Phospholipase A activity in growing Escherichia coli cells

Homeoviscous adaptation occurs with thermal acclimation in biological membranes from heart and gill, but not the brain, in the Antarctic fish Notothenia coriiceps

As temperatures continue to rise, adjustments to biological membranes will be key for maintenance of function. It is largely unknown to what extent Antarctic notothenioids possess the capacity to remodel their biological membranes in response to thermal change. In this study, physical and biochemical properties were examined in membranes prepared from gill epithelia (plasma membranes), cardiac ventricles (microsomes, mitochondria), and brains (synaptic membranes, myelin, mitochondria) from Notothenia coriiceps following acclimation to 5 °C (or held at ambient temperature, 0 °C) for a minimum of 6 weeks. Fluidity was measured between 0 and 30 °C in all membranes, and polar lipid compositions and cholesterol contents were analyzed in a subset of biological membranes from all tissues. Osmotic permeability was measured in gills at 0 and 4 °C. Gill plasma membranes, cardiac mitochondria, and cardiac microsomes displayed reduced fluidity following acclimation to 5 °C, indicating compensation for elevated temperature. In contrast, no fluidity changes with acclimation were observed in any of the membranes prepared from brain. In all membranes, adjustments to the relative abundances of major phospholipid classes, and to the extent of fatty acid unsaturation, were undetectable following thermal acclimation. However, alterations in cholesterol contents and acyl chain length, consistent with the changes in fluidity, were observed in membranes from gill and cardiac tissue. Water permeability was reduced with 5 °C acclimation in gills, indicating near-perfect homeostatic efficacy. Taken together, these results demonstrate a homeoviscous response in gill and cardiac membranes, and limited plasticity in membranes from the nervous system, in an Antarctic notothenioid.

Neisseria gonorrhoeae MlaA influences gonococcal virulence and membrane vesicle production

The six-component maintenance of lipid asymmetry (Mla) system is responsible for retrograde transport of phospholipids, ensuring the barrier function of the Gram-negative cell envelope. Located within the outer membrane, MlaA (VacJ) acts as a channel to shuttle phospholipids from the outer leaflet. We identified Neisseria gonorrhoeae MlaA (ngo2121) during high-throughput proteomic mining for potential therapeutic targets against this medically important human pathogen. Our follow-up phenotypic microarrays revealed that lack of MlaA results in a complex sensitivity phenome. Herein we focused on MlaA function in cell envelope biogenesis and pathogenesis. We demonstrate the existence of two MlaA classes among 21 bacterial species, characterized by the presence or lack of a lipoprotein signal peptide. Purified truncated N. gonorrhoeae MlaA elicited antibodies that cross-reacted with a panel of different Neisseria. Little is known about MlaA expression; we provide the first evidence that MlaA levels increase in stationary phase and under anaerobiosis but decrease during iron starvation. Lack of MlaA resulted in higher cell counts during conditions mimicking different host niches; however, it also significantly decreased colony size. Antimicrobial peptides such as polymyxin B exacerbated the size difference while human defensin was detrimental to mutant viability. Consistent with the proposed role of MlaA in vesicle biogenesis, the ΔmlaA mutant released 1.7-fold more membrane vesicles. Comparative proteomics of cell envelopes and native membrane vesicles derived from ΔmlaA and wild type bacteria revealed enrichment of TadA–which recodes proteins through mRNA editing–as well as increased levels of adhesins and virulence factors. MlaA-deficient gonococci significantly outcompeted (up to 16-fold) wild-type bacteria in the murine lower genital tract, suggesting the growth advantage or increased expression of virulence factors afforded by inactivation of mlaA is advantageous in vivo. Based on these results, we propose N. gonorrhoeae restricts MlaA levels to modulate cell envelope homeostasis and fine-tune virulence.

