Direct Monolayer Packing Imbalance and Phospholipid Flip-Flop: Two Mechanisms of Local Bilayer Deformation. Application to Mitochondrial Cristae of Wild-Type and Cardiolipin-Deficient Mutant

Abstract

We have shown that a spatially localized transmembrane pH gradient, produced by HCl micro-injection near the external surface of cardiolipin(CL)-containing giant unilamellar vesicles (GUVs), promotes the formation of tubules that retract after collapse of this gradient. Such tubules have morphologies similar to mitochondrial cristae. The tubulation effect is due to direct phospholipid packing modification in the outer leaflet, that is associated with protonation of CL headgroups (1). We have also compared the case of CL-containing GUVs with that of phosphatidylglycerol (PG)-containing GUVs. Local HCl micro-injection also promotes formation of tubules in the latter. However, compared to CL-containing GUVs the tubules are longer, undergo a visible pearling and have a much longer retraction time after acid micro-injection is stopped (2). We attribute these differences to an additional mechanism that increases monolayer lipid density imbalance, namely inward PG flip-flop promoted by the local transmembrane pH-gradient. Simulations using a fully non-linear membrane model, that describes the bilayer by its midsurface shape and by a lipid density field for each monolayer (3), confirm this hypothesis. Interestingly, among yeast mutants deficient in CL biosynthesis, only the crd1-null mutant, which accumulates PG, displays significant mitochondrial activity, in agreement with our data. Our work emphasizes the importance of the transmembrane proton gradient in cristae morphology as well as the salient role of specific lipids in mitochondrial function.1. Khalifat, N., N. Puff, S. Bonneau, J. B. Fournier, and M. I. Angelova. 2008. Biophys. J. 95:4924-4933.2. Khalifat, N.; Rahimi, M.; Bitbol, A. F.; Seigneuret, M.; Fournier, J. B.; Puff, N.; Arroyo, M.; Angelova, M. I. 2014 Biophys. J. 107: 879-8903. Rahimi, M. and Arroyo, M., 2012. Physical Review E, 86: 011932.