Last updated: March 13, 2019
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The outer membrane of Gram-negative bacteriums is non a inactive construction, but can be remodeled in response to environmental conditions that allow bacteriums to last and map in hostile conditions. PagP, an outer membrane enzyme, plays a important function in its remodeling by reassigning a palmitate concatenation from phospholipids to the proximal glucosamine unit of lipid A. This alteration protects bacteriums from host immune defences by reenforcing the outer membrane permeableness barrier and rarefying the host immune signalling. A new function of PagP in signal transduction in response to outer membrane emphasis was late conceptualized in add-on to its old function as palmitoyltransferase.

The Gram-negative cell envelope and permeableness barrier

The Gram-negative cell envelope is a alone and complex cellular construction that is composed of an interior membrane and outer membrane separated by a gelatinlike part called the periplasmic infinite [ 1 ] The interior membrane is a phospholipid bilayer involved in selective alimentary consumption, protein translocation, lipid biogenesis and oxidative phosphorylation. The periplasmic infinite contains soluble proteins and the peptidoglycan exoskeleton, which provides the structural model for the cell and an ground tackle for certain outer membrane lipoproteins [ 2 ] .

The outer membrane of Gram negative bacterium is a alone asymmetric bilayer and its interior cusp comprises phospholipids, including phosphatidylethanolamine, phoshatidylglycerol and cardiolipin whereas the outer cusp is composed preponderantly of lipopolysaccharide ( LPS ) [ 1 ] . LPS comprises of three parts, proximal lipid A ground tackle, distal O-antigen polyose and nucleus oligosachcharides linking them ( Fig. 1 ( A ) ) . Lipid A is phosphoglycolipid that is polyacylated by concentrated fatty acid ironss and makes up the outer monolayer of the Gram negative cell envelope [ 3 ] .

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LPS has phosphate groups and acidic sugars that make it negatively charged. These negatively charged molecules are bridge by bivalent cations, chiefly Mg2+ , to stabilise electrostatic repulsive force between next molecules. Six or seven saturated acyl ironss of lipoid A aid in take downing the fluidness of the outer membrane [ 4 ] . These tight sidelong interactions and decreased fluidness makes the outer membrane extremely impermeable to hydrophobic compounds in comparing with a regular phospholipid bilayer [ 5 ] .

The Gram-negative response to envelope emphasis

Bacterias brush with legion hostile conditions in nature and their hosts [ 6 ] . To last in the host environment and to do infection enteral bacteriums like E. coli have evolved a figure of signaling webs, including the alternate extracytoplasmic map sigma factor ?E and the two component systems such as CpxAR and PhoPQ. Bacteria activate the ?E tract in response to the emphasiss that interfere with outer membrane protein biosynthesis [ 7 ] . Anti-sigma factor RseA and its periplasmic opposite number RseB render ?E inactive in the normal status [ 8 ] . Extracytoplasmic emphasis activates the peptidase activity of DegS, which cleaves RseA on a cytoplasmatic site and therefore releases ?E into the cytol. Activated ?E causes the transcriptional activation of a set of cistrons involved in outer membrane biosynthesis [ 7 ] .

The two constituent system CpxAR responds to emphasize that adversely affect the assembly of surface molecules like pili and certain outer membrane proteins [ 9 ] . These environmental cues provide the stimulation for dissociation of periplasmic inhibitor CpxP from CpxA and trigger the autophosphorylation of a conserved histidine residue on its cytoplasmatic sphere. Phosphorylated CpxP so transfers phosphoryl groups to the response regulator CpxR, which finally causes the written text of cistrons involved in envelope protein care [ 8 ] .

The PhoPQ two constituent system consists of PhoQ as the membrane edge detector histidine kinase and PhoP as the blood relation response regulator. PhoQ binds to divalent cations like Mg2+ via its periplasmic sphere in its pent-up province [ 10 ] . Cationic antimicrobic peptides displace the Mg2+ from PhoQ [ 11 ] and promotes phosphorylation of PhoP. PhoP so regulates the written text of cistrons related to lipid A construction, opposition to antimicrobic peptides and phagosome change [ 12 ] .

