Mutant of Escherichia coli K-12 deficient for detergent-resistant phospholipase A

Outer-membrane phospholipase A: known structure, unknown biological function

Outer-membrane phospholipase A (OMPLA) is one of the few enzymes present in the outer membrane of Gram-negative bacteria. The enzymatic activity of OMPLA is strictly regulated to prevent uncontrolled breakdown of the surrounding phospholipids. The activity of OMPLA can be induced by membrane perturbation and concurs with dimerization of the enzyme. The recently elucidated crystal structures of the inactive, monomeric and an inhibited dimeric form of the enzyme provide detailed structural insight into the functional properties of the enzyme. OMPLA is a serine hydrolase with a unique Asn-156-His-142-Ser-144 catalytic triad. Only in the dimeric state, complete substrate binding pockets and functional oxyanion holes are formed. A model is proposed for the activation of OMPLA in which membrane perturbation causes the formation of non-bilayer structures, resulting in the presentation of phospholipids to the active site of OMPLA and leading to the formation of the active dimeric species. Possible roles for OMPLA in maintaining the cell envelope integrity and in pathogenicity are discussed.

Bacterial phospholipases and pathogenesis

Phospholipases are produced from a diverse group of bacterial pathogens causing very different diseases. In some cases, secreted phospholipases appear to be the major cause of pathophysiological effects. Yet in other cases, phospholipases are key virulence factors, contributing to bacterial survival or dissemination without causing tissue destruction. Perhaps the most intriguing aspect of phospholipases as virulence factors is their potential to interfere with cellular signaling cascades and to modulate the host immune response.

Molecular characterization of enterobacterial pldA genes encoding outer membrane phospholipase A

The pldA gene of Escherichia coli encodes an outer membrane phospholipase A. A strain carrying the most commonly used mutant pldA allele appeared to express a correctly assembled PldA protein in the outer membrane. Nucleotide sequence analysis revealed that the only difference between the wild type and the mutant is the replacement of the serine residue in position 152 by phenylalanine. Since mutants that lack the pldA gene were normally viable under laboratory conditions and had no apparent phenotype except for the lack of outer membrane phospholipase activity, the exact role of the enzyme remains unknown. Nevertheless, the enzyme seems to be important for the bacteria, since Western blotting (immunoblotting) and enzyme assays showed that it is widely spread among species of the family Enterobacteriaceae. To characterize the PldA protein further, the pldA genes of Salmonella typhimurium, Klebsiella pneumoniae, and Proteus vulgaris were cloned and sequenced. The cloned genes were expressed in E. coli, and their gene products were enzymatically active. Comparison of the predicted PldA primary structures with that of E. coli PldA revealed a high degree of homology, with 79% of the amino acid residues being identical in all four proteins. Implications of the sequence comparison for the structure and the structure-function relationship of PldA protein are discussed.

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.

Expression of the kil gene of the ColE1 plasmid in Escherichia coli Kilr mutants causes release of periplasmic enzymes and of colicin without cell death

Expression of the kil gene of the ColE1 plasmid in certain classes of Escherichia coli mutants (Kilr) resistant to kil-caused cell death brought about release of periplasmic enzymes and of colicin. Phospholipase A was present but was not activated by kil expression in any of the mutants. This indicates that in these mutants the various effects of kil gene expression have become dissociated.

Effects of the bactericidal/permeability-increasing protein of polymorphonuclear leukocytes on isolated bacterial cytoplasmic membrane vesicles

The bactericidal/permeability-increasing protein (BPI) of polymorphonuclear leukocytes is a potent bactericidal agent specific for gram-negative bacteria. The protein blocks bacterial multiplication without substantially inhibiting the uptake and incorporation of macromolecular precursors, suggesting that the cytoplasmic membrane escapes early injury. Because greater than 90% of bound BPI can be removed from the bacterial surface sites after irreversible loss of viability, it was uncertain whether BPI reaches the cytoplasmic membrane and, if so, affects its functions. This study shows that BPI caused similar dose-dependent inhibition of O2 consumption and metabolic energy-dependent amino acid transport by cytoplasmic membrane vesicles of both gram-negative (Escherichia coli) and gram-positive (Bacillus subtilis) bacteria. Near maximal inhibition occurred at BPI doses that caused complete killing of an equivalent number of intact E. coli, with binding of BPI to membrane vesicles that was less than or equal to 10% of binding to intact (BPI-sensitive) bacteria. The effects of BPI and of the membrane-disruptive peptide antibiotic polymyxin B on membrane vesicles were distinctly different, indicating that the two agents affect membrane function by different mechanisms. BPI also rapidly inhibited O2 consumption by intact E. coli, with minimal impairment of bacterial protein synthesis. Thus, BPI is capable of damaging the cytoplasmic membrane of both gram-negative and gram-positive bacteria and of inhibiting at least one cytoplasmic membrane-associated function in intact E. coli. The relationship between these effects and the mechanism of bacterial killing by BPI remains to be established.

