Partially reversible adsorption of annexin A1 on POPC/POPS bilayers investigated by QCM measurements, SFM, and DMC simulations

The kinetics of annexin A1 binding to solid-supported lipid bilayers consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS; 4:1) has been investigated as a function of the calcium ion concentration in the bulk phase. Quartz crystal microbalance measurements in conjunction with scanning force microscopy, fluorescence microscopy, and computer simulations indicate that at a given Ca2+ concentration annexin A1 adsorbs irreversibly on membrane domains enriched in POPS. By contrast, annexin A1 adsorbs reversibly on the POPC-enriched phase, which is composed of single POPS molecules embedded within a POPC matrix. The overall area occupied by the POPS-enriched phase is controlled by the CaCl2 concentration. Monte Carlo simulations suggest that the area of the POPS-enriched phase increases by a factor of 7 when the Ca2+ concentration is changed from 0.01 to 1 mM.