Photoliposomes; Suitable Platforms for Studying the Order and Dynamics of Lipid Bilayers and Integral Proteins by MAS-NMR Spectroscopy
Authors:   , Doroudgar Mahmoudreza 
Published: May 2022

The phospholipid bilayers are the primary constituents of the membrane in living cells in which lipids are hold together in bilayer leaflets through a combination of different forces into the liquid crystalline (Lα) phase. Despite their thin fragile formations, the phospholipid bilayers are responsible for performing a variety of important tasks in the cells, some of which are carried out directly by the lipid bilayers and some by various integral proteins embedded within the bilayers. There have been continues efforts over the past decades to replicate the compound biophysical properties of living cell membranes in model lipid bilayers. An important question remains unanswered: is it possible to replicate physical properties under “non-equilibrium” conditions as found in cell membranes in model lipid bilayers? In almost all previous studies, the model lipid bilayers were under static conditions – for instance, at zero lateral pressure. However, in living organisms, the cell membranes are involved in continuous (nonequilibrium) exchange and (or) transport of lipid species with the surrounding environment which consequently leads them to experience continuous lateral pressure variations. One suitable in vitro approach is to spatiotemporally control the model lipid bilayers over a time period during which they can be spatially stimulated at a level compatible to that found under in vivo conditions. This can be achieved with high spatiotemporal resolution by making lipids light-dependent through implementation of azobenzene photoswitch in their structures. In this study, a specific azobenzene containing photolipid (AzoPC) is integrated into POPE:POPG bilayers (POPE: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine, POPG: 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol)) at ~14 mol% to construct a photo responsive model bilayers entitled as photoliposomes. Magic angle spinning solid-state NMR spectroscopy (MAS-NMR) at high field (850 MHz) is the measurement technique of choice by which it is possible to pursue the dynamics (fluidity) of the bulk lipids within the photoliposomes at atomistic resolution. It is shown that the AzoPCs undergo an efficient trans-to-cis isomerization (~85%) within the photoliposomes as the result of UV light absorption, and thermally relax back to the trans state during a period of ~65 h under the MAS measurement conditions. The order parameter measurements based on the C−H dipolar couplings reveal that the non-equilibrium cis-to-trans thermal isomerization impact of AzoPC on the fluidity of the bulk lipid is highly localized – the fluidity perturbations originate from specific order parameter changes in the middle section of the bulk lipid acyl chains. Further 1H NOESY measurements confirm VI the hypothesis that the azoswitch topologies in either cis and trans conformer of the photolipid is the key parameter in localized alteration of the C−H order parameters along the bulk lipid acyl chains. Diacylglycerol kinase (DgkA) from E. coli is an enzyme responsible for the phosphorylation of diacylglycerol to phosphatidic acid, at the expense of adenosine triphosphate. Structurally, DgkA is a homo oligomer composed of three symmetric 14 kDa protomers, each of which has three transmembrane helices and one surface helix. Upon embedding within the photoliposomes, it is shown that DgkA enhances the AzoPC localization impact on the fluidity of the bulk lipids. In this regard, the results of a series of statistical simulations of lipid lateral diffusions along the bilayer leaflets in presence and absence of embedded proteins are accompanied with those of experimentally measured based upon which it is justified that membrane proteins markedly limit lipid lateral diffusions in the bilayers. In case of the DgkA proteo-liposomes with lipid-to-protein ratio of 50, it is estimated that the diffusion coefficient of lipids is above 2-fold lower compared to that of the protein free liposomes. The cis-to-trans AzoPC isomerization and its following consequence in localized alteration of the bulk lipid fluidity is further investigated on the structural dynamics and enzymatic functionality of the embedded DgkA within the proteo-photoliposomes. It is revealed that DgkA structural dynamics are perturbated in a multi-scale, complex manner. The dynamics of residues located in different regions of DgkA changes with the light-induced AzoPC isomerization, but their time courses differ from residue to residue. For example, 29Ala, a residue on the hinge between the surface helix and membrane helix-1, exhibits the steepest time-dependent cross peak intensity changes in time-resolved NCA spectra. The impact of the lasting membrane fluidity perturbation on the enzymatic functionality of the embedded DgkA is subsequently measured which demonstrates a significant variation under cis- and trans-AzoPC conformations within the proteo-photoliposomes.