The Gram-negative outer membrane is a formidable barrier, primarily because of its asymmetric composition. A layer of lipopolysaccharide is exposed to the external environment and phospholipids are on the internal face of the outer membrane. MlaA is part of a bacterial system that prevents phospholipid accumulation within the lipopolysaccharide layer. If MlaA is removed, membrane asymmetry is disrupted and bacteria become more vulnerable to certain antimicrobials. Neisseria gonorrhoeae causes millions of infections worldwide annually. A growing number are resistant to available antibiotics. Improving our understanding of gonococcal pathogenicity and basic biological processes is required to facilitate the discovery of new weapons against gonorrhea. We investigated the role of MlaA in N. gonorrhoeae and found that when MlaA was absent, bacteria were more sensitive to antibiotics and human defensins. However, the mutant bacteria produced more membrane vesicles–packages of proteins wrapped in membrane material. Mutant vesicles and cell envelopes were enriched in proteins that contribute to disease. These alterations significantly increased mutant fitness during experimental infection of the female mouse genital tract. Our results provide new insights into the processes N. gonorrhoeae uses to fine-tune its ability to stay fit in the hostile environment of the genital tract.

Interplay between Colistin Resistance, Virulence and Fitness in Acinetobacter baumannii

Acinetobacter baumannii is an important opportunistic nosocomial pathogen often resistant to multiple antibiotics classes. Colistin, an “old” antibiotic, is now considered a last-line treatment option for extremely resistant isolates. In the meantime, resistance to colistin has been reported in clinical A. baumannii strains. Colistin is a cationic peptide that disrupts the outer membrane (OM) of Gram-negative bacteria. Colistin resistance is primarily due to post-translational modification or loss of the lipopolysaccharide (LPS) molecules inserted into the outer leaflet of the OM. LPS modification prevents the binding of polymyxin to the bacterial surface and may lead to alterations in bacterial virulence. Antimicrobial pressure drives the evolution of antimicrobial resistance and resistance is often associated with a reduced bacterial fitness. Therefore, the alterations in LPS may induce changes in the fitness of A. baumannii. However, compensatory mutations in clinical A. baumannii may ameliorate the cost of resistance and may play an important role in the dissemination of colistin-resistant A. baumannii isolates. The focus of this review is to summarize the colistin resistance mechanisms, and understand their impact on the fitness and virulence of bacteria and on the dissemination of colistin-resistant A. baumannii strains.

Biogenesis, transport and remodeling of lysophospholipids in Gram-negative bacteria

Lysophospholipids (LPLs) are metabolic intermediates in bacterial phospholipid turnover. Distinct from their diacyl counterparts, these inverted cone-shaped molecules share physical characteristics of detergents, enabling modification of local membrane properties such as curvature. The functions of LPLs as cellular growth factors or potent lipid mediators have been extensively demonstrated in eukaryotic cells but are still undefined in bacteria. In the envelope of Gram-negative bacteria, LPLs are derived from multiple endogenous and exogenous sources. Although several flippases that move non-glycerophospholipids across the bacterial inner membrane were characterized, lysophospholipid transporter LplT appears to be the first example of a bacterial protein capable of facilitating rapid retrograde translocation of lyso forms of glycerophospholipids across the cytoplasmic membrane in Gram-negative bacteria. LplT transports lyso forms of the three bacterial membrane phospholipids with comparable efficiency, but excludes other lysolipid species. Once a LPL is flipped by LplT to the cytoplasmic side of the inner membrane, its diacyl form is effectively regenerated by the action of a peripheral enzyme, acyl-ACP synthetase/LPL acyltransferase (Aas). LplT-Aas also mediates a novel cardiolipin remodeling by converting its two lyso derivatives, diacyl or deacylated cardiolipin, to a triacyl form. This coupled remodeling system provides a unique bacterial membrane phospholipid repair mechanism. Strict selectivity of LplT for lyso lipids allows this system to fulfill efficient lipid repair in an environment containing mostly diacyl phospholipids. A rocker-switch model engaged by a pair of symmetric ion-locks may facilitate alternating substrate access to drive LPL flipping into bacterial cells. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

The induction and identification of novel Colistin resistance mutations in Acinetobacter baumannii and their implications

Acinetobacter baumannii is a significant cause of opportunistic hospital acquired infection and has been identified as an important emerging infection due to its high levels of antimicrobial resistance. Multidrug resistant A. baumannii has risen rapidly in Vietnam, where colistin is becoming the drug of last resort for many infections. In this study we generated spontaneous colistin resistant progeny (up to >256 μg/μl) from four colistin susceptible Vietnamese isolates and one susceptible reference strain (MIC <1.5 μg/μl). Whole genome sequencing was used to identify single nucleotide mutations that could be attributed to the reduced colistin susceptibility. We identified six lpxACD and three pmrB mutations, the majority of which were novel. In addition, we identified further mutations in six A. baumannii genes (vacJ, pldA, ttg2C, pheS and conserved hypothetical protein) that we hypothesise have a role in reduced colistin susceptibility. This study has identified additional mutations that may be associated with colistin resistance through novel resistance mechanisms. Our work further demonstrates how rapidly A. baumannii can generate resistance to a last resort antimicrobial and highlights the need for improved surveillance to identified A. baumannii with an extensive drug resistance profile.

