Liposomes were prepared by freeze-thaw and extrusion through 200 nm polycarbonate filters [66] , and were stored at 4uC in TAN buffer under N 2 and used within 2 weeks of preparation. Liposomes were composed of a 1:1:1:3 molar ratio of 1-palmitoyl-2-oleoylsn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-snglycero-3-phosphoethanolamine (POPE), sphingomyelin (bovine brain) (Avanti Polar Lipids; Alabaster, AL), and cholesterol (Steraloids, Inc.; Wilton, NH), plus trace amounts of 3 H-cholesterol (Amersham; Arlington Heights, IL).