INTERACTIONS OF ANNEXINS WITH MEMBRANE PHOSPHOLIPIDS

The annexins are proteins that bind to membranes and can aggregate vesicles and modulate fusion rates in a Ca2+-dependent manner. In this study, experiments are presented that utilize a pyrene derivative of phosphatidylcholine to examine the Ca2+-dependent membrane binding of soluble human annexin V and other annexins. When annexin V and other annexins were bound to liposomes containing 5 mol % acyl chain labeled 3-palmitoyl-2-(1-pyrenedecanoyl)-L-alpha-phosphatidylcholine, a decrease in the excimer-to-monomer fluorescence ratio was observed, indicating that annexin binding may decrease the lateral mobility of membrane phospholipids without inducing phase separation. The observed increases of monomer fluorescence occurred only with annexins and not with other proteins such as parvalbumin or bovine serum albumin. The extent of the increase of monomer fluorescence was dependent on the protein concentration and was completely and rapidly reversible by EDTA. Annexin V binding to phosphatidylserine liposomes was consistent with a binding surface area of 59 phospholipid molecules per protein. Binding required Ca2+ concentrations ranging between approximately 10 and 100-mu-M, where there was no significant aggregation or fusion of liposomes on the time scale of the experiments. The polycation spermine also displaced bound annexins, suggesting that binding is largely ionic in nature under these conditions.