Properties of a membrane-bound cardiolipin synthetase from Lactobacillus plantarum

Isolation, purification and characterization of Cardiolipin synthase from Mycobacterium phlei {PRIVATE}

It has been observed that mycobacterial species has high content of cardiolipin (CL) in their cell membranes more so pathogenic mycobacteria and in bacteria CL activates polymerases, gyrases by removing the bound ADP. Therefore, in the present study cardiolipin synthase (cls) which catalyses the formation of CL was isolated purified and characterized from the cell membrane of Mycobacterium phlei. The purified cls obtained from C-18 RP-HPLC column had a molecular weight of 58 kDa with an isoelectric point of 4.5. The enzyme activity (11.5+0.15 µM of CL phosphorous. ml-1 minute-1 for PG as substrate and 14+0.35µM of CL phosphorous. ml-1 minute-1 for CDP-DG as substrate) was optimal at pH 4.8 and showed K_M values of 55+0.05µM and 2.56+0.04µM for phosphatidyl glycerol and CDP-diacylglycerol, respectively, with an absolute requirement of Mg²⁺ and Mn²⁺ ions for its activity however, Ca²⁺ ions inhibited the activity of the cls. The partial amino acid sequence of cls showed significant homology with pgsA3 gene of M. tuberculosis and in this organism the CL biosynthesis is very high having three genes coding for PLs biosynthesis therefore, enzymes involved in CL biosynthesis may be an attractive drug target in the development of new antimycobacterial drugs.

Characterization of Staphylococcus aureus cardiolipin synthases 1 and 2 and their contribution to accumulation of cardiolipin in stationary phase and within phagocytes

In many bacteria, including Staphylococcus aureus, progression from the logarithmic to the stationary phase is accompanied by conversion of most of bacterial membrane phosphatidylglycerol (PG) to cardiolipin (CL). Phagocytosis of S. aureus by human neutrophils also induces the conversion of most bacterial PG to CL. The genome of all sequenced strains of S. aureus contains two open reading frames (ORFs) predicting proteins encoded with ∼30% identity to the principal CL synthase (cls) of Escherichia coli. To test whether these ORFs (cls1 and cls2) encode cardiolipin synthases and contribute to CL accumulation in S. aureus, we expressed these proteins in a cls strain of E. coli and created isogenic single and double mutants in S. aureus. The expression of either Cls1 or Cls2 in CL-deficient E. coli resulted in CL accumulation in the stationary phase. S. aureus with deletion of both cls1 and cls2 showed no detectable CL accumulation in the stationary phase or after phagocytosis by neutrophils. CL accumulation in the stationary phase was due almost solely to Cls2, whereas both Cls1 and Cls2 contributed to CL accumulation following phagocytosis by neutrophils. Differences in the relative contributions of Cls1 and Cls2 to CL accumulation under different triggering conditions suggest differences in the role and regulation of these two enzymes.

Cardiolipin synthase from yeast

Cardiolipin synthase catalyzes the synthesis of the mitochondrial phospholipid cardiolipin. Cardiolipin synthase is a unique membrane-bound enzyme in that it utilizes two phospholipids, both insoluble in water, as substrates. Kinetic analysis suggests that the enzyme forms a ternary complex with the two lipid substrates, and that a divalent metal ion directly associates with cardiolipin synthase to form the active enzyme. While little is known about the regulation of cardiolipin synthase in yeast, activity is reduced in mutants in which the mitochondrial genome is deleted, and in mutants with defective respiratory complexes. In p0 mutants, which contain no mitochondrial DNA and are defective in the assembly of many mitochondrial membrane protein complexes, cardiolipin synthase activity is reduced by 50%. Mutants defective in respiratory complexes, particularly those incapable of cytochrome oxidase assembly, also have reduced cardiolipin synthase activity. Thus it is likely that respiration and cardiolipin formation are interdependent. The enzyme was recently purified from the budding yeast Saccharomyces cerevisiae. Enzyme activity was associated with a 25-30-kDa protein. The amino acid sequence of this protein, combined with the availability of the complete yeast genome sequence, will hopefully lead to the identification of the structural gene for this enzyme in the near future.

