Fluid-Phase Chain Unsaturation Controlling Domain
Microstructure and Phase in Ternary Lipid Bilayers
Containing GalCer and Cholesterol
Wan-Chen Lin 1, Craig D. Blanchette 1 and Marjorie L. Longo 1*
1 University of California, Davis
* To whom correspondence should be addressed. E-mail: mllongo{at}ucdavis.edu.
Submitted on August 18, 2006
Revised on October 8, 2006
Accepted on 28 December 2006
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Abstract |
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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 (SLBs) of the ternary mixtures were imaged by atomic force microscopy (AFM) and GalCer-rich domains were characterized by area to perimeter ratios (A/P). GalCer domain phase transitions from solid (S) to liquid (L) phase were verified by domain behavior in giant unilamellar vesicles (GUVs), which displayed similar two-dimensional microstructure to SLBs. As cholesterol concentration was increased, we observed ~2.5, ~10, and ~ 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 ~10 mol% cholesterol for bilayers containing DOPC or POPC and was accompanied by maintenance of a constant A/P. L-L phase coexistence does 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.
Key Words:
AFM, GUV, HIV, raft, sphingolipid, supported lipid bilayer
