Physico-chemical properties of phosphatidylglycerol in membranes of Acholeplasma laidlawii. A study with phospholipase A2

Topology and transport of membrane lipids in bacteria

The last two decades have witnessed a break-through in identifying and understanding the functions of both the proteins and lipids of bacterial membranes. This development was parallelled by increasing insights into the biogenesis, topology, transport and sorting of membrane proteins. However, progress in research on the membrane distribution and transport of lipids in bacteria has been slow in that period. The development of novel biochemical in vitro approaches and recent genetic studies have increased our understanding of these subjects. The aim of this review is to present an overview of the current knowledge of the distribution and transport of lipids in both Gram-positive and Gram-negative bacteria. Special attention is paid to recently obtained results, which are expected to inspire further research to finally unravel these poorly understood phenomena.

Transmembrane movement of phosphatidylglycerol and diacylglycerol sulfhydryl analogues

Transmembrane movement of phospholipids is a fundamental step in the process of biological membrane assembly and intracellular lipid sorting. To facilitate study of transmembrane movement, we have synthesized analogues of phosphatidylglycerol and diacylglycerol in which a sulfhydryl group replaces a hydroxyl group in the polar head group. A rapid, continuous assay for the movement of phospholipids across single-walled lipid vesicles was developed that exploits the reactivity of these analogues toward 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), a nonpenetrating, colorimetric, sulfhydryl reagent. In the reaction of DTNB with vesicles containing phosphatidylthioglycerol, a phosphatidylglycerol analogue, two kinetic phases were seen, which represent the reaction of DTNB with phosphatidylthioglycerol in the outer and inner leaflets of the bilayer. Analysis of the slow second phase indicated that the half-time for phosphatidylthioglycerol transbilayer movement was in excess of 8 days. In a similar experiment using dioleoylthioglycerol, a diacylglycerol analogue, the reaction was complete within 15 s. The large difference in translocation rates between these two lipids indicates that the primary barrier to transmembrane movement is the polar head group and implies that phospholipid translocation events in biological membranes may not be unlike those for molecules similar to the polar head groups alone.

Photo-induced dimerization of anthracene phospholipids for the study of the lateral distribution of lipids in membranes

It is shown that photo-induced cross-linking reaction between anthracene-labelled phospholipids can be used for studying, at a molecular level, their lateral distribution in bilayer structures. A simple and versatile method is proposed. It is based on the property of anthracene to form covalently bound dimers upon irradiation in the near ultraviolet (360 nm) and on the possibility of separating the lipid photo-dimers from the lipid monomers by thin-layer chromatography. Identification of the photo-dimers is easily achieved since, upon illumination at a shorter wavelength (250-280 nm), they partially dissociate to the native monomer molecules. The feasibility of the method was tested by checking the effects of cations (sodium, calcium) on the homogeneity of 1/1 mixtures of anthracene-phosphatidylcholine, i.e. 1-acyl-2-[9-(2-anthryl)-nonanoyl]-sn-glycero-3-phosphocholin e (Anthr-PC) with anthracene-phosphatidic acid (Anthr-PA) and with anthracene-phosphatidylglycerol (Anthr-PG) in the form of liposomes. These lipids were anthracene-labelled by acylation of their glycerol backbone at the sn-2 position with the synthetic 9-(2-anthryl)-nonanoic acid. Data presented indicate a good miscibility of these lipids in the presence of sodium. For each lipid mixture, the lipid heterodimers were clearly identified and, quantitatively, they dominated the lipid homodimers, as expected for a regular distribution of the lipids in the 1/1 mixture. Addition of calcium ions to the lipid suspensions did not alter the miscibility properties of Anthr-PC and Anthr-PG. In contrast, calcium triggered a clear-cut phase separation in the Anthr-PC/Anthr-PA mixture as, in this case, only traces of the heterodimer form of the lipids remained observable on the chromatogram. The three anthracene-phospholipids, pure or mixed together, exhibit a clear-cut gel-to-liquid phase transition which was detectable by fluorescence intensity measurements. The analysis of the corresponding phase-transition temperatures confirms, at a 'macroscopic' level, the effects of sodium and calcium on the mixing properties of the anthracene phospholipids which were revealed at a 'microscopic' level by the dimerization procedure.

Evidence for a homogeneous lateral distribution of lipids in a bacterial membrane. A photo cross-linking approach using anthracene as a photoactivable group

A new photo cross-linking method has been developed for the study of the lateral distribution of lipids in natural membranes, which uses anthracene as a photoactivable group. This method, which rests on the potentiality of anthracene to form covalently bound dimers upon irradiation around 340-380 nm has been applied to the membrane lipids (dimannosyl diacylglycerol, phosphatidylglycerol, phosphatidylinositol) of the bacterium Micrococcus luteus. These glyco- and phospholipids were anthracene labelled by metabolically incorporating the synthetic 9-(2-anthryl)nonanoic acid. The following sequential procedure was used: dimerization of the anthracene-labelled lipids in the membrane by irradiation of the intact cells at 360 nm; extraction of the lipids and thin-layer chromatography in the first dimension to separate the various lipid dimers from the monomers; partial dedimerization of the lipid dimers by illumination of the chromatogram at around 250-280 nm; chromatography in the second dimension to separate the native lipid monomers from the corresponding residual lipid dimers. On account of the occurrence of the 3 hetero dimers phosphatidylglycerol-dimannosyl diacylglycerol, phosphatidylinositol-dimannosyl diacylglycerol and phosphatidylglycerol-phosphatidylinositol after irradiating the cells, it is concluded that in this bacterial membrane, dimannosyl diacylglycerol, phosphatidylglycerol and phosphatidylinositol are homogeneously distributed.