A Metagenomic Investigation of the Duodenal Microbiota Reveals Links with Obesity

Background

Few studies have tested the small intestine microbiota in humans, where most nutrient digestion and absorption occur. Here, our objective was to examine the duodenal microbiota between obese and normal volunteers using metagenomic techniques.

Methodology/Principal Findings

We tested duodenal samples from five obese and five normal volunteers using 16S rDNA V6 pyrosequencing and Illumina MiSeq deep sequencing. The predominant phyla of the duodenal microbiota were Firmicutes and Actinobacteria, whereas Bacteroidetes were absent. Obese individuals had a significant increase in anaerobic genera (p < 0.001) and a higher abundance of genes encoding Acyl-CoA dehydrogenase (p = 0.0018) compared to the control group. Obese individuals also had a reduced abundance of genes encoding sucrose phosphorylase (p = 0.015) and 1,4-alpha-glucan branching enzyme (p = 0.05). Normal weight people had significantly increased FabK (p = 0.027), and the glycerophospholipid metabolism pathway revealed the presence of phospholipase A1 only in the control group (p = 0.05).

Conclusions/Significance

The duodenal microbiota of obese individuals exhibit alterations in the fatty acid and sucrose breakdown pathways, probably induced by diet imbalance.

The Vps/VacJ ABC transporter is required for intercellular spread of Shigella flexneri

The Vps/VacJ ABC transporter system is proposed to function in maintaining the lipid asymmetry of the outer membrane. Mutations in vps or vacJ in Shigella flexneri resulted in increased sensitivity to lysis by the detergent sodium dodecyl sulfate (SDS), and the vpsC mutant showed minor differences in its phospholipid profile compared to the wild type. vpsC mutants were unable to form plaques in cultured epithelial cells, but this was not due to a failure to invade, to replicate intracellularly, or to polymerize actin via IcsA for movement within epithelial cells. The addition of the outer membrane phospholipase gene pldA on a multicopy plasmid in a vpsC or vacJ mutant restored its resistance to SDS, suggesting a restoration of lipid asymmetry to the outer membrane. However, the pldA plasmid did not restore the mutant's ability to form plaques in tissue culture cells. Increased PldA levels also failed to restore the mutant's phospholipid profile to that of the wild type. We propose a dual function of the Vps/VacJ ABC transporter system in S. flexneri in both the maintenance of lipid asymmetry in the outer membrane and the intercellular spread of the bacteria between adjacent epithelial cells.

Structural biology of membrane-intrinsic beta-barrel enzymes: sentinels of the bacterial outer membrane

The outer membranes of Gram-negative bacteria are replete with integral membrane proteins that exhibit antiparallel beta-barrel structures, but very few of these proteins function as enzymes. In Escherichia coli, only three beta-barrel enzymes are known to exist in the outer membrane; these are the phospholipase OMPLA, the protease OmpT, and the phospholipidColon, two colonslipid A palmitoyltransferase PagP, all of which have been characterized at the structural level. Structural details have also emerged for the outer membrane beta-barrel enzyme PagL, a lipid A 3-O-deacylase from Pseudomonas aeruginosa. Lipid A can be further modified in the outer membrane by two beta-barrel enzymes of unknown structure; namely, the Salmonella enterica 3'-acyloxyacyl hydrolase LpxR, and the Rhizobium leguminosarum oxidase LpxQ, which employs O(2) to convert the proximal glucosamine unit of lipid A into 2-aminogluconate. Structural biology now indicates how beta-barrel enzymes can function as sentinels that remain dormant when the outer membrane permeability barrier is intact. Host immune defenses and antibiotics that perturb this barrier can directly trigger beta-barrel enzymes in the outer membrane. The ensuing adaptive responses occur instantaneously and rapidly outpace other signal transduction mechanisms that similarly function to restore the outer membrane permeability barrier.

