Lipid headgroup spacing and peptide penetration, but not peptide oligomerization, modulate peptide-induced fusion

In this study, the mechanism by which an amphipathic negatively charged pep tide consisting of 11 amino acids (WAE) induces fusion of liposomal phospha tidylcholine membranes is investigated. WAE-induced fusion, which only occu rs when the peptide is covalently attached to the bilayer, shows a highly r emarkable dependence on naturally occurring phosphatidylcholine species. Th e initial rate of fusion increased in the order l-palmitoyl 2-arachidonoyl PC (PAPC) > 1-palmitoyl 2-oleoyl PC (POPC) > 1-stearoyl 2-oleoyl PC (SOPC) > dioleoyl PC (DOPC) > egg yolk PC. interestingly, the susceptibility of th e various PC species toward WAE-induced fusion matched a similar order of i ncrease in intrinsic lipid headgroup spacing of the target membrane. The de gree of spacing, in turn, was found to be related to the extent by which th e fluorescence quantum yield of the Trp residue increased, which occurred u pon the interaction of WAE with target membranes. Therefore, these results demonstrate an enhanced ability for WAE to engage in hydrophobic interactio ns when headgroup spacing increases. Thus, this latter parameter most likel y regulates the degree of penetration of WAE into the target membrane. Apar t from penetrating, WAE oligomerizes at the site of fusion as revealed by m onitoring the self-quenching of the fluorescently derivatized lipid anchor to which WAE is attached. Clustering appears specifically related to the pr ocess of membrane fusion and nor membrane aggregation. This is indicated by the fact that fusion and clustering, but not aggregation, display the same strict temperature dependence. However, evidence is presented indicating t hat clustering is an accompanying event rather than a prerequisite for fusi on. The notion that various biologically relevant fusion phenomena are acco mpanied by protein clustering and the specific PC-species-dependent regulat ion of membrane fusion emphasize the biological significance of the peptide in serving as a model for investigating mechanisms of protein-induced fusi on.