Membrane asymmetry. A survey and critical appraisal of the methodology. II. Methods for assessing the unequal distribution of lipids

In the companion paper, I have reviewed the techniques employed for assessment of the asymmetric distribution and orientation of membrane proteins. This article deals with methods applicable to the investigation of the unequal distribution of lipids between the two membrane leaflets. Among the techniques I will discuss are the use of immunological techniques and lectins, chemical reagents, enzymatic isotopic labeling and degradation of membrane lipids, exchange proteins and physical techniques. Whenever appropriate, problems of crypticity and non-availability of lipids to interact with the appropriate ligands, reagents, modifying enzymes or exchange proteins have been envisaged. It appears that in many case, highly discordant results, sometimes with the same biological material, have been obtained. Some of the difficulties encountered presumably stem from the reported existence of non-bilayer arrangements and isotropic movement of lipids as evidenced by freeze-fracture and NMR studies. Other problems may be related to the induction of such arrangements, especially the inverted micellar arrangement, by the modifying agents, particularly degradation enzymes or exchange proteins when they cause severe unilateral modification of the lipids of the exposed leaflet. In addition, the situation is complicated by the role of the induced increase in the flip-flop rate under different experimental conditions and by modification of the rearrangement of lipid molecules as a result of the metabolic state of the cell or ghost preparation and of the reactivity of lipids as a consequence of temperature changes. Here, more so than with proteins, one must be cautious in interpreting experimental results. Moreover, it would appear that the use of different techniques in conjunction and the consequent comparison of results should be recommended. It has been emphasized that 'general rules' do not hold and that each new material should be assay again. To give one example, it is not pertinent to state that proteins enhance the flip-flop rate in lipid vesicles (and hence in membranes). This holds true for glycophorin from erythrocyte membrane, but could not be proved when mitochondrial cytochrome oxidase was used. There seems to be no rule for the distribution of lipids between the two leaflets of different membranes. For example, even for different strains of the same bacterial species, highly divergent results have been reported. It is generally (and probably under the influence of different studies with erythrocytes) believed that in mammalian plasma membranes, choline phospholipids are enriched in the outer leaflet and aminophospholipids in the inner leaflet. Though this contention may prove to be correct, different instances of contradictory results have been given in the text. This shows that if rules do exist, they remain to be discovered or established...

Effect of membrane cholesterol on action of phospholipase A2 in Mycoplasma mycoides var. Capri. Evidence for lysophospholipase activity

Cells of Mycoplasma mycoides var. Capri grown in a medium containing 10 micrograms/ml cholesterol (native organisms) or in cholesterol-free medium (adapted organisms) were treated with phospholipase A2. Hydrolysis of polar lipids (phosphatidylglycerol and diphosphatidylglycerol) only occurred in the adapted cells. Cholesterol replenishment of the membranes of these adapted cells in vitro which involves an increase from 7 micrograms to 66 micrograms cholesterol/mg membrane protein, completely abolished hydrolysis of polar lipid pools by phospholipase A2. This suggests that cholesterol incorporated either during growth or under conditions in vitro has an identical disposition and function in the membrane. This observation further indicates that cholesterol incorporation in M. mycoides var. Capri can be explained in terms of a simple physical adsorption process. Polar-lipid breakdown products resulting from phospholipase A2 action on intact cells, isolated membranes and lipids extracted from adapted organisms were analyzed. In experiments with intact cells [14C]oleic lysoderivatives but not [3H]palmitic lysoderivatives are accumulated within the membranes. In membrane preparations, again only [14C]oleic lysoderivatives are accumulated but transiently. Finally, both [14C]oleic and [3H]palmitic lysoderivatives were produced in phosphatidylglycerol-diphosphatidylglycerol liposomal preparations. From these results it can be concluded that: (a) 80% of the phosphatidylglycerol and diphosphatidylglycerol have an unusual positional distribution of their fatty acid (unsaturated oleic acid in position 1) and (b) membranes of M. mycoides var. Capri contain an active lysophospholipase which more efficiently hydrolyzes palmitic-acid-containing lysoderivatives.

The distribution of molecular classes of phosphatidylglycerol in the membrane of Acholeplasma laidlawii

A double-label technique has been applied to study the distribution of different molecular classes of phosphatidylglycerol in the membrane of Acholeplasma laidlawii. After growth on oleic acid, 16% of the total phosphatidylglycerol contains two oleic acid residues and 84% contains one oleic acid and one saturated fatty acid. The dioleoyl phosphatidylglycerol is present in equal amounts in the outer and the inner layer of the membrane. Phosphatidylglycerol which is associated with membrane proteins consists exclusively of the class containing only one oleic acid.