beta(2)-glycoprotein I-dependent alterations in membrane properties

beta (2)-Glycoprotein I (beta (2)GP1), a 50 kDa serum glycoprotein, binds a nionic phospholipids and plays a role in phosphatidylserine (PS)-dependent coagulation and apoptotic processes. To characterize the molecular conseque nces that occur to target membranes upon binding of beta (2)GP1, the intera ction between beta (2)GP1 and PS-containing vesicles was investigated by fl uorescent spectroscopy. Membranes containing pyrene-labeled lipid showed th at binding of beta (2)GP1 induced a decrease in excimer/monomor ratios (E/M ) of the target membrane. Although these membrane alterations occurred in i sotonic buffer, the effects were greater in low ionic strength buffer and w ere coincident to membrane precipitation. In contrast, increases in membran e polarization were only seen in low ionic strength buffer. Analysis of bet a (2)GP1 binding kinetics by resonance energy transfer between fluorescein- labeled beta (2)GP1 and rhodamine-containing PS vesicles revealed a two-com ponent process: (1) a primary and rapid binding via the C-terminus that occ urred < 2 s in both isotonic and low ionic strength buffers, and (2) a sequ ential binding of the N-terminus that was <similar to> 100-fold slower in l ow ionic strength solution. Taken together, these data suggest that beta (2 )GP1 alters the fluidity and membrane polarization of its target membrane, which in low ionic strength buffer is of sufficient magnitude to induce pre cipitation. (C) 2000 Elsevier Science B.V. All rights reserved.