Vesicle-independent extracellular release of a proinflammatory outer membrane lipoprotein in free-soluble form

Background

Aggregatibacter actinomycetemcomitans is an oral bacterium associated with aggressively progressing periodontitis. Extracellular release of bacterial outer membrane proteins has been suggested to mainly occur via outer membrane vesicles. This study investigated the presence and conservation of peptidoglycan-associated lipoprotein (AaPAL) among A. actinomycetemcomitans strains, the immunostimulatory effect of AaPAL, and whether live cells release this structural outer membrane lipoprotein in free-soluble form independent of vesicles.

Results

The pal locus and its gene product were confirmed in clinical A. actinomycetemcomitans strains by PCR-restriction fragment length polymorphism and immunoblotting. Culturing under different growth conditions revealed no apparent requirement for the AaPAL expression. Inactivation of pal in a wild-type strain (D7S) and in its spontaneous laboratory variant (D7SS) resulted in pleiotropic cellular effects. In a cell culture insert model (filter pore size 0.02 μm), AaPAL was detected from filtrates when strains D7S and D7SS were incubated in serum or broth in the inserts. Electron microscopy showed that A. actinomycetemcomitans vesicles (0.05–0.2 μm) were larger than the filter pores and that there were no vesicles in the filtrates. The filtrates were immunoblot negative for a cytoplasmic marker, cyclic AMP (cAMP) receptor protein. An ex vivo model indicated cytokine production from human whole blood stimulated by AaPAL.

Conclusion

Free-soluble AaPAL can be extracellularly released in a process independent of vesicles.

Functional importance of calcium binding sites in outer membrane phospholipase A

Outer membrane phospholipase A (OMPLA) is an integral membrane enzyme that hydrolyses phospholipids requiring Ca(2+) as cofactor. In vitro studies have shown that OMPLA is only active as a dimer. The structures of monomeric and dimeric OMPLA provided possible clues to the activation process. In the inhibited dimeric species calcium ions are located at the dimer interface ideally suited to stabilise the oxyanion intermediates formed during catalysis. The side chain hydroxyl function of Ser152 is one of the ligands of this interfacial calcium. In the crystal structure of monomeric OMPLA the interfacial calcium site is lacking, but calcium was found to bind at a site involving the carboxylates of Asp149 and Asp184. In the current study the relevance of the identified calcium sites has been studied by site-directed mutagenesis. The Ser152Asn variant confirmed the importance of the interfacial calcium site for catalysis, and also demonstrated that this site is essentially involved in the dimerisation process. Replacements of the ligands in monomeric OMPLA, i.e. Asp149Asn, Asp149Ala and Asp184Asn, only showed minor effects on catalytic activity and dimerisation. A stronger effect observed for the variant Asp184Ala was explained by the proximity of Asp184 to the catalytically important Ser152 residue. We propose that Asp149 and Asp184 provide an electronegative funnel that may facilitate Ca(2+) transfer to the interfacial calcium site.

Substrate interferes with dimerisation of outer membrane phospholipase A

Outer membrane phospholipase A (OMPLA) activity is regulated by reversible dimerisation with the dimer being the active species. Observed lag phases in activity indicated that dimerisation may be slow relative to turnover. A covalent OMPLA dimer indeed did not display lag phase behaviour. A model for OMPLA kinetics was proposed accounting for a slow dimerisation step. Preincubation conditions determined the initial amount of monomer and influenced both lag times and final activities. Under the conditions used, substrate concentrations higher than 50 mol% inhibited OMPLA activity and increased lag times. Our results may shed more light on mechanisms controlling OMPLA activity in vivo.

Absence of the outer membrane phospholipase A suppresses the temperature-sensitive phenotype of Escherichia coli degP mutants and induces the Cpx and sigma(E) extracytoplasmic stress responses

