Electron microscopic investigation of the morphology and calcium-induced fusion of lipid vesicles with an oligomerised inner leaflet

The lipid head groups in the inner leaflet of unilamellar bilayer vesicles of the synthetic lipids DHPBNS and DDPBNS can be selectively oligomerised. Earlier studies have established that these vesicles: fuse much slower and less extensively upon oligomerisation of the lipid head groups. The morphol ogy and calcium-induced fusion of vesicles of DHPBNS and DDPBNS were invest igated using cryo-electron microscopy. DHPBNS vesicles are not spherical bu t Battened, ellipsoidal structures. Upon addition of CaCl2, DHPBNS vesicles with an oligomerised inner leaflet were occasionally observed in an arrest ed hemifused state. However. the evidence for hemifusion is not equivocal d ue to potential artefacts of sample preparation. DDPBNS vesicles show the e xpected spherical morphology. Upon addition of excess CaCl2, DDPBNS vesicle s fuse into dense aggregates that show a regular spacing corresponding to t he bilayer width. Upon addition of EDTA. the aggregates readily disperse in to large unilamellar vesicles. At low concentration of calcium ion. DDPBNS vesicles with an oligomerised inner leaflet form small multilamellar aggreg ates. in which a spacing corresponding to the bilayer width appears. Additi on of excess EDTA results: in slow dispersal of the Ca2+-lipid aggregates i nto a heterogeneous mixture of bilamellar, spherical vesicles and networks of thread-like vesicles. These lipid bilayer rearrangements are discussed w ithin the context of shape transformations and fusion of lipid membranes. ( C) 2001 Elsevier Science Ireland Ltd. All rights reserved.