The PhoPQ two constituent system besides regulates the outer membrane ?-barrel enzyme involved in lipid A alteration called PagP. Of the many enzymes involved in the alteration of lipoid A, PagP is the lone known enzyme that is located in the OM of E. coli. It catalyzes the transportation of palmitate from phospholipid to lipid A thereby change overing hexa acylated lipid A to hepta acylated lipid A [ 13 ] . This apparently minor alteration neutralizes the challenge of assorted cationic antimicrobic peptide and attenuate host innate immune signaling through TLR4 [ 14 ] , and one survey reported that PagP can trip the ?E response in E. coli but merely under unnatural growing conditions [ 15 ] .

PagP construction

Built-in outer membrane protein of gram negative bacteriums discover so far shows ? barrel designer. PagP being an outer membrane protein is no exclusion, and has 8 strands to transverse the outer membrane and short N- end point amphipathic ?-helix on the periplasmic side [ 16 ] . Recent research indicates that the i??-helix Acts of the Apostless as a station assembly clinch which helps in stabilising PagP in the membrane one time foldable is complete [ 17 ] . Unlike other outer proteins, its barrel axis is at an angle of 25 & A ; deg ; with regard to membrane normal and atilt barrel is supported by the aromatic residues. [ 16 ] . PagP exists in two dynamically distinguishable provinces, termed R ( relaxed ) which facilitate the substrate entry and T ( tense ) which play function in contact action [ 18 ] .

PagP has usual flexible extracellular cringles but has an unusual Centre with the lower periplasmic-exposed half being hydrophilic, and upper half being hydrophobic. The upper hydrophobic part consists of a lipid binding pocket called the hydrocarbon swayer and a individual molecule of detergent helps place its place. PagP selects the concentrated 16 C fatty acerb palmitate concatenation among all the fatty acids nowadays in the membrane lipid pool and reassign it to lipid A [ 16 ] . The deepness of the hydrocarbon swayer can be modified to choose shorter acyl ironss by mutating the glycine 88 residue that lies on the base of hydrocarbon swayer. Gly88 permutation generated by a combination of site directed mutagenesis and chemical alkylation have generated modified PagP enzymes that transportation acyl concatenation in individual C increases from C16 to C10 [ 19 ]

Extenuation of outer membrane emphasis by Palmitoylation

Asymmetric bilayer composing of outer membrane confine phospholipids on interior cusp but PagP ‘s active site is located on outer cusp. Therefore, phospholipids can merely entree PagP ‘s lipid adhering pocket from outer membrane outer cusp. Equally long as phospholipid remain on the interior cusp PagP remains in hibernating status. It seems that PagP does n’t play any function in migrating phospholipids from inner to outer cusp but merely react to the externally mediated disturbance in the outer membrane dissymmetry by palmitoylating the lipoid A [ 20 ] .

When cationic antimicrobic peptides attack bacteriums, the Mg2+ ions bridging next LPS molecules are displaced. As a consequence, negatively charged LPS begins to drive each other, thereby leting phospholipids to migrate from the inner to the outer cusp. This breach in the asymmetric distribution of outer membrane phospholipids allow the hydrophobic antibiotics and detergents to come in freely into the periplasm [ 10 ] . This stress status of outer membrane makes phospholipids to be available in the outer membrane along with LPS and both this substrate can now entree to the active site of PagP.

It has been shown that Phospholipid and lipid A acquire entree to the PagP ‘s lipid binding pocket through sidelong diffusion. PagP possess some proline residues which ca n’t take part in H bonding because they do n’t hold amide proton to donate and expose weakened H adhering in the next strands. Gatewaies so formed are called scallops and port, which provide sidelong paths for entry and emersion of phospholipid and lipid A severally [ 21 ] .

