Characterization of new membrane lipoproteins and their precursors of Escherichia coli

The multifarious roles of Tol-Pal in Gram-negative bacteria

In the 1960s several groups reported the isolation and preliminary genetic mapping of Escherichia coli strains tolerant towards the action of colicins. These pioneering studies kick-started two new fields in bacteriology; one centred on how bacteriocins like colicins exploit the Tol (or more commonly Tol-Pal) system to kill bacteria, the other on the physiological role of this cell envelope-spanning assembly. The following half century has seen significant advances in the first of these fields whereas the second has remained elusive, until recently. Here, we review work that begins to shed light on Tol-Pal function in Gram-negative bacteria. What emerges from these studies is that Tol-Pal is an energised system with fundamental, interlinked roles in cell division – coordinating the re-structuring of peptidoglycan at division sites and stabilising the connection between the outer membrane and underlying cell wall. This latter role is achieved by Tol-Pal exploiting the proton motive force to catalyse the accumulation of the outer membrane peptidoglycan associated lipoprotein Pal at division sites while simultaneously mobilising Pal molecules from around the cell. These studies begin to explain the diverse phenotypic outcomes of tol-pal mutations, point to other cell envelope roles Tol-Pal may have and raise many new questions.

Degradation of complex arabinoxylans by human colonic Bacteroidetes

Some Bacteroidetes and other human colonic bacteria can degrade arabinoxylans, common polysaccharides found in dietary fiber. Previous work has identified gene clusters (polysaccharide-utilization loci, PULs) for degradation of simple arabinoxylans. However, the degradation of complex arabinoxylans (containing side chains such as ferulic acid, a phenolic compound) is poorly understood. Here, we identify a PUL that encodes multiple esterases for degradation of complex arabinoxylans in Bacteroides species. The PUL is specifically upregulated in the presence of complex arabinoxylans. We characterize some of the esterases biochemically and structurally, and show that they release ferulic acid from complex arabinoxylans. Growth of four different colonic Bacteroidetes members, including Bacteroides intestinalis, on complex arabinoxylans results in accumulation of ferulic acid, a compound known to have antioxidative and immunomodulatory properties.

Functional Studies of β-Glucosidases of Cytophaga hutchinsonii and Their Effects on Cellulose Degradation

Cytophaga hutchinsonii can rapidly digest crystalline cellulose without free cellulases or cellulosomes. Its cell-contact cellulose degradation mechanism is unknown. In this study, the four β-glucosidase (bgl) genes in C. hutchinsonii were singly and multiply deleted, and the functions of these β-glucosidases in cellobiose and cellulose degradation were investigated. We found that the constitutively expressed BglB played a key role in cellobiose utilization, while BglA which was induced by cellobiose could partially make up for the deletion of bglB. The double deletion mutant ΔbglA/bglB lost the ability to digest cellobiose and could not thrive in cellulose medium, indicating that β-glucosidases were important for cellulose degradation. When cultured in cellulose medium, a small amount of glucose accumulated in the medium in the initial stage of growth for the wild type, while almost no glucose accumulated for ΔbglA/bglB. When supplemented with a small amount of glucose, ΔbglA/bglB started to degrade cellulose and grew in cellulose medium. We inferred that glucose might be essential for initiating cellulose degradation, and with additional glucose, C. hutchinsonii could partially utilize cellulose without β-glucosidases. We also found that there were both cellulose binding cells and free cells when cultured in cellulose. Since direct contact between C. hutchinsonii cells and cellulose is necessary for cellulose degradation, we deduced that the free cells which were convenient to explore new territory in the environment might be fed by the adherent cells which could produce cello-oligosaccharide and glucose into the environment. This study enriched our knowledge of the cellulolytic pathway of C. hutchinsonii.

