Osmotic shock stimulates de novo synthesis of two cardiolipins in an extreme halophilic archaeon

Abstract

The present report illustrates the response to osmotic stress of an extreme halophilic archaeon, Halorubrum sp., isolated from the saltern ponds of Margherita di Savoia in southern Italy. The hypotonic stress induces relevant changes in the membrane lipid composition: archaeal cardiolipin content markedly increases, whereas phosphatidylglycerol (PG) decreases. Membranes isolated from this archaeon after cell disruption by osmotic shock are highly enriched in archaeal cardiolipin and reveal the presence of a novel phospholipid. Electrospray ionization mass spectrometry and NMR analyses revealed that this novel lipid has the structure of a sulfo-diglyco-diether-phosphatidic acid, i.e., a phospholipid dimer or a novel cardiolipin analogue. As NMR analyses showed that the sugars in the novel phospholipid dimer are the same and in the same order of a sulfated diglycosyl diphytanylglycerol diether (S-DGD-5) present as a major lipid component in the archaeon membranes, the novel phospholipid dimer was named S-DGD-5-PA. We conclude that osmotic shock induces a specific increase in the membrane content of the two cardiolipins and suggest that PG and S-DGD-5 are intermediates for the de novo synthesis of archaeal cardiolipin and S-DGD-5-PA, respectively. -Lopalco, P., S. Lobasso, F. Cardiolipin, a dimeric phospholipid discovered in heart mitochondria, plays an important role in the function and stability of many enzymatic complexes of the respiratory chain (1). A number of substituted cardiolipins or cardiolipin analogues have been described in bacteria (2-4); the function of these cardiolipin analogues in bacteria is still unclear. Both authentic cardiolipin and cardiolipin analogues are composed of two diacylglycerol moieties and therefore possess four hydrophobic chains.Cardiolipins have also been found in the Archaea world (5, 6). The first evidence of the presence of archaeal cardiolipin was found in Halobacterium salinarum , in which a bisphosphatidylglycerol (BPG) and another phospholipid dimer composed of the glycolipid sulfated triglycosyl diphytanylglycerol diether-1 (S-TGD-1) esterified to phosphatidic acid (PA) have been described [S-TGD-1-PA, also called glycocardiolipin (GlyC)] (5).GlyC, a major lipid component of the purple membrane (PM) of H. salinarum , plays an important role in the function and stability of bacteriorhodopsin (BR), the photoactivated proton pump (7), whereas BPG is only a minor lipid of PM (5, 8). Interestingly, it has been shown that the amount of GlyC in the PM of H. salinarum increases when the cells are exposed to hypotonic shock and that the formation of GlyC occurs at the expense of S-TGD-1 (9).In the present report, we show that a halo-archaeal isolate collected from the salterns of Margherita di Savoia in southern Italy, which is closely related to Halorubrum trapanicum , actively synthesizes high amounts of the archaeal cardiolipin BPG under osmotic shock. Membranes isolated after cell disruption by osmotic shock are highly enriched in this archaeal cardiolipin and reve...

From genomes to function: haloarchaea as model organisms

“…Thus haloarchaea are excellent models to study osmoadaptation over an extreme range of salt concentrations. Several recent studies have identified genes that are differentially expressed at different salt concentrations (Bidle, 2003;Choi et al, 2005;Jäger et al, 2002) or detected de novo synthesis of dimeric lipids upon an osmotic downshift (Lopalco et al, 2004), but the opportunities are clearly underexploited.…”

MALDI-TOF/MS analysis of archaebacterial lipids in lyophilized membranes dry-mixed with 9-aminoacridine

“…6A reports the TLC lipid profi le of red membranes and lipid assignments based on data in previous literature and mass spectrometric analysis. Experimental lipid masses obtained by MALDI-TOF/MS analysis correspond well to calculated lipid masses and to those previously obtained by ESI-MS analysis ( 12,13 ). It can be seen that in the MALDI spectra of membrane/matrix pellets most of archaebacterial lipids are represented by their sodium adducts.…”

“…A fragment of the pellet was used for MALDI-TOF/MS analysis. The resulting MALDI-TOF/MS lipid profi le was perfectly superimposable onto the lipid profi le obtained by previous TLC and ESI-MS analysis ( 12,13 ). Fig.…”

“…6 illustrates the MALDI-TOF/MS analysis of lyophilized red membranes isolated from the strain MdS1, another extremely halophilic microorganism of the Archaea world. MdS1 red membranes are highly enriched in BPG ( 12,13 ).…”

“…The MdS1 strain, a representative of Halorubrum sp. , was isolated from the salterns Margherita di Savoia and grown in our laboratory as previously described ( 12 ).…”