Computational insight in the role of fusogenic lipopeptides at the onset of liposome fusion

• We used coarse-grained molecular dynamics (MARTINI) for pioneering simulations of fusogenic lipid–PEG–peptide hybrids.
• Lipopeptide interactions with membranes of varying composition were studied.
• Single lipopeptides always quickly bind to the membrane used in fusion experiments.
• We anticipate a key role for unbound lipopeptide aggregates in the locking mechanism prior to fusion.
• The observed partitioning–folding coupling highlights the role of the membrane as a template.

We performed an extensive computational study to obtain insight in the molecular mechanisms that take place prior to membrane fusion. We focused on membrane-anchored hybrid macromolecules (lipid–polymer–oligopeptide) that mimic biological SNARE proteins in terms of liposome fusion characteristics [H. Robson Marsden et al., 2009]; efficient micro-second simulation was enabled by combining validated MARTINI force fields for the molecular building blocks in coarse-grained molecular dynamics (CGMD). We find that individual peptide domains in the hybrid macromolecules bind and partially integrate parallel to the membrane surface, in agreement with experimental findings. By varying several experimental design parameters, we observe that peptide domains remain in the solvent phase only in two cases: (1) for solitary lipopeptides (low concentration), below a threshold area per lipid in the membrane, and (2) when the lipopeptide concentration is high enough for the peptide domains to self-assemble into tetrameric homo-complexes. The peptide–membrane binding is not affected by solvent-induced peptide unfolding, which we mimicked by relaxing the usual MARTINI helix constraints. Remarkably, in this case, a reverse transition to a helical secondary structure is observed after binding, highlighting the role of the membrane as a template (partitioning–folding coupling). Our findings undermine the current view of the initial stages towards fusion, in which membranes are thought to be kept in close apposition via dimerization of individual complementary peptides in the solvent phase. Although we did not study actual fusion, our simulations show that the formation of homomers, which is suppressed in experimental peptide-pair design and therefore believed to be insignificant for fusion, by peptides anchored to the same membrane does play a key role in this locking mechanism and potentially also in membrane destabilization that precede fusion.

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