PagP mitigate the outer membrane emphasis cause by the onslaught of antimicrobic peptides by palmitoylating the lipoid A. Extra acyl concatenation present as a consequence of palmitoyl transferase lower the fluidness of outer membrane and reconstruct the outer membrane permeableness barrier. PagP specifically use the palmitate concatenation from sn-1 place of phospholipid to reassign it to the hydroxyl group of R-3 hydroxymyristate concatenation at place 2 of lipoid A.

Normally, pagP is transcribed by PhoPQ in response to Mg2+-limitation. When cells are treated with EDTA, which strips a fraction of LPS from the cell surface and promotes the migration of phospholipids into the OM outer cusp, palmitoylation occurs excessively quickly to be dependent on written text, and was shown to happen independent of de novo protein synthesis [ 22 ] . PagP is alone in its ability to modify lipid A independent of de novo protein or LPS biogenesis in E. coli. Due to its location and ability to respond instantly to outer membrane hurt, PagP represents a signifier of first-line defence for many Gram-negative bacteriums.

Lipid A palmitoylation eases the endurance of bacteriums in host

Lipid A is extremely immunogenic in mammalian cells. Picomolar degrees of the molecule are sufficient to trip the innate immune response via the TLR4 signaling pathway [ 3 ] . Signal transduction occurs through the mammalian LPS receptor, a composite of TLR4, MD2, and CD14 that assembles on the surface of a figure of cell types, including macrophages and dendritic cells. Signing through TLR4 finally leads to the translocation of NF-i?«B into the karyon, where it initiates the written text of proinflammatory cytokines [ 23 ] . Activation of innate unsusceptibility through TLR4 consequences in the production of cationic antimicrobic peptides. The interaction of cationic antimicrobic peptides with the outer membrane induces an amphipathic secondary construction that allows them to perforate the outer membrane, and so infix into the interior membrane. Cationic antimicrobic peptides are deadly to bacteriums through their break of the electrochemical potency across the inner membrane [ 24 ] .

Human TLR4 is extremely sensitive to the construction of lipid A, and alterations to lipid A ‘s acylation form can rarefy its ability to trip mammalian innate unsusceptibility through the TLR4 tract. Palmitoylated lipid A was observed to be 30-fold less active in exciting the activity of NF-i?«B than un-palmitoylated lipid A [ 25 ] . On the other manus, lessening in membrane fluidness resulted from the palmitoylation of lipoid A protect bacterium against cationic antimicrobic peptides [ 26 ] . PagP ‘s ability to rarefy signaling through TLR4 and protect bacteriums from immune effecters eases the ability of bacteriums to set up their infection in their host. PagP ‘s narrow distribution amongst chiefly infective beings like Salmonella enterica, L. pneumophila, B. bronchiseptica, Yersiniae pestis clearly indicate the importance of the enzyme in pathogenesis [ 27 ]

Due to their partial activation of unconditioned unsusceptibility, modified constructions of lipoid A are being used as immune adjuvants [ 28 ] . The Glaxo Smith Kline Cervarix vaccinum used in human populations includes the monophosphoryl lipid A adjuvant that depends on palmitate incorporated by PagP.

New function of PagP as an apical sensory transducer

Recently, a new function of PagP in trans-envelope signalling was elucidated in E. coli O157: H7. Deficiency of myristate concatenation in distal glucosamine unit of lipid A, due to the omission of msbB, was sufficient to transgress the outer membrane dissymmetry in E.coli O157: H7 and trip PagP. msbB deficient mutations besides showed additions susceptibleness to the serum intervention, proposing that O-antigen part of lipopolysaccharide might be losing. Further analysis of lipopolysaccharide isolated from msbB mutation by cataphoresis, supports the absence of O-antigen repetitions usually present in this serotype of E. coli. Nature of the nucleus shortness was revealed by mass spectrometry, gas chromatography of alditol ethanoates, and NMR spectrometry, indicated that shortness was happening at the degree of the first outer nucleus glucose residue, which is added in the cytol by WaaG [ 29 ] . Presence of smooth lipopolysaccharide when pagP is absent and truncated lipopolysaccharide when pagP is present supports the function of PagP in nucleus shortness.

