title: Subnanometer Structure of an Asymmetric Model Membrane: Interleaflet Coupling Influences Domain Properties.
abstract: Cell membranes possess a complex three-dimensional architecture, including nonrandom lipid lateral organization within the plane of a bilayer leaflet, and compositional asymmetry between the two leaflets. As a result, delineating the membrane structure-function relationship has been a highly challenging task. Even in simplified model systems, the interactions between bilayer leaflets are poorly understood, due in part to the difficulty of preparing asymmetric model membranes that are free from the effects of residual organic solvent or osmotic stress. To address these problems, we have modified a technique for preparing asymmetric large unilamellar vesicles (aLUVs) via cyclodextrin-mediated lipid exchange in order to produce tensionless, solvent-free aLUVs suitable for a range of biophysical studies. Leaflet composition and structure were characterized using isotopic labeling strategies, which allowed us to avoid the use of bulky labels. NMR and gas chromatography provided precise quantification of the extent of lipid exchange and bilayer asymmetry, while small-angle neutron scattering (SANS) was used to resolve bilayer structural features with subnanometer resolution. Isotopically asymmetric POPC vesicles were found to have the same bilayer thickness and area per lipid as symmetric POPC vesicles, demonstrating that the modified exchange protocol preserves native bilayer structure. Partial exchange of DPPC into the outer leaflet of POPC vesicles produced chemically asymmetric vesicles with a gel/fluid phase-separated outer leaflet and a uniform, POPC-rich inner leaflet. SANS was able to separately resolve the thicknesses and areas per lipid of coexisting domains, revealing reduced lipid packing density of the outer leaflet DPPC-rich phase compared to typical gel phases. Our finding that a disordered inner leaflet can partially fluidize ordered outer leaflet domains indicates some degree of interleaflet coupling, and invites speculation on a role for bilayer asymmetry in modulating membrane lateral organization.
authors.forename: Frederick A
authors.initials: FA
authors.lastname: Heberle
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Drew
authors.initials: D
authors.lastname: Marquardt
authors.affiliation: Tri-Institutional PhD Program in Computational Biology and Medicine, Weill Cornell Medical College , New York, New York 10065, United States.|Department of Molecular Biology and Genetics, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Milka
authors.initials: M
authors.lastname: Doktorova
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Barbara
authors.initials: B
authors.lastname: Geier
authors.forename: Robert F
authors.initials: RF
authors.lastname: Standaert
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Peter
authors.initials: P
authors.lastname: Heftberger
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Kollmitzer
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Nickels
authors.affiliation: Department of Molecular Biology and Genetics, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Dick
authors.affiliation: Department of Molecular Biology and Genetics, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Gerald W
authors.initials: GW
authors.lastname: Feigenson
authors.forename: John
authors.initials: J
authors.lastname: Katsaras
authors.affiliation: Department of Biochemistry and Cell Biology, Stony Brook University , Stony Brook, New York 11794, United States.
authors.forename: Erwin
authors.initials: E
authors.lastname: London
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Georg
authors.initials: G
authors.lastname: Pabst
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D010713:Phosphatidylcholines / Q000737:chemistry; D053835:Unilamellar Liposomes / Q000737:chemistry*
chemical_list: D010713:Phosphatidylcholines; D053835:Unilamellar Liposomes; D015060:1,2-Dipalmitoylphosphatidylcholine; C017251:1,2-oleoylphosphatidylcholine