Interaction of beta2-Glycoprotein I with Members of the Low Density Lipoprotein Receptor Family.

The antiphospholipid syndrome (APS) is a non-inflammatory disease characterized by arterial and/or venous thrombosis and/or pregnancy morbidity in the presence of auto-antibodies that recognize beta2-glycoprotein I (beta2GPI) bound to cardiolipin. We have previously demonstrated that dimerization of beta2GPI by auto-antibodies induces platelet activation. This effect has been shown to be mediated by apoER2′. Here, we show that dimeric beta2GPI, and not plasma beta2GPI interacts with three different LDL-receptor family members; apolipoprotein E receptor 2′ (apoER2′), the low LDL-receptor related protein (LRP) and megalin. Interaction between dimeric beta2GPI and apoER2′ was best described with a one-site binding model (K_D(app) 25 nM), whereas for LRP and megalin the interaction with dimeric beta2GPI was best described with a two-site binding model, representing a high- (3.1 nM) and a low-affinity site (192.1 and 241.2 nM, respectively). Binding kinetics of a dimeric beta2GPI mutant, that binds poorly to anionic phospholipids were similar to the binding kinetics of full length dimeric beta2GPI for all three LDL-receptor family members. Upon deletion of domain I or domain II of dimeric beta2GPI no changes in the binding kinetics were observed. Deletion of domain V however, significantly decreased the affinity for apoER2′, LRP and megalin. In conclusion, our data show that dimeric beta2GPI can interact with different LDL-receptor family members. This interaction is dependent on a recognition site in domain V of beta2GPI, which does not overlap with the phospholipid-binding site.