−β2GPI complexes from these membranes.