DegP is a periplasmic protease that is a member of both the sigma(E) and Cpx extracytoplasmic stress regulons of Escherichia coli and is essential for viability at temperatures above 42 degrees C. [U-(14)C]acetate labeling experiments demonstrated that phospholipids were degraded in degP mutants at elevated temperatures. In addition, chloramphenicol acetyltransferase, beta-lactamase, and beta-galactosidase assays as well as sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated that large amounts of cellular proteins are released from degP cells at the nonpermissive temperature. A mutation in pldA, which encodes outer membrane phospholipase A (OMPLA), was found to rescue degP cells from the temperature-sensitive phenotype. pldA degP mutants had a normal plating efficiency at 42 degrees C, displayed increased viability at 44 degrees C, showed no degradation of phospholipids, and released far lower amounts of cellular protein to culture supernatants. degP and pldA degP mutants containing chromosomal lacZ fusions to Cpx and sigma(E) regulon promoters indicated that both regulons were activated in the pldA mutants. The overexpression of the envelope lipoprotein, NlpE, which induces the Cpx regulon, was also found to suppress the temperature-sensitive phenotype of degP mutants but did not prevent the degradation of phospholipids. These results suggest that the absence of OMPLA corrects the degP temperature-sensitive phenotype by inducing the Cpx and sigma(E) regulons rather than by inactivating the phospholipase per se.

Structural investigations of calcium binding and its role in activity and activation of outer membrane phospholipase A from Escherichia coli

Outer membrane phospholipase A (OMPLA) is an integral membrane enzyme that catalyses the hydrolysis of phospholipids. Enzymatic activity is regulated by reversible dimerisation and calcium-binding. We have investigated the role of calcium by X-ray crystallography. In monomeric OMPLA, one calcium ion binds between two external loops (L3L4 site) at 10 A from the active site. After dimerisation, a new calcium-binding site (catalytic site) is formed at the dimer interface in the active site of each molecule at 6 A from the L3L4 calcium site. The close spacing and the difference in calcium affinity of both sites suggests that the L3L4 site may function as a storage site for a calcium ion, which relocates to the catalytic site upon dimerisation. A sequence alignment demonstrates conservation of the catalytic calcium site but evolutionary variation of the L3L4 site. The residues in the dimer interface are conserved as well, suggesting that all outer membrane phospholipases require dimerisation and calcium in the catalytic site for activity. For this family of phospholipases, we have characterised a consensus sequence motif (YTQ-X(n)-G-X(2)-H-X-SNG) that contains conserved residues involved in dimerisation and catalysis.

Bacterial phospholipase A: structure and function of an integral membrane phospholipase

Within the large family of lipolytic enzymes, phospholipases constitute a very diverse subgroup with physiological functions such as digestion and signal transduction. Most phospholipases may associate with membranes at the lipid-water interface. However, in many Gram-negative bacteria, a phospholipase is present which is located integrally in the bacterial outer membrane. This phospholipase (outer membrane phospholipase A or OMPLA) is involved in transport across the bacterial outer membrane and has been implicated in bacterial virulence. OMPLA is calcium dependent and its activity is strictly regulated by reversible dimerisation. Recently the crystal structure of this integral membrane enzyme has been elucidated. In this review, we summarise the implications of these structural data for the understanding of the function and regulation of OMPLA, and discuss a mechanism for phospholipase dependent colicin release in Escherichia coli.

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.

Bacteriocin release protein triggers dimerization of outer membrane phospholipase A in vivo

Bacteriocin release protein is known to activate outer membrane phospholipase A (OMPLA), which results in the release of colicin from Escherichia coli. In vivo chemical cross-linking experiments revealed that the activation coincides with dimerization of OMPLA. Permeabilization of the cell envelope and dimerization were characterized by a lag time of 2 h.

Dimerization regulates the enzymatic activity of Escherichia coli outer membrane phospholipase A

The outer membrane phospholipase A (OMPLA) of Escherichia coli is present in a dormant state in the cell envelope. The enzyme is activated by various processes, which have in common that they perturb the outer membrane. Kinetic experiments, chemical cross-linking, and analytical ultracentrifugation were carried out with purified, detergent-solubilized OMPLA to understand the underlying mechanism that results in activation. Under conditions in which the enzyme displayed full activity, OMPLA was dimeric. High detergent concentrations or very dilute protein concentrations resulted in low specific activity of the enzyme, and under those conditions the enzyme was monomeric. The cofactor Ca2+ was required for dimerization. Covalent modification of the active site serine with hexadecylsulfonylfluoride resulted in stabilization of the dimeric form and a loss of the absolute calcium requirement for dimerization. The results of these experiments provide evidence for dimerization as the molecular mechanism by which the enzymatic activity of OMPLA is regulated. This dimerization probably plays a role in vivo as well. Data from chemical cross-linking on whole cells indicate that OMPLA is present in the outer membrane as a monomer and that activation of the enzyme induces dimerization concurrent with the appearance of enzymatic activity.

