The aggregation state of mellitin in lipid bilayers. An energy transfer study.

We used fluorescence non-radiative energy transfer to measure the self-association of melittin in solution and when bound to lipid bilayers. Energy transfer occurred from the tryptophan residue of unlabeled melittin to an N-methyl anthraniloyl residue covalently bound to a basic lysine residue on melittin. The extent of energy transfer from tryptophan to the label was found to increase severalfold upon the salt-induced tetramerization of melittin. When bound to vesicles of dimyristoyl-L-alpha-phosphatidylcholine, the extent of energy transfer was found to be equivalent to that of monomeric melittin, irrespective of the presence of monomeric or tetrameric melittin in the aqueous phase. We conclude that membrane-bound melittin is monomeric.