Fusogenic Tilted Peptides Induce Nanoscale Holes In Supported Phosphatidylcholine Bilayers

Tilted peptides are known to insert in lipid bilayers with an oblique
orientation, thereby destabilizing membranes and facilitating membrane fusion
processes. Here, we report the first direct visualization of the interaction of
tilted peptides with lipid membranes using in situ atomic force microscopy (AFM)
imaging. Phase-separated supported
dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) bilayers
were prepared by fusion of small unilamellar vesicles and imaged in buffer
solution, in the absence and in the presence of the simian immunodeficiency virus
(SIV) peptide. The SIV peptide was shown to induce the rapid appearance of
nanometer scale bilayer holes within the DPPC gel domains, while keeping the
domain shape unaltered. We attribute this behavior to a local weakening and
destabilization of the DPPC domains due to the oblique insertion of the peptide
molecules. These results were directly correlated with the fusogenic activity of
the peptide as determined using fluorescently labeled DOPC/DPPC liposomes. By
contrast, the nontilted ApoE peptide did not promote liposome fusion and did not
induce bilayer holes but caused slight erosion of the DPPC domains. In
conclusion, this work provides the first direct evidence for the production of
stable, well-defined nanoholes in lipid bilayer domains by the SIV peptide, a
behavior that we have shown to be specifically related to the tilted character of
the peptide. A molecular mechanism underlying spontaneous insertion of the SIV
peptide within lipid bilayers and the subsequent removal of bilayer patches is
proposed, and its relevance to membrane fusion processes is discussed.