Intermediates in membrane fusion and bilayer/nonbilayer phase transitions imaged by time-resolved cryo-transmission electron microscopy.

Bilayer-to-nonbilayer phase transitions in phospholipids occur by means of poorly characterized intermediates. Many have proposed that membrane fusion can also occur by formation of these intermediates. Structures for such intermediates were proposed in a recent theory of these transition mechanisms. Using time-resolved cryo-transmission electron Microscopy (TRC-TEM), we have directly visualized the evolution of inverted phase micro-structure in liposomal aggregates. We have identified one of the proposed intermediates, termed an interlamellar attachment (ILA), which has the structure and dimensions predicted by the theory. We show that ILAs are likely to be the structure corresponding to "lipidic particles" observed by freeze-fracture electron microscopy. ILAs appear to assemble the inverted cubic (III) phase by formation of an ILA lattice, as previously proposed. ILAs are also observed to mediate membrane fusion in the same systems, on the same time scale, and under nearly the same conditions in which membrane fusion was observed by fluorescence methods in earlier studies. These earlier studies indicated a linkage between a membrane fusion mechanism and III phase formation. Our micrographs suggest that the same intermediate structure mediates both of those processes.