High affinity binding of β2-glycoprotein I to human endothelial cells is mediated by annexin II

beta(2)-Glycoprotein I (beta(2)GPI) is an abundant plasma phospholipid-binding protein and an autoantigen in the antiphospholipid antibody syndrome. Binding of beta(2)GPI to endothelial cells targets them for activation by anti-beta(2)GPI antibodies, which circulate and are associated with thrombosis in patients with the antiphospholipid antibody syndrome. However, the binding of beta(2)GPI to endothelial cells has not been characterized and is assumed to result from association of beta(2)GPI with membrane phospholipid, Here, we characterize the binding of beta(2)GPI to endothelial cells and identify the beta(2)GPI binding site. I-125-beta(2)GPI bound With high affinity (K-d similar to 18 nM) to human umbilical vein endothelial cells (HUVECs), Using affinity purification, we isolated beta(2)GPI-binding proteins of similar to 78 and similar to 36 kDa from HUVECs and EAHY.926 cells. Amino acid sequences of tryptic peptides from each of these were identical to sequences within annexin II. A role for annexin II in binding of beta(2)GPI to cells was confirmed by the observations that annexin II-transfected HEK 293 cells bound similar to 10-fold more I-125-beta(2)GPI than control cells and that anti-annexin II antibodies inhibited the binding of I-125 beta(2)GPI to HUVECs by similar to 90%. Finally, surface plasmon resonance studies revealed high affinity binding between annexin II and beta(2)GPI. These results demonstrate that annexin II mediates the binding of beta(2)GPI to endothelial cells.