A new type of lipid organization is observed in mixtures of phosphatidyl-choline with cardiolipin (in the presence of Ca2+), monoglycosyldiglyceride and phosphatidylethanolamine (in the presence of cholesterol). This phase is characterised by an isotropic 31P NMR signal and is visualised by freeze-fracturing as particles and pits on the fracture faces of the lipid bilayer. As the most favourable model for this phase we propose the inverted micelle sandwiched in between the two monolayers of the lipid bilayer.

SummaryThe addition of Ca 2÷ to small unilamellar vesicles of an equimolar mixture of egg phosphatidylcholine and cardiolipin induces fusion of these vesicles in association with the appearance of lipidic particles on the fusion sites.Membrane fusion clearly requires that participating lipids adopt transitory non-bilayer configurations during the intermediate stages. The nature of the possible intermediate structures (such as micellar [1] or inverted micellar [2,3] ) and their relation to the physical properties of membrane lipids remain, however, a matter of some speculation. Recently, it has been argued that endogenous lipids which preferentially adopt the hexagonal (HII) phase under certain conditions may be directly involved in fusion events [4]. In this regard it has been established that the presence of Ca 2÷ is vital to the fusion process [ 5] and that the addition of Ca 2÷ can trigger formation of the hexagonal (HII) phase in model membrane systems consisting of certain pure [6] or mixed [7] species of naturally occurring phospholipid. It is therefore of interest to first establish that Ca 2÷ can also induce fusion between model systems partly comprised of such phospholipids, and secondly to establish that such fusion events proceed via the non-bilayer structures engendered by the presence of Ca 2.. In this work we present results obtained employing model systems comprised of an equimolar mixture of bovine heart cardiolipin and egg yolk phosphatidylcholine in conjunction with

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The adsorption constant of the irreversible adsorption of the bacteriophage K3 to Escherichia coli K12 bacteria is strongly dependent on the incubation temperature. Two inflection points are observed in an Arrhenius plot. For cells grown at 37 "C the inflection points are found at 20 "C and 28 "C whereas these inflection points shift to 10 "C and 19 "C for cells grown at 12 "C.To study the lipid environment of the receptor the temperature dependence of the inactivation of bacteriophage K3 was measured in vitro in the presence of various lipids. The Arrhenius plots of the rate of inactivation of phage K3 by complexes of protein d and lipopolysaccharide are very similar to those observed for whole cells. With lipopolysaccharide isolated from cells grown at 37 "C inflection points are observed at 20 "C and 28 "C. With lipopolysaccharide from cells grown at 12 "C the inflection points are found at 10 "C and 21 "C. These results show that the environment of protein d in vivo can be mimicked perfectly in vitro by protein d/lipopolysaccharide complexes.The fatty acid composition of lipopolysaccharide isolated from cells grown at 37 "C and at 12 "C differs in that in the latter case the amounts of mono-unsaturated fatty acids (mainly palmitoleic acid) are increased at the expense of lauric acid. This difference in fatty acid composition probably explains the difference in the phase transition temperatures caused by the two lipopolysaccharide preparations. A transition at the inflection point of the highest temperature is also found for lipopolysaccharide using light-scattering measurements and appears to be a thermal transition, since it is also observed in differential scanning calorimetry.Cells of mutant strain CE1071 lacking outer membrane proteins b and c and concomitantly containing phospholipids in the outer leaflet of the outer membrane, adsorb phage K3 with an almost normal rate, but the shape of the Arrhenius plot differs from the curve of wild-type cells. The characteristics of the adsorption of phage K3 to these mutant cells can be mimicked in vitro by the incorporation of phospholipid into protein d/lipopolysaccharide complexes, indicating that phospholipids are part of the environment of the phage K3 receptor in cells of this mutant, but not in wildtype cellsThe outer membrane of gram-negative bacteria contains lipopolysaccharide, phospholipids and proteins. Lipopolysaccharide is located in the outer monolayer [l] whereas phospholipids are probably exclusively present in the inner monolayer [2,3]. One of the major proteins of the outer membrane is the heat-modifiable protein d [4,5], which is also referred to as protein 3a [6], 1I* [7] and 0-10 [8]. This protein has been purified in its non-heat modified form [7,9]. In complex with lipopolysaccharide this protein inactivates bacteriophages K3 [9] and TuII* [lo, 111. In the latter case the lipid A part of lipopolysaccharide is as active as intact lipopolysaccharide in generating phage receptor activity [l 11. Phospholipid or detergent cannot replace lipopol...

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