Surface-Exposed Lipoproteins: An Emerging Secretion Phenomenon in Gram-Negative Bacteria

Bacterial lipoproteins are hydrophilic proteins that are anchored to a cell membrane by N-terminally linked fatty acids. It is widely believed that nearly all lipoproteins produced by Gram-negative bacteria are either retained in the inner membrane (IM) or transferred to the inner leaflet of the outer membrane (OM). Lipoproteins that are exposed on the cell surface have also been reported but are generally considered to be rare. Results from a variety of recent studies, however, now suggest that the prevalence of surface-exposed lipoproteins has been underestimated. In this review we describe the evidence that the surface exposure of lipoproteins in Gram-negative bacteria is a widespread phenomenon and discuss possible mechanisms by which these proteins might be transported across the OM.

Heat shock proteins IbpA and IbpB are required for NlpI-participated cell division in Escherichia coli

Lipoprotein NlpI of Escherichia coli is involved in the cell division, virulence, and bacterial interaction with eukaryotic host cells. To elucidate the functional mechanism of NlpI, we examined how NlpI affects cell division and found that induction of NlpI inhibits nucleoid division and halts cell growth. Consistent with these results, the cell division protein FtsZ failed to localize at the septum but diffused in the cytosol. Elevation of NlpI expression enhanced the transcription and the outer membrane localization of the heat shock protein IbpA and IbpB. Deletion of either ibpA or ibpB abolished the effects of NlpI induction, which could be restored by complementation. The C-terminus of NlpI is critical for the enhancement in IbpA and IbpB production, and the N-terminus of NlpI is required for the outer membrane localization of NlpI, IbpA, and IbpB. Furthermore, NlpI physically interacts with IbpB. These results indicate that over-expression of NlpI can interrupt the nucleoids division and the assembly of FtsZ at the septum, mediated by IbpA/IbpB, suggesting a role of the NlpI/IbpA/IbpB complex in the cell division.

Export of a hyperexpressed mammalian globular cytochrome b5 precursor in Escherichia coli is dramatically affected by the nature of the amino acid flanking the secretory signal sequence cleavage bond

A chimeric mammalian globular cytochrome b(5) fused to Escherichia coli alkaline phosphatase signal sequence (SS) was used as a model probe to investigate the influence of substituting each one of the standard 20 amino acids at its N-terminus on the Sec-dependent export of the precursor to the periplasmic space of E. coli. Substituting the native Met(+1) of the passenger protein flanking the SS with any one of the remaining 19 amino acids introduced significant changes in the export of cytochrome b(5) without jamming the Sec-dependent translocon. Acidic and hydrophilic residues proved to be the most efficient promoters of export. Small, nonbulky and basic residues yielded intermediate levels of the hemoprotein export. Replacement with a Cys(+1) residue generated significant quantities of both monomeric and disulfide-linked dimeric forms. However, bulky, aromatic and hydrophobic residues caused a significant decline in the rates of secretion. In expectation with their absences in the natural periplasmically secreted proteins, Pro and Ile-tagged cytochrome b(5) precursors failed to generate any detectable secreted recombinant products. Although Ala, amongst the native E. coli periplasmic proteins, is the preferred X(+1) residue with an occurrence of 50% frequency, it proved half as effective in promoting export when inserted proximally to the SS of cytochrome b(5). The mechanisms involved for these export variations are discussed. The findings will prove beneficial for high-level generation of recombinant proteins by secretory means for pharmaceutical and related biotechnological applications.

Biogenesis and Membrane Targeting of Lipoproteins

Bacterial lipoproteins represent a unique class of membrane proteins, which are anchored to membranes through triacyl chains attached to the amino-terminal cysteine. They are involved in various functions localized in cell envelope. Escherichia coli possesses more than 90 species of lipoproteins, most of which are localized in the outer membrane, with others being in the inner membrane. All lipoproteins are synthesized in the cytoplasm with an N-terminal signal peptide, translocated across the inner membrane by the Sec translocon to the periplasmic surface of the inner membrane, and converted to mature lipoproteins through sequential reactions catalyzed by three lipoprotein-processing enzymes: Lgt, LspA, and Lnt. The sorting of lipoproteins to the outer membrane requires a system comprising five Lol proteins. An ATP-binding cassette transporter, LolCDE, initiates the sorting by mediating the detachment of lipoproteins from the inner membrane. Formation of the LolA-lipoprotein complex is coupled to this LolCDE-dependent release reaction. LolA accommodates the amino-terminal acyl chain of lipoproteins in its hydrophobic cavity, thereby generating a hydrophilic complex that can traverse the periplasmic space by diffusion. Lipoproteins are then transferred to LolB on the outer membrane and anchored to the inner leaflet of the outer membrane by the action of LolB. In contrast, since LolCDE does not recognize lipoproteins possessing Asp at position +2, these lipoproteins remain anchored to the inner membrane. Genes for Lol proteins are widely conserved among gram-negative bacteria, and Lol-mediated outer membrane targeting of lipoproteins is considered to be the general lipoprotein localization mechanism.