Overproduction and high-yield purification of phospholipase A from the outer membrane of Escherichia coli K-12

The cloned gene for the outer-membrane-bound phospholipase A from Escherichia coli was placed under control of the strong PL promoter of phage lambda. Induction of PL resulted in a 250-fold overexpression up to about 2% total cellular protein. This overproduced enzyme was indistinguishable from the wild-type enzyme. A homogeneous phospholiphase A preparation was obtained in high yield from overproducing bacteria, using the zwitterionic detergent C12-Sulfobetaine and anion-exchange chromatography. Detergent gradients were found to exert great influence on the elution characteristics. Considerations for the choice of optimal detergent gradients are discussed. The purified enzyme migrated as a single 29-kDa band in SDS/polyacrylamide gels, and required Ca(II) for activity. Maximum activity was displayed by enzyme samples taken from solutions with detergent concentrations near the critical micelle concentration. However, upon switching from high to optimal detergent concentration, maximum activity was restored in several hours, probably reflecting a slow conformational transition of the protein. Because the final pure protein was found to hydrolyze phospholipids in the intact erythrocyte membrane, a densely packed bilayer, we assume that this protein is in its biological native state.

Molecular cloning of pldA, the structural gene for outer membrane phospholipase of E. coli K12

The pldA gene of Escherichia coli K12, which is involved in the synthesis of an outer membrane (OM) phospholipase, has been cloned using a cosmid cloning system. For detection of the cloned gene a newly developed, in vivo phospholipase assay was used. Subsequent cloning of the pldA gene was performed into the multicopy plasmid vectors pBR322 and pACYC184. The gene was localised on these hybrid plasmids by the analysis of in vitro-constructed deletion plasmids and mutant plasmids generated by transposon gamma delta-insertions. Analysis of plasmid-encoded proteins in a minicell system showed that the pldA gene product is a polypeptide with apparent molecular weight of 29,000. This apparent molecular weight changes from 29,000 to 26,000 when the denaturing temperature is changed from 95 degrees C to 37 degrees C. These data are in agreement with those on purified OM phospholipase (Nishijima et al. 1977), and therefore strongly suggest that pldA is the structural gene for this phospholipase. From the minicell experiments the direction of transcription of pldA could be established relative to the metE gene, which is also cloned on the same hybrid plasmids. Strains carrying the pldA gene on these high copy vectors do not appear to be affected by the product with respect to cell growth in any way. However they do harbour increased amounts of 29 K protein in cell envelope fractions, indicating that gene expression and product translocation to the OM are proportional to the increased gene copy number. We therefore conclude that phospholipase enzymatic activity is strictly regulated at the protein level.

31P nuclear magnetic resonance and freeze-fracture electron microscopy studies on Escherichia coli. III. The outer membrane

1. The outer membrane of a phospholipase A-deficient mutant of Escherichia coli K12, isolated without the use of EDTA and lysozyme, showed the same freeze-fracture morphology as that seen in cells and remained stable for hours as observed by 31P-NMR. 2. 31P-NMR spectroscopy of the isolated outer membranes revealed that the lipopolysaccharide exists in the same physical state as in phospholipid-lipopolysaccharide liposomes and is most probably arranged in a bilayer at 37 degrees C. The outer membrane contains most or all of the phospholipids at 37 degrees C, and all the phospholipids at 20 degrees C, as a bilayer. 3. The 31P-NMR spectroscopy of the outer membranes from a mutant strain lacking the major outer membrane protein b, c and d (60% of the total outer membrane protein) yields virtually the same spectrum as the wild-type outer membranes, although most of the particles and pits which were observed in wild-type outer membranes in freeze-fracture electron microscopy were absent. 4. Whereas treatment of wild-type outer membranes with calcium ions has no effect on the 31P-NMR spectrum, treatment with EDTA results in more motion of the lipopolysaccharide.

