INFLUENCE OF TRANSBILAYER AREA ASYMMETRY ON THE MORPHOLOGY OF LARGE UNILAMELLAR VESICLES

The morphological consequences of differences in the monolayer surface areas of large unilamellar vesicles (LUVs) have been examined employi ng cryoelectron microscopy techniques. Surface area was varied by indu cing net transbilayer transport of dioleoylphosphatidylglycerol (DOPG) in dioleoylphosphatidylcholine (DOPC):DOPG (9:1, mol:mol) LUVs in res ponse to transmembrane pH gradients. It is shown that when DOPG is tra nsported from the inner to the outer monolayer, initially invaginated LUVs are transformed to long narrow tubular structures, or spherical s tructures with one or more protrusions. Tubular structures are also se en in response to outward DOPG transport in DOPC:DOPG:Chol (6:1:3, mol :mol:mol) LUV systems, and when lyse-PC is allowed to partition into t he exterior monolayer of DOPC:DOPG (9:1, mol:mol) LUVs in the absence of DOPG transport. Conversely, when the inner monolayer area is expand ed by the transport of DOPG from the outer monolayer to the inner mono layer of non-invaginated LUVs, a reversion to invaginated structures i s observed. The morphological changes are well described by an elastic bending theory of the bilayer. identification of the difference in re laxed monolayer areas and of the volume-to-area ratio of the LUVs as t he shape-determining factors allows a quantitative classification of t he observed morphologies. The morphology seen in LUVs supports the pos sibility that factors leading to differences in monolayer surface area s could play important roles in intracellular membrane transport proce sses.