Bls. Mui et al., INFLUENCE OF TRANSBILAYER AREA ASYMMETRY ON THE MORPHOLOGY OF LARGE UNILAMELLAR VESICLES, Biophysical journal, 69(3), 1995, pp. 930-941
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.