Water dispersions of phospholipid: cholesterol: diacylglycerol may, under certain conditions, originate either the lipid- and water-permeable Q224 cubic phase, or the lipid-permeable but water-impermeable Q227 cubic phase. These results are discussed within the framework of the phospholipase C-induced fusion of liposomes [Nieva et al. (1993) Biochemistry 32, 1054]. It is suggested that the cubic phases Q224 and Q227 represnt two classes of lipid organization, one promoting, the other hindering the mixing of aqueous contents that is characteristic of membrane fusion. In this context, inverted micelles appear to be the end point of the fusion process, rather than fusion intermediates.