Prediction of lipoprotein signal peptides in Gram-positive bacteria with a Hidden Markov Model

We present a Hidden Markov Model method for the prediction of lipoprotein signal peptides of Gram-positive bacteria, trained on a set of 67 experimentally verified lipoproteins. The method outperforms LipoP and the methods based on regular expression patterns, in various data sets containing experimentally characterized lipoproteins, secretory proteins, proteins with an N-terminal TM segment and cytoplasmic proteins. The method is also very sensitive and specific in the detection of secretory signal peptides and in terms of overall accuracy outperforms even SignalP, which is the top-scoring method for the prediction of signal peptides. PRED-LIPO is freely available at http://bioinformatics.biol.uoa.gr/PRED-LIPO/, and we anticipate that it will be a valuable tool for the experimentalists studying secreted proteins and lipoproteins from Gram-positive bacteria.

Proteomic analysis of and immune responses to Ehrlichia chaffeensis lipoproteins

Ehrlichia chaffeensis is an obligately intracellular gram-negative bacterium and is the etiologic agent of human monocytic ehrlichiosis (HME). Although E. chaffeensis induces the generation of several cytokines and chemokines by leukocytes, E. chaffeensis lacks lipopolysaccharide and peptidoglycan. Bioinfomatic analysis of the E. chaffeensis genome, however, predicted genes encoding 15 lipoproteins and 3 posttranslational lipoprotein-processing enzymes. The present study showed that by use of multidimensional liquid chromatography followed by tandem mass spectrometry, all predicted lipoproteins as well as lipoprotein-processing enzymes were expressed by E. chaffeensis cultured in the human promyelocytic leukemia cell line HL-60. Consistent with this observation, a signal peptidase II inhibitor, globomycin, was found to inhibit E. chaffeensis infection and lipoprotein processing in HL-60 cell culture. To study in vivo E. chaffeensis lipoprotein expression and host immune responses to E. chaffeensis lipoproteins, 13 E. chaffeensis lipoprotein genes were cloned into a mammalian expression vector. When the DNA constructs were inoculated into naïve dogs, or when dogs were infected with E. chaffeensis, the animals developed delayed-type hypersensitivity reactions at cutaneous sites of the DNA construct deposition and serum antibodies to these lipoproteins. This is the first demonstration of lipoprotein expression and elicitation of immune responses by a member of the order Rickettsiales. Multiple lipoproteins expressed by E. chaffeensis in vitro and in vivo may play key roles in pathogenesis and immune responses in HME.

Inactivation of Lgt allows systematic characterization of lipoproteins from Listeria monocytogenes

Lipoprotein anchoring in bacteria is mediated by the prolipoprotein diacylglyceryl transferase (Lgt), which catalyzes the transfer of a diacylglyceryl moiety to the prospective N-terminal cysteine of the mature lipoprotein. Deletion of the lgt gene in the gram-positive pathogen Listeria monocytogenes (i) impairs intracellular growth of the bacterium in different eukaryotic cell lines and (ii) leads to increased release of lipoproteins into the culture supernatant. Comparative extracellular proteome analyses of the EGDe wild-type strain and the Delta lgt mutant provided systematic insight into the relative expression of lipoproteins. Twenty-six of the 68 predicted lipoproteins were specifically released into the extracellular proteome of the Delta lgt strain, and this proved that deletion of lgt is an excellent approach for experimental verification of listerial lipoproteins. Consequently, we generated Delta lgt Delta prfA double mutants to detect lipoproteins belonging to the main virulence regulon that is controlled by PrfA. Overall, we identified three lipoproteins whose extracellular levels are regulated and one lipoprotein that is posttranslationally modified depending on PrfA. It is noteworthy that in contrast to previous studies of Escherichia coli, we unambiguously demonstrated that lipidation by Lgt is not a prerequisite for activity of the lipoprotein-specific signal peptidase II (Lsp) in Listeria.

