FUSION OF DIOLEOYLPHOSPHATIDYLCHOLINE VESICLES INDUCED BY AN AMPHIPHILIC CATIONIC PEPTIDE AND OLIGOPHOSPHATES AT NEUTRAL PH

Peptide E5 is an analogue of the fusion peptide of influenza virus hem agglutinin and K5 is a cationic peptide which has an arrangement of el ectric charges complementary to that of E5. We reported that a stoichi ometric mixture of E5 and K5 caused fusion of large unilamellar vesicl es (LUV) of neutral phospholipids (Murata, M., Kagiwada, S., Takahashi , S. and Ohnishi, S. (1991) J. Biol. Chem. 266, 14353-14358). K5 cause d fusion of LUV composed of dioleoylphosphatidylcholine (DOPC) at pH > 10, but not at neutral pH. In the presence of oligophosphates, such a s 1 mM ATP, GTP, or polyphosphate, K5 caused rapid and efficient fusio n of DOPC LUV at neutral pH without hydrolysis of oligophosphate group s, but another anions such as citrate, acetate, AMP, phosphate, or EDT A were ineffective. The peptide/oligophosphate-induced fusion behavior s have been investigated by a fluorescence resonance energy transfer a ssay for lipid mixing of LUV and negative staining electron microscopy . At higher ionic strengths (> 0.3 M KCl) or in the presence of 5.0 mM MgCl2, the fusion was inhibited. Even at the inhibitory conditions, t he association of K5 with lipid vesicles at neutral pH was directly co nfirmed by the Ficoll gradient assay method and by blue shifts of the tryptophan fluorescence of the peptide. A nonhydrolyzable GTP analogue , GTPgammaS, also induced fusion. These observations suggested that th e electrostatic interactions between the positive and negative charges of K5 and oligophosphate, respectively, induced complex formation, tr iggering membrane fusion.