As Ca2+ and phosphatidylserine (PS) are known to induce the adhesion of bilayer vesicles and
form collapsed multibilayer structures in vitro, it was the aim of this study to examine how
that interaction and the resultant structures might be modified by neutral lipid species. X-ray
diffraction data from multilamellar systems suggest that phosphatidylcholine (PC) and
diacylglycerol (DG) might be in the collapsed phase up to a concentration of -30 mole % and
that above this concentration these neutral lipids may modify Ca2+-induced bilayer
interactions. Using large unilamellar vesicles and long incubations in excess Ca2+ to ensure
equilibration, similar preliminary results were again obtained with PC, and also with
phosphatidylethanolamine (PE). A combination of X-ray diffraction, thin-layer
chromatography, density gradient centrifugation and freeze-fracture electron microscopy, used
in conjunction with an osmotic stress technique, showed that (i) -30 mole % PC can be
accomodated in the Ca(DOPS)2 phase; and (ii) higher PC levels modify Ca2+-induced bilayer
interactions resulting in single lamellar phases of larger dimension and reduced tendency for
REV collapse. Importantly, the data suggest that PC is dehydrated during the rapid collapse
process leading. to Ca(DOPS)2 formation and exists with this dehydrated phase. Similar results
were obtained using PS isolated from bovine brain. Preliminary studies using two different
phosphatidylethanolamine (PE) species indicated accomodation by Ca(DOPS)2 of -25-30 mole
0/0 PE and bulk phase separation, of species favouring a non-bilayer phase, at higher levels.
Significantly, all PS/PE vesicles appear to undergo a complete Ca2+-induced collapse, even with
contents of up to 90 mole % PE. These data suggest that PE may have an important role in fusion
mechanisms in vivo. In sum the data lend both structural and stoichiometric evidence for th~
existence of laterally segregated neutral lipid molecules within the same bilayers as PS domains
exposed to Ca2+.
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