Depletion of apolipoprotein N-acyltransferase causes mislocalization of outer membrane lipoproteins in Escherichia coli

Lipoproteins in Gram-negative Enterobacteriaceae carry three fatty acids on the N-terminal cysteine residue, two as a diacylglyceride and one through an N-linkage following signal peptide cleavage. Most lipoproteins are anchored in the outer membrane, facing the periplasm, but some lipoproteins remain in the plasma membrane, depending on the amino acid at position +2, immediately after the fatty-acylated cysteine. In vitro, the last step in lipoprotein maturation, N-acylation of apolipoproteins by the plasma membrane apolipoprotein N-acyltransferase (Lnt), is necessary for efficient recognition of outer membrane lipoproteins by the Lol system, which transports them from the plasma to the outer membrane (Fukuda, A., Matsuyama, S.-I., Hara, T., Nakayama, J., Nagasawa, H., and Tokuda, H. (2002) J. Biol. Chem. 277, 43512-43518). To study the role of Lnt in vivo, we constructed a conditional lnt mutant of Escherichia coli. The apo-form of peptidoglycan-anchored major lipoprotein (Lpp) and two other outer membrane lipoproteins accumulated in the plasma membrane when lnt expression was reduced. We also found that Lnt is an essential protein in E. coli and that the lethality is partially because of the retention of apoLpp in the plasma membrane. Topology mapping of Lnt with beta-galactosidase and alkaline phosphatase fusions indicated the presence of six membrane-spanning segments. The lnt gene in a mutant of Salmonella enterica displaying thermosensitive Lnt activity (Gupta, S. D., Gan, K., Schmid, M. B., and Wu, H. C. (1993) J. Biol. Chem. 268, 16551-16556) was found to carry a mutation causing a single glutamate to lysine substitution at a highly conserved position in the last predicted periplasmic loop of the protein.

The two murein lipoproteins of Salmonella enterica serovar Typhimurium contribute to the virulence of the organism

Septic shock due to Salmonella and other gram-negative enteric pathogens is a leading cause of death worldwide. The role of lipopolysaccharide in sepsis is well studied; however, the contribution of other bacterial outer membrane components, such as Braun (murein) lipoprotein (Lpp), is not well defined. The genome of Salmonella enterica serovar Typhimurium harbors two copies of the lipoprotein (lpp) gene. We constructed a serovar Typhimurium strain with deletions in both copies of the lpp gene (lpp1 and lpp2) by marker exchange mutagenesis. The integrity of the cell membrane and the secretion of the effector proteins through the type III secretion system were not affected in the lpp double-knockout mutant. Subsequently, the virulence potential of this mutant was examined in a cell culture system using T84 intestinal epithelial and RAW264.7 macrophage cell lines and a mouse model of salmonellosis. The lpp double-knockout mutant was defective in invading and inducing cytotoxic effects in T84 and RAW264.7 cells, although binding of the mutant to the host cell was not affected when compared to the wild-type (WT) serovar Typhimurium. The motility of the mutant was impaired, despite the finding that the number of flagella was similar in the lpp double knockout mutant and the WT serovar Typhimurium. Deletion in the lpp genes did not affect the intracellular survival and replication of Salmonella in macrophages and T84 cells. Induction of the proinflammatory cytokines tumor necrosis factor alpha and interleukin-8 (IL-8) was significantly reduced in macrophages and T84 cells infected with the lpp double-knockout mutant. The levels of IL-8 remained unaffected in T84 cells when infected with either live or heat-killed WT and lpp mutant, indicating that invasion was not required for IL-8 production and that Toll-like receptor 2 signaling might be affected in the Lpp double-knockout mutant. These effects of the Lpp protein could be restored by complementation of the isogenic mutant. The lpp double-knockout mutant was avirulent in mice, and animals infected with this mutant were protected from a lethal challenge dose of WT serovar Typhimurium. The severe combined immunodeficient mice, on the other hand, were susceptible to infection by the lpp double-knockout mutant. The serovar Typhimurium mutants from which only one of the lpp (lpp1 or lpp2) genes was deleted were also avirulent in mice. Taken together, our data indicated that Lpp specifically contributed to the virulence of the organism.