Lipopolysaccharides are known to play important function in folding of outer membrane protein [ 30 ] and there is the possibility that shortness is an artefact of bring forthing an untypical lipoid A species that does n’t back up OM protein turn uping. Similarly, the lyso-phospholipid, bi-product of PagP contact action bird back to the cytol for re-acylation, and PagP could pass on shortness during this procedure. However, a catalytically inactive pagPser77ala allelomorph was every bit capable of bring forthing shortness, bespeaking that PagP ‘s palmitoyltransferase activity, and hence the production of an untypical lipoid A species, is non responsible for shortness. Therefore, PagP is pass oning with the cytol through separate mechanism via different sphere of PagP. [ 29 ]

The periplasmic side of PagP if inspected closely contains 3 amino acids i.e. Asp 61, His 67 and Tyr 87 which form a tight charge-relay web stick outing from the ?-barrel outside that is buried beneath the ?-helix in the enzyme ‘s inactive conformation, the R province. When PagP gets activated and transform to the T province, it is possible that the spiral being nomadic might alter its place [ 31 ] and Asp 61, His 67 and Tyr 87 become exposed. This expose proteolytic three might split lipoprotein and let go of it into the periplasm to originate the signal transduction.

Evidence of these residues to work together as a catalytic three becomes stronger because of their spacial agreement. Asp61, His 67 and Tyr 87 residues of PagP has about superimposable agreement with the catalytic three of Chymotrypsin ( Asp 102, His 57, Ser 195 ) . Chymotrypsin, a serine peptidase, contains serine residue which serves as a nucleophile and affects the contact action with its hydroxyl group. Catalytic three of PagP contains instead than tyrosine alternatively of serine but there is no any biochemical ground that hydroxyl group of tyrosin could non work proteolytically. In fact, the extra resonance stabilisation afforded by tyrosine ‘s phenyl ring really reduces its pKa relation to serine, theoretically bettering tyrosine ‘s ability to proteolytically split a mark protein. Furthermore, while Asp 61 and His 67 are extremely conserved amongst homologs of pagP from 8 genera, Tyr 87 is perfectly conserved [ 27 ] .

The novitiate PagP mediated uncover so far in E.coli O157: H7 can be summarize as, myristate deficient lipid A someway glycosylated and conveyance to the outer membrane and causes the disturbance of outer membrane symmetricalness which activates PagP. This activation causes the spiral to swing outward towards outer membrane-periplasm interface and exposes the catalytic three. This expose catalytic three somehow helps in transducing and the signal to impact the cytoplasmatic events of R3 nucleus biogenesis. Therefore, It seems sensible to presume that that signal transduction is happening because of outer membrane emphasis.


PagP, an outer membrane enzyme, is a investigation for outer membrane dissymmetry. Aberrant migration of phospholipid to the outer cusp, due to the outer membrane emphasis, is sense by PagP. Activated PagP appreciation palmitate concatenation from phospholipid and provides excess acyl concatenation to the lipoid A. This hepta acylated lipid A of restores the permeableness in the outer membrane and besides helps in weakening the host immune signalling that arise in response to lipid A.

PagP, antecedently thought to work merely as an acyltransferase, has been observed to act upon cytoplasmatic stairss of nucleus biogenesis in the absence of msbB [ 29 ] . Although rare, outer membrane enzymes able to pass on with the cytol have been reported. For illustration, the ferrous citrate receptor, FecA, mediates the synthesis of constituents of the Fe consumption system in response to extracellular ligand adhering [ 32 ] .

The probe of this fresh tract has been shifted to the research lab strain of E.coli, which will give the clear image of whether PagP mediated signalling is regular phenomenon or is specific to E.coli O157: H7. The find of putative catalytic three on the periplasmic face of PagP might supply a clear penetration in to the spheres involve in signalling. The possible that SlyB, the lone PhoPQ-controlled lipoprotein common to both E. coli and S. typhimurium, may interact with PagP to intercede signaling is presently being investigated [ 33 ] . Since PagP mediate signaling is looking because of outer membrane emphasis, its nexus to other outer membrane emphasis response systems is besides plausible.