Abstract
Complexes formed between beta2GPI (beta2-glycoprotein I), a human plasma protein, and biological membranes are considered to be targets of macrophages and antiphospholipid autoantibodies involved in autoimmune diseases, such as antiphospholipid syndrome or systemic lupus erythematosus. The positively charged lysine-rich fifth domain of beta2GPI facilitates its interaction with phospholipid membranes containing acidic phospholipids, which normally become exposed by apoptotic processes. In the present study, atomic force microscopy was applied to visualize the binding of beta2GPI to a mixed phospholipid model membrane at physiological ionic strength. On supported lipid bilayers the formation of supramolecular assemblies of the protein with a height of approx. 3.3 nm was observed, suggesting a lateral agglomeration of beta2GPI. Detailed analysis of kinetic constants using surface plasmon resonance revealed that the binding can be described by a two-state reaction model, i.e. a very fast interaction step, depending on the content of acidic phospholipids in the bilayer, and a second step with significantly lower k(on) and k(off) values. Taken together, our results suggest a biphasic interaction mechanism: a fast step of beta2GPI binding to negatively charged lipids, mainly based on electrostatic interactions, and a slower phase of agglomeration of the protein on the bilayer surface accompanied by a protein-induced rigidification of the membrane, as revealed by electron paramagnetic resonance.