Transfer of phosphoethanolamine residues from phosphatidylethanolamine to the membrane-derived oligosaccharides of Escherichia coli

The Power of Asymmetry: Architecture and Assembly of the Gram-Negative Outer Membrane Lipid Bilayer

Determining the chemical composition of biological materials is paramount to the study of natural phenomena. Here, we describe the composition of model gram-negative outer membranes, focusing on the predominant assembly, an asymmetrical bilayer of lipid molecules. We also give an overview of lipid biosynthetic pathways and molecular mechanisms that organize this material into the outer membrane bilayer. An emphasis is placed on the potential of these pathways as targets for antibiotic development. We discuss deviations in composition, through bacterial cell surface remodeling, and alternative modalities to the asymmetric lipid bilayer. Outer membrane lipid alterations of current microbiological interest, such as lipid structures found in commensal bacteria, are emphasized. Additionally, outer membrane components could potentially be engineered to develop vaccine platforms. Observations related to composition and assembly of gram-negative outer membranes will continue to generate novel discoveries, broaden biotechnologies, and reveal profound mysteries to compel future research.

Diversity of non-stoichiometric substitutions on the lipopolysaccharide of E. coli C demonstrated by electrospray ionization single quadrupole mass spectrometry

The lipopolysaccharide (LPS) of enterobacteria frequently contains various numbers of charged non-stoichiometric substituents such as phosphate (P) and ethanolamine (EtN) groups and a third residue of 3-deoxy-D-manno-2-octulosonic acid (KDO) on the R-core polysaccharide backbone. These substituents can modify the biological activities of LPS including varying the stability of the outer membrane, tolerance to cationic antibiotics, pathogenicity, and sensitivity to enterobacteria bacteriophages. These diverse substituents can be clearly detected in degraded samples of LPS from E. coli C using electrospray ionization single quadrupole mass spectrometry (ESI-Q-MS) from a 0.1 mg/mL solution in a 50:50 mixture of methanol and 10 mM ammonium acetate (pH 6.8). The O-deacylated derivative showed multiple peaks of [M-3H](3-) ions which corresponded to species having up to eight phosphates, two ethanolamines, and an additional KDO on the backbone of Hex(5) Hep(3) KDO(2) GlcN(2) C14:0(3-OH)(2). The major components of the O,N-deacylated derivative were the species associated with four and five phosphates on Hex(5) Hep(3) KDO(2) GlcN(2). The polysaccharide portion of LPS also revealed species which corresponded to Hex(5) Hep(3) KDO associated with two to four phosphates and an ethanolamine. The present method was proved to be useful to investigate the structural diversity of enterobacterial LPS.

Genetic and functional analyses of PptA, a phospho-form transferase targeting type IV pili in Neisseria gonorrhoeae

The PilE pilin subunit protein of Neisseria gonorrhoeae undergoes unique covalent modifications with phosphoethanolamine (PE) and phosphocholine (PC). The pilin phospho-form transferase A (PptA) protein, required for these modifications, shows sequence relatedness with and architectural similarities to lipopolysaccharide PE transferases. Here, we used regulated expression and mutagenesis as means to better define the relationships between PptA structure and function, as well as to probe the mechanisms by which other factors impact the system. We show here that pptA expression is coupled at the level of transcription to its distal gene, murF, in a division/cell wall gene operon and that PptA can act in a dose-dependent fashion in PilE phospho-form modification. Molecular modeling and site-directed mutagenesis provided the first direct evidence that PptA is a member of the alkaline phosphatase superfamily of metalloenzymes with similar metal-binding sites and conserved structural folds. Through phylogenetic analyses and sequence alignments, these conclusions were extended to include the lipopolysaccharide PE transferases, including members of the disparate Lpt6 subfamily, and the MdoB family of phosphoglycerol transferases. Each of these enzymes thus likely acts as a phospholipid head group transferase whose catalytic mechanism involves a trans-esterification step generating a protein-phospho-form ester intermediate. Coexpression of PptA with PilE in Pseudomonas aeruginosa resulted in high levels of PE modification but was not sufficient for PC modification. This and other findings show that PptA-associated PC modification is governed by as-yet-undefined ancillary factors unique to N. gonorrhoeae.