Determinants of activation by complement of group II phospholipase A2 acting against Escherichia coli

Prompt killing of many strains of Escherichia coli during phagocytosis in vitro by isolated polymorphonuclear leukocytes (PMN) requires the presence of nonlethal doses of nonimmune serum (B. A. Mannion, J. Weiss, and P. Elsbach, J. Clin. Invest. 86:631-641, 1990). Because this requirement is bypassed in a phospholipase A (PLA)-rich mutant (pldA ) of E. coli, we have examined the effect of serum on bacteria] phospholipid (PL) degradation during phagocytosis of wild-type (pldA+) and PLA-deficient (pldA) E. coli. In parallel with increased killing, nonlethal doses of serum increased the degradation of prelabeled bacterial PL during phagocytosis by two- to fivefold, to nearly the same levels (ca. 50 to 60%) as those produced during phagocytosis of E. coli pldA in the absence of serum. The effects on the E. coli pldA mutant imply that there is a serum-mediated enhancement of granule-associated group II PMN PLA2 activity. At the same doses, serum promoted action against E. coli in the presence of purified rabbit and human group II PLA2 but did not activate bacterial PLA. Related PLA2s that lack specific structural determinants needed for optimal activity against E. coli treated with the bactericidal/permeability-increasing protein (BPI) of PMN are also less active than wild-type group II PLA2 against serum-treated E. coli. Treatment of E. coli with C7- or C9-depleted serum did not enhance bacterial killing or PL degradation during phagocytosis or the action of purified PLA2. In summary, these findings suggest that (i) nonlethal assemblies of the membrane attack complex promote intracellular killing and destruction of E. coli ingested by PMN, in part by promoting the action of granule-associated PLA2 against ingested bacteria, and (ii) structural determinants first implicated in PLA2 action against BPI-treated E. coli are also important in PLA2 action in concert with other host defense systems, such as complement.

A conserved histidine residue of Escherichia coli outer-membrane phospholipase A is important for activity

Escherichia coli outer-membrane phospholipase A (OMPLA) is thought to be a member of the class of serine hydrolases, having a classical Asp-His-Ser catalytic triad [Horrevoets, A. J. G., Verheij, H. M. & de Haas, G. H. (1991) Eur. J. Biochem. 198, 247-253]. To identify the histidine residue that is important for catalytic activity, the four histidine residues in E. coli OMPLA that are conserved in other enterobacterial OMPLA enzymes were replaced by cysteine residues using PCR-directed, site-specific mutagenesis. The resulting mutant proteins were all well expressed and displayed heat modifiability, indicating that they were properly folded. Enzyme assays showed that only the His142Cys mutant protein was lacking enzymatic activity. In addition, a His142Gly mutant protein appeared to be inactive. These results show that His142 is important for the enzymatic activity of OMPLA.

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.

Purification and kinetics of extracellular phospholipase A of Salmonella newport

Attempts were made to purify and study the kinetics of extracellular phospholipase A of Salmonella newport (6,8, eb; 1,2). The enzyme was purified by salt precipitation followed by gel filtration, using different grades of Sephadex. The enzymically active purified preparation was found to be a protein, having molar mass ranging between 43 and 67 kDa. The enzyme had a pH optimum at 7.5, giving 18.2 micrograms of lysophosphatidylcholine per mg protein. Its activity was enhanced by all metal ions except potassium, by solvents and surfactants except sodium dodecyl sulfate. It hydrolyzed the membrane phospholipids of red blood cells and was inhibitory to the growth of other microorganisms.

Activation of reconstituted Escherichia coli outer-membrane phospholipase A by membrane-perturbing peptides results in an increased reactivity towards the affinity label hexadecanesulfonyl fluoride

