Protein-membrane interaction was monitored using a liposome co-floatation assay [29, 30] . E9 or DI/II proteins at a final concentration of 50 mg/ml were incubated in TAN buffer (pH 8.0) for 5 min at 28uC in the presence of 200 mg pr peptide/ml as indicated. Liposomes were then added to a final concentration of 1mM lipid and the samples were adjusted to pH 5.75 by the addition of 0.3 M MES or maintained at pH 8.0, and the incubation continued at 28uC for 30-60 min. The samples were then adjusted to 20% sucrose and loaded on top of a 300 ml cushion of 40% sucrose, then overlaid with 1.2ml 15% sucrose and 200 ml 5% sucrose. All sucrose solutions were at the same pH as the samples, and were wt/wt in TAN buffer at pH 8.0 or in MES buffer (50 mM MES, 100 mM NaCl) at pH 5.5. Gradients were centrifuged for 3 hr at 54,000 rpm at 4uC in a TLS55 rotor, and fractioned into the top 700 ml, middle 400 ml and bottom 1 ml. The 3 H-cholesterol marker was quantitated by scintillation counting. 200 ml of each fraction were precipitated with 10% trichloroacetic acid and analyzed by SDS-PAGE and western blotting [29] . Purified human secreted placental alkaline phosphatase with a ST affinity tag (Seap) was used as a control protein [67] , and was a kind gift from Yves Durocher, Biotechnology Research Institute, Montreal.