A phosphoethanolamine transferase specific for the outer 3-deoxy-D-manno-octulosonic acid residue of Escherichia coli lipopolysaccharide. Identification of the eptB gene and Ca2+ hypersensitivity of an eptB deletion mutant

Addition of a phosphoethanolamine (pEtN) moiety to the outer 3-deoxy-D-manno-octulosonic acid (Kdo) residue of lipopolysaccharide (LPS) in WBB06, a heptose-deficient Escherichia coli mutant, occurs when cells are grown in 5-50 mM CaCl2 (Kanipes, M. I., Lin, S., Cotter, R. J., and Raetz, C. R. H. (2001) J. Biol. Chem. 276, 1156-1163). A Ca2+-induced, membrane-bound enzyme was responsible for the transfer of the pEtN unit to the Kdo domain. We now report the identification of the gene encoding the pEtN transferase. E. coli yhjW was cloned and overexpressed, because it is homologous to a putative pEtN transferase implicated in the modification of the beta-chain heptose residue of Neisseria meningitidis lipo-oligosaccharide (Mackinnon, F. G., Cox, A. D., Plested, J. S., Tang, C. M., Makepeace, K., Coull, P. A., Wright, J. C., Chalmers, R., Hood, D. W., Richards, J. C., and Moxon, E. R. (2002) Mol. Microbiol. 43, 931-943). In vitro assays with Kdo2-4'-[32P]lipid A as the acceptor showed that YhjW (renamed EptB) utilizes phosphatidylethanolamine in the presence of Ca2+ to transfer the pEtN group. Stoichiometric amounts of diacylglycerol were generated during the EptB-catalyzed transfer of pEtN to Kdo2-lipid A. EptB is an inner membrane protein of 574 amino acid residues with five predicted trans-membrane segments within its N-terminal region. An in-frame replacement of eptB with a kanamycin resistance cassette rendered E. coli WBB06 (but not wild-type W3110) hypersensitive to CaCl2 at 5 mM or higher. Ca2+ hypersensitivity was suppressed by excess Mg2+ in the medium or by restoring the LPS core of WBB06. The latter was achieved by reintroducing the waaC and waaF genes, which encode LPS heptosyl transferases I and II, respectively. Our data demonstrate that pEtN modification of the outer Kdo protected cells containing heptose-deficient LPS from damage by high concentrations of Ca2+. Based on its sequence similarity to EptA(PmrC), we propose that the active site of EptB faces the periplasmic surface of the inner membrane.

Unique modifications with phosphocholine and phosphoethanolamine define alternate antigenic forms of Neisseria gonorrhoeae type IV pili

Several major bacterial pathogens and related commensal species colonizing the human mucosa express phosphocholine (PC) at their cell surfaces. PC appears to impact host-microbe biology by serving as a ligand for both C-reactive protein and the receptor for platelet-activating factor. Type IV pili of Neisseria gonorrhoeae (Ng) and Neisseria meningitidis, filamentous protein structures critical to the colonization of their human hosts, are known to react variably with monoclonal antibodies recognizing a PC epitope. However, the structural basis for this reactivity has remained elusive. To address this matter, we exploited the finding that the PilE pilin subunit in Ng mutants lacking the PilV protein acquired the PC epitope independent of changes in pilin primary structure. Specifically, we show by using mass spectrometry that PilE derived from the pilV background is composed of a mixture of subunits bearing O-linked forms of either phosphoethanolamine (PE) or PC at the same residue, whereas the wild-type background carries only PE at that same site. Therefore, PilV can influence pilin structure and antigenicity by modulating the incorporation of these alternative modifications. The disaccharide covalently linked to Ng pilin was also characterized because it is present on the same peptides bearing the PE and PC modifications and, contrary to previous reports, was found to be linked by means of 2,4-diacetamido-2,4,6-trideoxyhexose. Taken together, these findings provide new insights into Ng type IV pilus structure and antigenicity and resolve long-standing issues regarding the nature of both the PC epitope and the pilin glycan.

Identification of mdoD, an mdoG paralog which encodes a twin-arginine-dependent periplasmic protein that controls osmoregulated periplasmic glucan backbone structures

Osmoregulated periplasmic glucans (OPGs) of Escherichia coli are anionic and highly branched oligosaccharides that accumulate in the periplasmic space in response to low osmolarity of the medium. The glucan length, ranging from 5 to 12 glucose residues, is under strict control. Two genes that form an operon, mdoGH, govern glucose backbone synthesis. The new gene mdoD, which appears to be a paralog of mdoG, was characterized in this study. Cassette inactivation of mdoD resulted in production of OPGs with a higher degree of polymerization, indicating that OpgD, the mdoD product (according to the new nomenclature), controls the glucose backbone structures. OpgD secretion depends on the Tat secretory pathway. Orthologs of the mdoG and mdoD genes are found in various proteobacteria. Most of the OpgD orthologs exhibit a Tat-dependent secretion signal, while most of the OpgG orthologs are Sec dependent.

