Fluid-phase chain unsaturation controlling domain microstructure and phase in ternary lipid bilayers containing GalCer and cholesterol.
We report the microstructure and phase behavior of three ternary mixtures each containing a long-chain saturated glycosphingolipid, galactosylceramide (GalCer), and cholesterol at room temperature. The unsaturation level of the fluid-phase component was varied by lipid choice, i.e., saturated 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), singly unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), or doubly unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). GalCer was used because of its biological significance, for example, as a ligand in the sexual transmission of HIV and stimulator of natural killer T-cells. Supported lipid bilayers of the ternary mixtures were imaged by atomic force microscopy and GalCer-rich domains were characterized by area/perimeter ratios (A/P). GalCer domain phase transitions from solid (S) to liquid (L) phase were verified by domain behavior in giant unilamellar vesicles, which displayed two-dimensional microstructure similar to that of supported lipid bilayers. As cholesterol concentration was increased, we observed approximately 2.5, approximately 10, and approximately 20-fold decreases in GalCer domain A/P for bilayers in L-S phase coexistence containing DOPC, POPC, and DLPC, respectively. The transition to L-L phase coexistence occurred at approximately 10 mol % cholesterol for bilayers containing DOPC or POPC and was accompanied by maintenance of a constant A/P. L-L phase coexistence did not occur for bilayers containing DLPC. We systematically relate our results to the impact of chain unsaturation on the interaction of the fluid-phase lipid and cholesterol. Physiologically, these observations may give insight into the interplay of fatty acid chain unsaturation, sterol concentration, and lipid hydrophobic mismatch in membrane phenomena.
Lin
Wan-Chen
WC
Biophysics Graduate Group, University of California, Davis, California 95616, USA.
Blanchette
Craig D
CD
Longo
Marjorie L
ML
D002784:Cholesterol / Q000737:chemistry*; D005699:Galactosylceramides / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry*; D008081:Liposomes / Q000737:chemistry*; D008560:Membrane Fluidity; D021962:Membrane Microdomains / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D044367:Phase Transition; D012995:Solubility; D012996:Solutions
D005699:Galactosylceramides; D008051:Lipid Bilayers; D008081:Liposomes; D012996:Solutions; D002784:Cholesterol