DIACYLGLYCEROL AND THE PROMOTION OF LAMELLAR-HEXAGONAL AND LAMELLAR-ISOTROPIC PHASE-TRANSITIONS IN LIPIDS - IMPLICATIONS FOR MEMBRANE-FUSION

Changes in steady-state fluorescence anisotropy of (4-trimethylaminoph enyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) are applied to the detection of lamellar-hexagonal transitions in egg phosphatidylethanolamine. Ev en low (2 mole%) proportions of diacylglycerol decrease the hexagonal transition temperature considerably, as confirmed by differential scan ning calorimetry. Diacylglycerol is also found to promote a lamellar t o ''isotropic'' (Q(224) cubic) transition in mixtures of tidylcholine: phosphatidylethanolamine:cholesterol. This nonreversible transition is also observed by P-31 nuclear magnetic resonance and detected as a la rge increase in TMA-DPH steady-state anisotropy. The same technique re veals as well that lysophosphatidylcholine counteracts the effect of d iacylglycerol and stabilizes the lamellar phase in both transitions. D iacylglycerol and lysophosphatidylcholine are known to respectively pr omote and inhibit membrane fusion in a variety of systems. These data are interpreted in support of the hypothesis of a highly bent structur al fusion intermediate (''stalk''). They also show the interest of lip id-phase studies in predicting and rationalizing membrane fusion mecha nisms.