The mdoC gene of Escherichia coli encodes a membrane protein that is required for succinylation of osmoregulated periplasmic glucans

Osmoregulated periplasmic glucans (OPGs) of Escherichia coli are anionic oligosaccharides that accumulate in the periplasmic space in response to low osmolarity of the medium. Their anionic character is provided by the substitution of the glucosidic backbone by phosphoglycerol originating from the membrane phospholipids and by succinyl residues from unknown origin. A phosphoglycerol-transferase-deficient mdoB mutant was subjected to Tn5 transposon mutagenesis, and putative mutant clones were screened for changes in the anionic character of OPGs by thin-layer chromatography. One mutant deficient in succinylation of OPGs was obtained, and the gene inactivated in this mutant was characterized and named mdoC. mdoC, which encodes a membrane-bound protein, is closely linked to the mdoGH operon necessary for the synthesis of the OPG backbone.

Cyclic beta-glucans of members of the family Rhizobiaceae

Cyclic beta-glucans are low-molecular-weight cell surface carbohydrates that are found almost exclusively in bacteria of the Rhizobiaceae family. These glucans are major cellular constituents, and under certain culture conditions their levels may reach up to 20% of the total cellular dry weight. In Agrobacterium and Rhizobium species, these molecules contain between 17 and 40 glucose residues linked solely by beta-(1,2) glycosidic bonds. In Bradyrhizobium species, the cyclic beta-glucans are smaller (10 to 13 glucose residues) and contain glucose linked by both beta-(1,6) and beta-(1,3) glycosidic bonds. In some rhizobial strains, the cyclic beta-glucans are unsubstituted, whereas in other rhizobia these molecules may become highly substituted with moieties such as sn-1-phosphoglycerol. To date, two genetic loci specifically associated with cyclic beta-glucan biosynthesis have been identified in Rhizobium (ndvA and ndvB) and Agrobacterium (chvA and chvB) species. Mutants with mutations at these loci have been shown to be impaired in their ability to grow in hypoosmotic media, have numerous alterations in their cell surface properties, and are also impaired in their ability to infect plants. The present review will examine the structure and occurrence of the cyclic beta-glucans in a variety of species of the Rhizobiaceae. The possible functions of these unique molecules in the free-living bacteria as well as during plant infection will be 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.

Phosphatidylinositol, a phospholipid of ice-nucleating bacteria

The nature of the phospholipids of the various bacteria that have ice nucleation activity in supercooled water has been determined. The seven bacteria studied included Pseudomonas syringae, Erwinia herbicola, three Escherichia coli K-12 strains that are phenotypically Ice+ because they contain plasmids with different amounts of either P. syringae or E. herbicola cloned DNA, and two E. coli K-12 strains without cloned ice gene DNA. All five Ice+ bacterial strains contained small amounts (0.1 to 1.0% of the total phospholipids) of phosphatidylinositol (PI), a phospholipid not previously detected in E. coli, Pseudomonas, or Erwinia species. The Ice- E. coli strains also contained trace level of PI that amounted to 2 to 30% of the level found in the Ice+ E. coli strains. Extracts of Ice+ strains contained low but measurable activities of PI synthase, while the activities in Ice- strains amounted to only 8 to 12% or less of that found in extracts of Ice+ bacteria. The functioning of the ice gene apparently increased both the PI synthase activity and the PI content of Ice+ strains from low endogenous levels. The relative ice nucleation activity at -4 degrees C or above (class A nucleation activity) of all Ice+ strains was found to be proportional to their PI content. The addition of myo-inositol (5 x 10(-4) M) to synthetic culture media increased the class A nucleation activity of both Ice+ E. coli strains and P. syringae up to sevenfold but had no stimulating effect on ice nucleation at lower temperatures (class B and class C nucleation activities). If these cells after fusion with PI vesicles were incubated with an energy source, the class A nucleation activity increased 70-fold over that present before fusion. These results indicate that PI plays an important role in ice nucleation at warm temperatures and is a likely precursor or component of the class A structure.

Cyclic glucans produced by Agrobacterium tumefaciens are substituted with sn-1-phosphoglycerol residues

In a previous study (Miller, K.J., Kennedy, E.P. and Reinhold, V.N. (1986) Science 231, 48-51) it was reported that the biosynthesis of periplasmic cyclic beta-1,2-glucans by Agrobacterium tumefaciens is strictly osmoregulated in a pattern closely similar to that found for the membrane-derived oligosaccharides of Escherichia coli (Kennedy, E.P. (1982) Proc. Natl. Acad. Sci. USA 79, 1092-1095). In addition to the well-characterized neutral cyclic glucan, the periplasmic glucans were found to contain an anionic component not previously reported. Biosynthesis of the anionic component is osmotically regulated in a manner indistinguishable from that of the neutral cyclic beta-1,2-glucan. We now find that the anionic component consists of cyclic beta-1,2-glucans substituted with one or more sn-1-phosphoglycerol residues. The presence of sn-1-phosphoglycerol residues represents an additional, striking similarity to the membrane-derived oligosaccharides of E. coli.