Fusion of phospholipid vesicles produced by the anti-tumour protein α-sarcin

Abstract

The anti-tumour protein α-sarcin causes fusion of bilayers of phospholipid vesicles at neutral pH. This is demonstrated by measuring the decrease in the efficiency of the fluorescence energy transfer between N-(7-nitro-2-1,3-benzoxadiazol-4-yl)-dimyristoylphosphatidylethanolami ne (NDB-PE) (donor) and N-(lissamine rhodamine B sulphonyl)-diacylphosphatidylethanolamine (Rh-PE) (acceptor) incorporated in dimyristoylphosphatidylcholine (DMPG) vesicles. The effect of α-sarcin is a maximum at 0.15 M ionic strength and is abolished at basic pH. α-Sarcin promotes fusion between 1,6-diphenylhexa-1,3,5-triene (DPH)-labelled DMPG and dipalmitoyl-PG (DPPG) vesicles, resulting in a single thermotropic transition for the population of fused phospholipid vesicles. Bilayers composed of DMPG and DMPG, at different molar ratios in the range 1:1 to 1:10 PC/PG, are also fused by α-sarcin. Freeze-fracture electron micrographs corroborate the occurrence of fusion induced by the protein. α-Sarcin also modifies the permeability of the bilayers, causing the leakage of calcein in dye-trapped PG vesicles. All of the observed effects reach saturation at a 50:1 phospholipid/protein molar ratio, which is coincident with the binding stoichiometry previously described.