New phases of phospholipids and implications to the membrane fusion problem.

Membrane fusion is a ubiquitous process in eukaryotic cells. When two membranes fuse, lipid must undergo molecular rearrangements at the point of merging. To understand how lipid structure transitions occur, scientists studied the phase transition of lipid between the lamellar (L(alpha)) phase and the inverted hexagonal (H(II)) phase, based on the idea that lipid must undergo a similar rearrangement as in fusion. However, previous investigations on the system of dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE) did not reveal intermediate phases between the L(alpha) and H(II) phases. Recently, we found a rhombohedral phase of diphytanoylphosphatidylcholine between its L(alpha) and H(II) phases using substrate-supported samples. Here we report the observation of two new phases in the DOPC-DOPE system: a rhombohedral phase and a distorted hexagonal phase. The rhombohedral phase confirms the stalk hypothesis for the L(alpha)-H(II) transition, but the phase of stable stalks exists only for a certain range of spontaneous curvature. The distorted hexagonal phase exists only in a lipid mixture. It implies that lipids may demix to adjust its local spontaneous curvature in order to achieve energy minimum under stress.