Recognition of B2GPI by the Lipoprotein Receptors.

Abstract 851

Lipoprotein receptors of the LDLR family serve as clearance receptors for beta2-glycoprotein-I (B2GPI) and as signaling receptors for the B2GPI/antibody complexes in patients with antiphospholipid syndrome (APS). B2GPI binds lipoprotein receptors through its domain V (B2GPI-DV) and the lipoprotein receptors use the LA modules to interact with B2GPI. The LA modules, which are about 40 residues each, are arranged in sequence to form the ligand-binding domains of the receptors. The LA modules have little sequence homology both within a single receptor and between different receptors, but adopt a very similar three dimensional structure. We have investigated the structural features of the contact interface in the B2GPI-DV/LA complexes and the characteristic features of the LA modules required for efficient binding to B2GPI. Using solution NMR spectroscopy, we have demonstrated that three different LA modules bind to the same site on B2GPI-DV and that the binding site for B2GPI-DV on these LA modules is centered on the residues involved in coordination of the calcium ion. The dissociation constants measured for different LA complexes are in the range from 1 mM to approximately 100 mM. The difference in binding affinity is attributed to the auxiliary contacts that augment the interactions of the residues at the core of the binding interface. We have solved the structure of the B2GPI-DV/LA4 complex by molecular docking guided by NMR-derived restraints and have validated the structure by four independent experimental observations based on mutagenesis, solution NMR spectroscopy, fluorescence polarization and intrinsic tryptophan fluorescence measurements. The structure identified three lysine residues of B2GPI-DV, Lys308, Lys317 and Lys282, as critical for binding the LA modules. Our data shows that not all the LA modules can bind B2GPI-DV and that those LA modules capable of binding B2GPI-DV will interact with it in a similar fashion as observed in the B2GPI-DV/LA4 complex. Whether an LA module is capable of binding B2GPI could be distinguished based on the primary sequence in the vicinity of the calcium-coordinating residues. According to the current hypothetical model, the binding of B2GPI to anionic phospholipids in the presence of APS-related antibodies is the first event leading to the pathology of APS. However, we have shown that the LA modules interfere with binding of B2GPI-DV to cardiolipin indicating that B2GPI-DV cannot simultaneously bind to anionic phospholipids and lipoprotein receptors.