Cardiolipin synthase from Escherichia coli

Escherichia coli cardiolipin synthase catalyzes reversible phosphatidyl group transfer from one phosphatidylglycerol molecule to another to form cardiolipin (CL) and glycerol. The enzyme is specified by the cls gene, located at min 28.02 of the E. coli genetic map. Cells with mutations in cls have longer doubling times, tend to lose viability in the stationary phase, are more resistant to 3,4-dihydroxybutyl-1-phosphonate, and have an altered sensitivity to novobiocin. Although cls null mutants appear to lack CL synthase activity, they are still able to form trace quantities of CL. The enzyme appears to be regulated at both the genetic and enzymatic levels. CL synthase's molecular mass is 45-46 kDa, or about 8 kDa less than the polypeptide predicted by the gene sequence, suggesting that posttranslational processing occurs. CL synthase can use various polyols such as mannitol and arabitol to convert CL to the corresponding phosphatidylglycerol analog. When the amino acid sequences of four bacterial CL synthases are compared, three highly conserved regions are apparent. One of these regions contains a conserved pentapeptide sequence, RN(Q)HRK, and another has a conserved HXK sequence. These two sequences may be part of the active site. E. coli CL synthase has been studied by using a mixed micelle assay. The enzyme is inhibited by CL, the product of the reaction, and by phosphatidate. Phosphatidylethanolamine partially offsets inhibition caused by CL but not by phosphatidate. CDP-diacylglycerol does not appear to affect the activity of the purified enzyme but does stimulate the activity associated with crude membrane preparations.

Mitochondrial cardiolipin in diverse eukaryotes. Comparison of biosynthetic reactions and molecular acyl species

Cardiolipin, a unique dimeric phospholipid of bacteria and mitochondria, can be synthesized by two alternative pathways discovered in rat and Escherichia coli, respectively. In mitochondrial preparations from fungi (Saccharomyces cerevisiae, Neurospora crassa), higher plants (Phaseolus aureus), molluscs (Mytilus edulis) and mammals (rat liver, bovine adrenal gland), cardiolipin was synthesized from CDP-diacylglycerol and phosphatidylglycerol, suggesting a common eukaryotic mechanism of cardiolipin formation which is in contrast to the prokaryotic biosynthesis from two molecules of phosphatidylglycerol. All mitochondrial cardiolipin synthases were inhibited by lysophosphatidylglycerol, were insensitive to N-ethylmaleimide and required divalent cations, although they had different cation specificities. The molecular species of cardiolipin from rat liver, bovine heart, S. cerevisiae and N. crassa were analysed by high-performance liquid chromatography of the derivative 1,3-bis[3'-sn-phosphatidyl]-2-benzoyl-sn-glycerol dimethyl ester. Cardiolipins from these organisms contained mainly monounsaturated or diunsaturated chains with 16 or 18 carbon atoms, resulting in a relatively homogeneous distribution of double bonds and carbon numbers among the four acyl positions. About half of the molecular species were symmetrical, i.e. they combined two identical diacylglycerol moieties. In N. crassa, the same species pattern was found at growth temperatures of 25 degrees C and 37 degrees C. Tentative molecular models were created for the most abundant molecular species and subjected to energy minimization. Geometric data, derived from these models, suggested similarities in the gross structure of the major cardiolipin species from different sources.

Metabolic regulations and biological functions of phospholipids in Escherichia coli

Extensive genetic and biochemical studies in the last two decades have elucidated almost completely the framework of synthesis and turnover of quantitatively major phospholipids in E. coli. The knowledge thus accumulated has allowed to formulate a novel working model that assumes sophisticated regulatory mechanisms in E. coli to achieve the optimal phospholipid composition and content in the membranes. E. coli also appears to possess the ability to adapt phospholipid synthesis to various cellular conditions. Understanding of the functional aspects of E. coli phospholipids is now advancing significantly and it will soon be able to explain many of the hitherto unclear cell's activities on the molecular basis. Phosphatidylglycerol is believed to play the central role both in metabolism and functions of phospholipids in E. coli. The results obtained with E. coli should undoubtedly be helpful in the study of more complicated phospholipid metabolism and functions in higher organisms.

