CALCIUM-INDUCED INTERACTION AND FUSION OF ARCHAEOBACTERIAL LIPID VESICLES - A FLUORESCENCE STUDY

The lipids extracted from the membrane of the thermophilic archaeobact erium Sulfolobus solfataricus have an unusual bipolar structure. Each molecule is formed by two isoprenoid chains (with up to four cyclopent ane groups per chain) ether-linked at both ends to glycerol or nonitol groups. These groups can be variably substituted, mainly with complex sugars. Fluorescence resonance energy transfer, aqueous contents mixi ng and calcein release assays were employed to assess whether bipolar lipid vesicles were able to undergo a calcium-induced fusion process. The possibility of getting fusion depends strongly on the phase behavi our of the lipids. With vesicles formed by the natural polar lipid ext ract (PLE), a mixture showing a complex polymorphic behaviour, the fus ion process was observed above the temperature T similar or equal to 6 0 degrees C at 15 mM Ca2+. By contrast, no fusion was observed in vesi cles of P2, a fraction displaying only the lamellar phase. A dramatic change of the fusion process was observed when egg PC or P2 was added to PLE. In this case only lipid mixing, but not a real fusion process occurred at T greater than or equal to 60 degrees C. The dependence of such a process on ionic conditions has also been studied. Additional experiments involving surface tension measurements on monolayers have been performed to assess the importance of a surface tension increase to get fusion. In contrast to other monopolar lipid systems, no detect able change in surface tension has been observed in our bipolar lipids even in cases in which the fusion process is present.