The activity of the Escherichia coli outer-membrane phospholipase (OM PLA) is strictly regulated in its natural habitat, the E. coli outer membrane. OM PLA can be reconstituted in phospholipid bilayers, resulting in low specific activity of the enzyme compared to its activity on mixed lipid/detergent micelles. The enzyme can be activated by the addition to these vesicles of the membrane-perturbing peptides polymyxin B, melittin or cardiotoxin resulting in hydrolysis of mainly the sn-1 ester bond of the phospholipids as is also observed in vivo. We used the affinity label hexadecanesulfonyl fluoride to probe the influence of lipid environment on the activity of OM PLA. In detergent and substrate micelles, the rate constant for the sulfonylation of the active-center serine of the purified OM PLA by the affinity label hexadecanesulfonyl fluoride depends on amphiphile concentration. We have reported a similar influence of amphiphile concentration on the activity of the enzyme [Horrevoets, A. J. G. et al. (1989) Biochemistry 28, 1139-1147]. Analysis of the rates of inactivation of OM PLA by hexadecanesulfonyl fluoride in vesicles composed of various phospholipids indicated that activation of the enzyme by membrane-perturbing peptides can be accurately quantified with this affinity label. Our results show that the affinity label hexadecanesulfonyl fluoride can be used to monitor the state of activation of OM PLA in different lipid environments, including non-hydrolyzable substrate analogues. Implications for the in vivo situation are discussed.

Inactivation of Escherichia coli outer-membrane phospholipase A by the affinity label hexadecanesulfonyl fluoride. Evidence for an active-site serine

The Escherichia coli outer-membrane phospholipase A (OM PLA) is a membrane-bound acyl hydrolase with a broad substrate specificity. In order to obtain more insight into the mechanism of action of this enzyme, we designed an active-site-directed inhibitor for OM PLA on the basis of the known substrate specificity as a first step in the elucidation of the catalytic mechanism of this enzyme. The inhibitor, hexadecanesulfonyl fluoride, consists of a long hydrocarbon chain for high-affinity binding by the enzyme and a sulfonyl fluoride moiety as a reactive group. The kinetics of the inactivation of OM PLA by hexadecanesulfonyl fluoride were studied in Triton X-100 micelles. Inactivation is very fast, specific and shows the same characteristics with respect to acyl specificity, pH profile and metal ion requirement as the activity of OM PLA on substrates. Incubation of OM PLA with a stoichiometric amount of hexadecanesulfonyl fluoride leads to a total and irreversible loss of enzyme activity, resulting from the sulfonylation of Ser144. This Ser144, which we suggest to be the active-site serine of OM PLA, is part of the sequence HDSNG, whereas in the water-soluble serine proteases and lipases the structural motif GXSXG is normally encountered. On the basis of the kinetics of inactivation of OM PLA by hexadecanesulfonyl fluoride, we discuss a possible catalytic mechanism of the enzyme.

A comparison of brush-border membranes prepared from rabbit small intestine by procedures involving Ca2+ and Mg2+ precipitation

Brush-border membranes were isolated from rabbit small intestine by procedures involving precipitation of undesired membranes with either 10 mM MgCl2 or 10 mM CaCl2. The membranes were compared on the basis of marker enzyme content and lipid composition. Ca2+-prepared membranes displayed a greater enrichment of alkaline phosphatase and sucrase activity compared to homogenate than did the Mg2+-prepared membranes. The former also displayed an impoverishment of (Na+ + K+)-ATPase activity, the specific activity of which increased several-fold in Mg2+-prepared membranes. Membranes prepared with Ca2+ were characterized by a lower phosphoacylglycerol-protein ratio and a higher phosphatidylethanolamine-phosphatidylcholine ratio. Although lysophosphoacylglycerols accounted for about 6% of the total phospholipids in these membranes compared to 2% in Mg2+-prepared membranes, the free fatty acid content was similar in both types of membranes. It was concluded that Ca2+ prepared membranes were less contaminated by basolateral membranes than were Mg2+-prepared membranes and the use of Ca2+ did not notably enhance degradation of endogenous lipids by brush-border membrane phospholipase A.

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.

Degraded and stable phosphatidylglycerol in Escherichia coli inner and outer membranes, and recycling of fatty acyl residues