Effect of 3,4-dihydroxybutyl-1-phosphonate on cardiolipin synthesis in B. subtilis

Endogenous phosphatidylglycerol is rapidly transformed into cardiolipin when B. subtilis 168 cells were incubated in a buffer without an energy source. Upon addition of 3,4-dihydroxybutyl-1-phosphonate (DHBP), a synthetic glycerol 3-phosphate analogue, this synthesis was completely blocked after a short lag; if the cells were grown in the presence of the analogue, there was no lag. When membrane fractions were incubated with exogenous [32P]phosphatidylglycerol, free DHBP and glycerol 3-phosphate had no effect on [32P]cardiolipin synthesis, but phosphatidyl-DHBP and phosphatidylglycerolphosphate were potent inhibitors. These results are consistent with our hypothesis that phosphatidylglycerolphosphate, the phosphatidylglycerol precursor, might also be a physical inhibitor of cardiolipin synthesis.

Bacillus subtilis mutant with temperature-sensitive net synthesis of phosphatidylethanolamine

Bacillus subtilis mutants with temperature-sensitive growth on complex media were screened for defects in phospholipid metabolism. One mutant was isolated that showed temperature-sensitive net synthesis of phosphatidylethanolamine. The mutant did not accumulate phosphatidylserine at the nonpermissive temperature. In the presence of hydroxylamine, wild-type B. subtilis accumulated phosphatidylserine at both 32 and 45 degrees C, whereas the mutant did only at 32 degrees C. In vitro phosphatidylethanolamine synthesis with bacterial membranes is no more temperature sensitive with mutant membranes than with wild-type membranes. The mutation probably affects the synthesis indirectly, possibly by altering a membrane protein. The mutant bacteria grew at the nonpermissive temperature, 45 degrees C, in a phosphate buffer-based minimal medium, although net synthesis of phosphatidylethanolamine was also temperature sensitive in this medium. One mutation caused both temperature-sensitive growth on complex media and temperature-sensitive net synthesis of phosphatidylethanolamine. The mutation is linked to aroD by transformation.

Effect of inhibition of protein synthesis on lipid metabolism in Lactobacillus plantarum

In Lactobacillus plantarum 17-5, lipid synthesis appears to be correlated with protein synthesis. Inhibition of protein synthesis by chloramphenicol (50 mug/ml) caused the nearly simultaneous inhibition of incorporation of radioactive oleic acid into polar lipids before the cessation of growth. In addition, de novo fatty acid synthesis, as determined by the incorporation of radioactive acetate into cellular lipids, was also inhibited. Removal of the antibiotic resulted in the resumption of growth, protein synthesis, and polar lipid synthesis. Inhibition of protein synthesis by leucine deprivation also produced a marked reduction in the incorporation of radioactive oleic acid into the total polar lipids at about the same time that growth stopped (30 to 60 min after the removal of leucine). However, the different classes of lipids behaved differently. For example, the incorporation of oleic acid into cardiolipin was inhibited immediately upon removal of leucine from the cultures, whereas incorporation into phosphatidyl-glycerol was maintained at near normal rates for 60 min after the removal of leucine and then ceased. In contrast, the accumulation of radioactive oleic acid in a neutral lipid identified as diglyceride occurred to a much greater extent in leucine-deprived cultures than in control (+ leucine) cultures. Upon addition of leucine to leucine-deprived cultures, the rates of synthesis of phosphatidyl-glycerol and cardiolipin returned to normal; the amount of radioactivity in the diglyceride fraction decreased to normal levels concomitantly with increased phospholipid synthesis.