β₂-Glycoprotein I-dependent alterations in membrane properties

β₂-Glycoprotein I (β₂GP1), a 50 kDa serum glycoprotein, binds anionic phospholipids and plays a role in phosphatidylserine (PS)-dependent coagulation and apoptotic processes. To characterize the molecular consequences that occur to target membranes upon binding of β₂GP1, the interaction between β₂GP1 and PS-containing vesicles was investigated by fluorescent spectroscopy. Membranes containing pyrene-labeled lipid showed that binding of β₂GP1 induced a decrease in excimer/monomor ratios (E/M) of the target membrane. Although these membrane alterations occurred in isotonic buffer, the effects were greater in low ionic strength buffer and were coincident to membrane precipitation. In contrast, increases in membrane polarization were only seen in low ionic strength buffer. Analysis of β₂GP1 binding kinetics by resonance energy transfer between fluorescein-labeled β₂GP1 and rhodamine-containing PS vesicles revealed a two-component process: (1) a primary and rapid binding via the C-terminus that occurred < 2 s in both isotonic and low ionic strength buffers, and (2) a sequential binding of the N-terminus that was ~100-fold slower in low ionic strength solution. Taken together, these data suggest that β₂GP1 alters the fluidity and membrane polarization of its target membrane, which in low ionic strength buffer is of sufficient magnitude to induce precipitation. (C) 2000 Elsevier Science B.V.