Mapping Interactions Between B2GPI and the Lipoprotein Receptors by Solution NMR Spectroscopy.

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by venous or arterial thrombosis and recurrent pregnancy loss. Beta2 glycoprotein I (B2GPI) is the major target for these antibodies. Current evidence suggests that B2GPI acquires pathological properties leading to APS after association with antibodies. It was demonstrated that APS related antibodies induce activation of endothelial cells, monocytes and platelets, although the identity of a cell-surface receptors that transmit the antibody-induced signaling inside cells remain unclear. Biochemical studies have shown that the receptors of the low-density lipoprotein receptor (LDLR) family are all capable to bind B2GPI/antibody complexes. These receptors are abundantly expressed by many cell types. The ligand-binding activity of the lipoprotein receptors is confined primarily to structurally homologous LA modules, which are arranged in sequence to form a ligand-binding domain. We investigated at the atomic level the features of the ligand-binding LA modules required for interaction with B2GPI. We sought to determine whether the same residues that form the ligand-binding pocket on the LA modules in the crystal structure of a RAP/LA3-4 complex also recognize B2GPI. We demonstrated that the same residues that form the ligand-binding pocket on LA4 in the crystal structure of the RAP/LA3-4 complex comprise the contact site for B2GPI. The domain 5 from B2GPI (B2GPI-D5) was expressed and labeled for NMR studies. The residues of B2GPI-D5 that are perturbed when B2GPI-D5 was titrated with LA4 are located at the C-terminus of domain 5. Using NMR relaxation measurements, we determined that B2GPI-D5 binds LA4 with 1:1 stoichiometry. The estimated Kd of the B2GPI-D5/LA4 complex is about 30 uM. NMR experimental data was used to build a docking model of the complex.