Helix-helix association of a lipid-bound amphipathic alpha-helix derived from apolipoprotein C-II

The interaction of a peptide derived from the sequence of apolipoprotein C- II (apoC-II) with a model lipid surface has been investigated by fluorescen ce spectroscopy. APoC-II19-391 labeled at the N-terminus with 7-nitrobenz-2 -oxa-1,3-diazole (NBD), bound to small unilamellar vesicles of phosphatidyl choline with a dissociation constant of 6 mu M. The lipid-bound NBD-labeled peptide exhibited a rededge excitation shift in its emission maximum and a nisotropy, consistent with insertion of the probe into the motionally restr icted, polar environment provided by the bilayer interface. The small Stoke s shift of the NBD fluorophore permits electronic energy homotransfer betwe en peptides on the lipid surface and results in depolarization of the NBD e mission. At high surface densities of lipid-bound peptide, the anisotropy o f the NBD probe was 33% lower than in corresponding samples in which electr onic energy homotransfer was prevented by the addition of an unlabeled pept ide. The efficiency of energy transfer between probes was not consistent wi th a random distribution of peptides on the lipid surface, indicating inste ad the self-association of lipid-bound apoC-II19-39. We propose that the ro le of this sequence in apoC-II is not only to mediate binding of protein to a lipid surface, but also to stabilize the lipoprotein complexes by associ ating with other amphipathic helices within apoC-II and with other apolipop roteins.