The Mechanism of Membrane Fusion and Lipid Flip/Flop Explored with Model Transmembrane Helices
Biochemistry
Final Report Abstract
Our results reveal that a conformationally flexible transmembrane helix promotes outer leaflet mixing and lipid splay more strongly than a conformationally rigid one. The lipid dependence of basal as well as of TMD-driven lipid mixing and splay suggests that the cone-shaped phosphatidylethanolamine stimulates basal fusion via enhancing lipid splay and that the negatively charged phosphatidylserine inhibits fusion via electrostatic repulsion. Phosphatidylserine also strongly differentiates basal and helix-driven fusion which is related to its preferred interaction with the conformationally more flexible transmembrane helix. Simulations confirm the differential flexibility of both TMDs in a bilayer and relate this to differential lipid association. Further, they suggest that differential lipid association translates into different rates of lipid diffusion around the TMDs and different extents of lipid splay. Altogether, our results show that the lipid splay hypothesis can be confirmed experimentally and the mechanism leading to TMD-mediated splay can be resolved computationally. Specifically, our MD simulations reproduce ratios of cross-relaxation rates in correspondence to ratios determined by 1H-1H NOESY NMR. This validates the simulations and allows to characterize the lipid dynamics underlying the decorrelation. The results from MD indicate that the number of close contacts between lipid tail methyl and DOPC choline methyl groups as well as dynamic features, such as lateral diffusion, play an important role for relative cross-relaxation rates. Further, the lipid distribution around the charged TMD flanks indicates that lipid preference might depends on the flexibility of the TMD helix. Finally, the fusogenic LV16 TMDs clearly enhance splay of PC tails relative to the L16 in a ternary bilayer. It is clear that protrusion probability, range of bilayer perturbation as well as the differential effects of the TMDs depend on the lipid composition. Since the activity of the SNARE protein synaptobrevin in cellular secretion is influenced by the conformational flexibility of its TMD, we expect that connecting the conformational flexibility of the model TMDs used here to their interaction with lipids, their induction of lipid splay, and membrane fusion may improve our mechanistic understanding of how TMDs contribute to the function of natural fusogens.
Publications
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(2018). Glycine perturbs local and global conformational flexibility of a transmembrane helix. Biochemistry, 57(8), 1326–1337
Högel, P., Götz, A., Kuhne, F., Ebert, M., Stelzer, W., Rand, K. D., Langosch, D.
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(2018). Transmembrane Helix Induces Membrane Fusion through Lipid Binding and Splay. J Phys Chem Lett 9, 3181-3186
Scheidt, H.A., Kolocaj, K., Veje Kristensen, J., Huster, D., and Langosch, D.
