Phosphatidylglycerol Asymmetry and Translocation in Lipid Membranes

Abstract

Our current understanding of the structure and dynamics of cellular membranes emerged in the early 1970 's. However, there is still much we do not know about the underlying lipid dynamics in cellular membranes, specifically the process of lipid translocation or flip-flop. The flip-flop rates of only a few lipid species, principally phosphatidylcholines (PCs), have been measured. Anionic lipids, such as phosphatidylseryine (PS), phosphatidylinositol (PI), and phosphatidylglycerol (PG), are known to be play active roles in membrane function, but almost nothing is known of the rates of translocation of these species. While PS and PI are the major anionic lipids of eukaryotic cellular membranes, PG is mainly found in prokaryotic membranes and comprises about 10% of the phospholipid content of the mitochondria membrane in eukaryotes. In the work presented here, the native flip-flop rates of 1,2-diasterol-sn-glycero-3-[phospho-(1′-rac-glycerol)] (DSPG) flip-flop in mixed DSPG: 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) bilayers have been investigated. Using methods of classical surface chemistry coupled with nonlinear optical methods, we have developed a novel analytical approach, using sum-frequency vibrational spectroscopy (SFVS), to selectively probe lipid compositional asymmetry in a planar supported lipid bilayer. This new method allows for the detection of lipid flip-flop kinetics and compositional asymmetry without the need for a fluorescent or spin-labeled lipid species by exploiting the coherent nature of SFVS. Using SFVS, the rates of DSPG flip-flop in a DSPC matrix have been examined for the first time. Analysis of the dynamics provides an assessment of the underlying energetic barrier to PG translocation. The results will be discussed in the framework of the protein-free energetic barriers to PG flip-flop and the role of electrostatics in this process.