Lipid-Interacting Properties Of The N-Terminal Domain Of Human Apolipoprotein C-III

The lipid-interacting properties of the N-terminal domain of human apolipoprotein
C-III (apo C-III) were investigated. By molecular modeling, we predicted that the
6-20 fragment of apo C-III is obliquely orientated at the lipid/water interface
owing to an asymmetric distribution of the hydrophobic residues when helical.
This is characteristic of 'tilted peptides' originally discovered in viral fusion
proteins and later in various proteins including some involved in lipoprotein
metabolism. Since most tilted peptides were shown to induce liposome fusion in
vitro, the fusogenic capacity of the 6-20 fragment of apo C-III was tested on
unilamellar liposomes and compared with the well characterized SIV fusion
peptide. Mutants were designed by molecular modeling to assess the role of the
hydrophobicity gradient in the fusion. FTIR spectroscopy confirmed the
predominantly helical conformation of the peptides in TFE solution and also in
lipid-peptide complexes. Lipid-mixing experiments showed that the apo C-III
(6-20) peptide is able to increase the fluorescence of a lipophilic fluorescent
probe. The vesicle fusion was confirmed by core-mixing and leakage assays. The
hydrophobicity gradient plays a key role in the fusion process because the mutant
with no hydrophobic asymmetry but the same mean hydrophobicity as the wild type
does not induce significant lipid fusion. The apo C-III (6-20) fragment is,
however, less fusogenic than the SIV peptide, in agreement with their respective
mean hydrophobicity. Since lipid fusion should not be the physiological function
of the N-terminal domain of apo CIII, we suggest that its peculiar distribution
of hydrophobic residues is important for the lipid-binding properties of apo
C-III and should be involved in apolipoprotein and lipid exchanges crucial for
triglyceride metabolism.