The use of a phospholipase A-less Escherichia coli mutant to establish the action of granulocyte phospholipase A on bacterial phospholipids during killing by a highly purified granulocyte fraction

Phospholipase A2 present in a highly purified, potently bactericidal, fraction from rabbit graulocytes produces net bacterial phospholipid degradation during killing of a phospholipase A-less strain of Escherichia coli. In the wild-type parent strain phospholipid breakdown is caused not only by the action of phospholipase A2 but also by phospholipase A1, indicating activation of the most prominent phospholipase of E. coli. This activation occurs as soon as the bacteria are exposed to the granulocyte fraction. Phospholipid breakdown by both phospholipases A is dose dependent but reaches a plateau after 30-60 min and at higher concentrations of the fraction. Phospholipid degradation is accompanied in both strains by an increase in permeability to actinomycin D that is also dose dependent. Even though net hydrolysis of phospholipids is greater in the parent strain than in the mutant, the increase in permeability is the same in the two strains. The addition of 0.04 M Mg2+, after the effects on phospholipids and permeability have become manifest, initiates in both strains the restoration of insensitivity to actinomycin D, the net resynthesis of phospholipids, and the disappearance of monoacylphosphatides and the partial disappearance of free fatty acids that had accumulated. Loss of ability to multiply is not reversed by Mg2+ in either strain. Less than 5 micrograms of granulocyte fraction causes loss of viability of from 90 to 99% of 1 X 10(8) microorganisms of both strains. However, at lower concentrations the parent strain is considerably more sensitive to the bactericidal effect of the granulocyte fraction than the mutant strain.

Detergent-resistant phospholipase A of Escherichia coli K-12. Purification and properties

Detergent-resistant phospholipase A, which is tightly bound to the outer membranes of Escherichia coli K-12 cells, was purified approximately 2000-fold to near homogeneity by solubilization with sodium dodecylsulfate and butan-1-ol, acid precipitation, acetone fractionation and column chromatographies on Sephadex G-100 in the presence of sodium dodecylsulfate and on DEAE-cellulose in the presence of Triton X-100. The final preparation showed a single band in the sodium dodecylsulfate gel system. The enzyme hydrolyzes both the 1-acyl and 2-acyl chains of phosphatidylethanolamine or phosphatidylcholine. It also attacks 1-acyl and 2-acylglycerylphosphorylethanolamine. Thus, this enzyme shows not only phospholipase A1 and lysophospholipase L1 activities but also phospholipase A2 and lysophospholipase L2 activities. The enzyme lost its activity completely on incubation at 80 degrees C for 5 min at either pH 6.4 or pH 8.0. It was stable in 0.5% sodium dodecylsulfate at below 40 degrees C. The enzyme was inactivated on incubation for 5 min at 90 degrees C in 1% sodium dodecylsulfate/1% 2-mercaptoethanol/4 M urea. The native and inactivated enzymes showed different protein bands with RF values corresponding to Mr 21 000 and Mr 28 000 respectively, in a sodium dodecylsulfate gel system. Triton X-100 seemed to protect the enzyme from inactivation. The purified enzyme was fully active on phosphatidylethanolamine in the presence of 0.0002% or 0.05% Triton X-100. The enzyme requires Ca2+. From its properties this enzyme seems to be identical with the enzyme purified from crude extracts of Escherichia coli B by Scandella and Kornberg. However, it differs from the latter in its positional specificity and susceptibility to sodium dodecylsulfate. Possible explanation of the difference of positional specificity of the two preparations is also described.

Phospholipase activity plays no role in the action of colicin K

1. A mutant lacking both detergent-resistant and detergent-sensitive phospholipase A activities is fully sensitive to colicin K. 2. In the absence of cellular phospholipases A, colicin K does not promote hydrolysis of phosphatidylethanolamine. 3. Cells of the colicin-treated mutant lacking lysophosphatidylethanolamine are as abnormally permeable to Co-2+ as the wind type is. 4. Increased levels of lysophosphatidylethanolamine in colicin-treated cells are not necessary for the increased sensitivity to sodium dodecylsulfate.

Genetic mapping of the locus for detergent-resistant phospholipase A(pldA) in Escherichia coli K-12

An Escherichia coli K-12 mutant deficient for detergent-resistant (DR) phospholipase A, a principal enzyme catalyzing the first step in phospholipid degradation, was characterized genetically. The mutation was found to affect the locus pldA (phospholipid degradation), which is cotransducible both with ilv and metE at a frequencies of 13 and 78%, respectively, and shown to lie between the ilv and metE loci on the E. coli chromosome. DR phospholipase A(1) and A(2) activities were simultaneously transduced with pldA(+) by phage P1; therefore it is proposed that DR phospholipase A has both activities.