Membrane-active properties of alpha-MSH analogs: aggregation and fusion of liposomes triggered by surface-conjugated peptides.
Reaction of the melanotropin hormone analogs [Nle(4),D-Phe(7)]-alpha-MSH and [Nle(4),D-Phe(7)]-alpha-MSH(4-10), which were extended at their N-terminus by a thiol-functionalized spacer arm, with preformed liposomes containing thiol-reactive (phospho)lipid derivatives resulted in the aggregation of the vesicles and in a partial leakage of their inner contents. This aggregation/leakage effect, which was only observed when the peptides were covalently conjugated to the surface of the liposomes, was correlated with the fusion of the vesicles as demonstrated by the observed decrease in resonance energy transfer between probes in a membrane lipid mixing assay. A limited fusion was confirmed by monitoring the mixing of the liposome inner contents (formation of 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylene bis(pyridinium bromide) complex). The membrane-active properties of the peptides could be correlated with changes in the fluorescence emission spectra of their tryptophan residue, which suggested that after their covalent binding to the outer surface of the liposomes they can partition within the core of the bilayers. A blue shift of 10 nm was observed for [Nle(4),D-Phe(7)]-alpha-MSH which was correlated with an increase in fluorescence anisotropy and with changes in the accessibility of the coupled peptide as assessed by the quenching of fluorescence of its tryptophan residue by iodide (Stern-Volmer plots). These results should be related to the previously described capacity of alpha-MSH, and analogs, to interact with membranes and with the favored conformation of these peptides which, via a beta-turn, segregate their central hydrophobic residues into a domain that could insert into membranes and, as shown here, trigger their destabilization.
de Souza
Debora Lima
DL
Laboratoire de Chimie Bioorganique, UMR 7514 CNRS/ULP, Université Louis Pasteur, Faculté de Pharmacie, Illkirch, France.
Frisch
Benoît
B
Duportail
Guy
G
Schuber
Francis
F
D000595:Amino Acid Sequence; D005452:Fluoresceins; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D008051:Lipid Bilayers / Q000737:chemistry*; D008081:Liposomes / Q000737:chemistry*; D008561:Membrane Fusion; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*; D000521:alpha-MSH / Q000031:analogs & derivatives*
D005452:Fluoresceins; D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D008081:Liposomes; D010455:Peptides; D000521:alpha-MSH; C007740:fluorexon