The metabolic fate of membrane phospholipids in exponentially growing Escherichia coli was reexamined by incorporation and chase of labeled precursors: [32P]phosphate, [2-3H]glycerol and 3H-labeled fatty acids. It was found that the well-known turnover of phosphatidylglycerol lasted only about two generation times; after which period, the remaining labeled phosphatidylglycerol, approximately one-third of the total, was stable for at least the subsequent two generation times. The location of the stable phosphatidylglycerol pool remaining after the turnover in the outer and inner membrane was investigated. Both envelopes were found to contain stable phosphatidylglycerol so that the existence of a stable portion cannot be ascribed to its exclusive location in one leaflet. In some experiments, a small loss of labeled phosphatidylethanolamine was also observed, and upon fractionation this was found to occur exclusively in the outer membrane. [32P]Phosphate and [2-3H]glycerol labels of the degraded phospholipids were lost from lipid-soluble material, whereas labeled fatty acid, palmitate or oleate was reincorporated into newly synthesized phosphatidylethanolamine and phosphatidylglycerol, so that total fatty acid label remained constant in (membrane) phospholipid during chase. In view of the amount of glycerol lost, the recycling of the fatty acids under the form of diacylglycerols to phosphatidic acid does not appear to be the predominant pathway of reincorporation. After double labeling with [32P]phosphate and [3H]palmitate, followed by chase, a complete balance sheet of loss and reincorporation of fatty acid, in the three phospholipids, in the two envelopes could be established. Results indicate that fatty acid was reincorporated essentially in the inner membrane phospholipids. Movements of phospholipids and of fatty acids from one membrane to another and in the plane of each layer are discussed in the light of the results.

Phospholipase A activity in supernatants from cultures of Bacteroides melaninogenicus

The phospholipase A activity in culture supernatants of two strains of Bacteroides melaninogenicus is described. The enzyme utilize phosphatidylcholine as substrate and produce mainly lysophosphatidylcholine and free fatty acids. The activities are Ca2+-independent, are not affected by the presence of a chelating agent, have a broad pH range (5-9) and an optimum temperature for activity of approx. 50 degrees C. The activity in a growing bacterial culture increases from the end of the lag phase to the late exponential phase of growth. Analysis of the products resulting from the actions of the enzymes on L-alpha-palmitoyl-beta-oleoyl[1-14C]phosphatidylcholine indicates that the enzymes are phospholipase A1 (EC 3.1.1.32).

Phospholipase A2 activity of the regularly arranged surface protein of Acinetobacter sp.199A

The regularly arranged surface protein, the a-protein, of Acinetobacter 199A has been shown to have phospholipase A2 activity. Since half of the a-protein synthesised by Acinetobacter 199A is secreted into the growth medium, the bacteria are producing extracellular phospholipase A2.

Phospholipid composition and phospholipase A activity of Neisseria gonorrhoeae

Exponential-phase cells of Neisseria gonorrhaeae 2686 were examined for phospholipid composition and for membrane-associated phospholipase A activity. When cells were harvested by centrifugation, washed, and lyophilized before extraction, approximately 74% of the total phospholipid was phosphatidylethanolamine, 18% was phosphatidylglycerol, 2% was cardiolipin, and 10% was lysophosphatidylethanolamine. However, when cells still suspended in growth medium were extracted, the amount of lysophosphatidylethanolamine decreased to approximately 1% of the phospholipid composition. This suggests that a gonococcal phospholipase A may be activated by conditions encountered during centrifugation and/or lyophilization of cells preceding extraction. Phospholipase A activity associated with cell membranes was assayed by measuring the conversion of tritiated phosphatidylethanolamine to lysophosphatidylethanolamine. Optimal activity was demonstrated in 10% methanol at pH 8.0 to 8.5, in the presence of calcium ions. The activity was both detergent sensitive and thermolabile. Comparisons of gonococcal colony types 1 and 4 showed no significant differences between the two types with respect to either phospholipid content or phospholipase A activity.

Polyglycerophosphatide metabolism in Escherichia coli

When Escherichia coli B cells were labelled with [14-C] glycerol and chased, there was a marked sparing of the phosphatidyl moiety compared to the nonacylated glycerol moiety of phosphatidylglycerol. When energy-depleted cells were restored to an energy-rich medium there resulted a conversion of 32-P-labelled cardiolipin to phosphatidylglycerol, a lack of phosphatidic acid accumulation and no loss in total polyglycerophosphatide counts. In cell-free extracts, phosphatidic acid produced from 32-P-labelled cardiolipin by the action of Escherichia coli phosphalipase D, was readily recycled to form poly-glycerophosphatide. In the presence of glycerol, such extracts displayed traansphosphatidylase activity by degrading cardiolipin to phosphatidyglycerol mainly. The results as a whole indicate that the enzyme synthesizing cardiolipin together with cardiolipin-hydrolyzing phospholipase D constitute a cycle which is normally involved in the turnover of polyglycerophosphatides in Escherichia coli.