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November 7, 2003 New Phases of Phospholipids and Implications to the Membrane Fusion ProblemL. Yang1, L. Ding2, and H.W. Huang2 Membrane fusion is involved in many biological processes, such as cellular transportation, signal transduction, and viral infection. A key element of the study of membrane fusion is to understand the behavior of the primary structural component of the membrane, the lipid bilayer, in order to elucidate fusion proteins' regulatory role. In this study, we use x-ray diffraction at beamline X21 to study how the lipid composition affects bilayer fusion.
However, previous investigations on the system of dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE) did not reveal intermediate phases between the L and HII phases. Recently, we found a rhombohedral phase (R) in diphytanoylphosphatidylcholine (DPhPC) between its L and HII phases using substrate-supported samples. Here we report the observation of the rhombohedral phase, as well as a previously not observed distorted hexagonal phase in DOPC-DOPE mixtures. The experiments were performed on model membranes that are made of hundreds of lipid bilayers prepared on silicon substrate. We observed the phase behavior of the lipid while varying the composition of the sample as well as its temperature and water content (Figure 1). The samples in general showed all three phases that were observed in DPhPC. However, depending on the sample composition, these three phases appeared at the different parts of the phase diagram. It has been shown that the rhombohedral phase of DPhPC contains a lattice of stalk structure that was thought to be an intermediate structure during the fusion process. The existence of this phase in DOPC/DOPE mixtures once again confirms the stalk hypothesis for the L-HII transition. Furthermore, the R phase exists only for a certain range of lipid composition (i.e. for a certain range of spontaneous curvature). This implies that the free energy barrier in the fusion pathway is directly determined by the spontaneous curvature of the lipid bilayer.
The samples with mixed DOPC and DOPE also showed (Figure 2) a distorted hexagonal phase (D). Though the detailed structure of this phase is yet to be determined, because of the symmetry of the structure and the fact that the D and HII phases are neighbors in the phase diagram, the D phase very likely contains distorted lipid tubes. This implies that, under stress, lipids may partially demix to adjust its local spontaneous curvature in order to achieve energy minimum. Therefore demixing, or lipid sorting, may be a mechanism to promote membrane fusion. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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When two lipid bilayers fuse, they must undergo molecular
rearrangements at the point of merging. The role of the fusion
proteins that strictly control membrane fusion is therefore most
likely to modify the relative free energy of different lipid
configurations and to exert mechanical work that shifts the system
over the energy barriers along the fusion pathway. In order to
understand the role of fusion proteins and how lipid structure
transitions occur, researchers studied the phase transition of pure
lipid membranes between the lamellar (L) phase (planar lipid layers)
and the inverted hexagonal (HII) phase (hexagonally stacked lipid
tubes), based on the idea that lipid must undergo a similar
rearrangement, as in fusion. 
