Interaction of β2-Glycoprotein 1 with Phosphatidylserine-Containing Membranes: Ligand-Dependent Conformational Alterations Initiate Bivalent Binding

Beta2-glycoprotein 1 (beta2GP1), a 50 kDa serum glycoprotein that binds anionic phospholipid-containing membranes, plays a regulatory role in physiology and pathology. The protein is a member of the short consensus repeat (SCR) superfamily containing four typical repeating domains and an aberrant fifth domain constructed into an SCR-like core at the C-terminus. To investigate the contribution of the individual domains to the binding of beta2GP1, a series of sequential domain-deleted recombinant protein fragments were generated and assessed for their interaction with PS-containing vesicles. Spectral analyses of lipid binding-dependent alterations in tryptophan emission spectra revealed that the (single) tryptophan residues of the individual domains underwent binding-dependent conformational alterations. Depending on the ionic strength, some domains moved from polar to nonpolar environments, while others moved from less polar to more polar environments. Analysis of a series of acrylamide quenching and resonance energy transfer experiments indicated that the binding of N-terminal domain 1 to PS membranes exists in two, ionic strength-dependent, conformations. At low ionic strengths, domain 1 bound to the vesicles and induced their precipitation and/or aggregation. At physiologic ionic strengths, domain 1 detached from the membrane surface while the remaining domains maintained their association with the membrane. Under these conditions, membrane-bound conformationally altered domain 1 projects away from the membrane surface, enabling it to interact with other proteins and/or cell surface ligands or receptors.