Structure–activity relationships of globomycin analogues as antibiotics

Globomycin (1a), a signal peptidase II inhibitor, and its derivatives show potent antibacterial activity against Gram-negative bacteria. The synthesis and antimicrobial activity of novel globomycin analogues are reported. The hydroxyl group in the L-Ser residue was essential for the antimicrobial activity and the length of the alkyl side chain greatly influenced the activity. In addition, derivatives that had a modified cyclic core exhibited weak activity. One of the analogues showed a wider antimicrobial spectrum, effective against not only Gram-negative but also Gram-positive bacteria.

Synthesis and antimicrobial activity of novel globomycin analogues

Globomycin, a signal peptidase II inhibitor, and its derivatives show potent antibacterial activity against Gram-negative bacteria. The synthesis and antimicrobial activity of novel globomycin analogues are reported. One of the analogues showed a more potent activity against Gram-negative bacteria than globomycin and also exhibited antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA).

Protein-peptidoglycan interactions modulate the assembly of the needle complex in the Salmonella invasion-associated type III secretion system

The invasion-associated type III secretion system of Salmonella enterica assembles as a supra-molecular structure, termed needle complex, which spans the bacterial envelope. Here, we present evidence for protein-peptidoglycan interactions that modulate the assembly of this organelle. The presence of major membrane components of the needle complex (PrgH, PrgK and InvG) and InvH, required for efficient assembly of the organelle, was examined in peptidoglycan purified by extensive boiling of bacteria in 4% SDS. InvH, PrgH and PrgK, but not InvG, were detected in this purified material. InvH was present in the peptidoglycan in higher relative amounts than PrgH or PrgK, and was the only protein efficiently bound to peptidoglycan in cross-linking experiments. Analysis in mutants defective for needle complex proteins showed that the needle proteins PrgI and PrgJ and, to a lesser extent, InvH, sustain the association of PrgH and PrgK with peptidoglycan. In contrast, the association of InvH with peptidoglycan did not necessitate other needle complex proteins. Functional analysis showed that the association of InvH, PrgH and PrgK with peptidoglycan is abolished in live bacteria carrying structural modifications in the peptidoglycan. The loss of these interactions caused a marked reduction in the number of needle complexes and, concomitantly, in protein secretion and bacterial invasion of cultured eukaryotic cells. Altogether, these data provide the first evidence for an association between proteins of the Salmonella needle complex and the peptidoglycan. In addition, we demonstrate that these protein-peptidoglycan interactions are critical for an efficient and correct assembly of this specialized organelle.

A novel strategy for protective Actinobacillus pleuropneumoniae subunit vaccines: detergent extraction of cultures induced by iron restriction

We have characterized antigens from Actinobacillus (A.) pleuropneumoniae grown under iron restriction with respect to their immunogenic and protective potential. Antigens were the cell-free culture supernatants (CFS) obtained after treatment of A. pleuropneumoniae broth cultures with sodium deoxycholate. Using the iron-repressible transferrin-binding lipoprotein TbpB and the constitutively expressed outer membrane lipoprotein OmlA as markers, we have shown that the detergent extraction enriched the CFS with lipoproteins from the outer membrane (OM). Extractions with 0.05% of sodium deoxycholate increased the lipoprotein contents in the CFS, but did not affect the integrity of the OM. This was demonstrated by the absence of the iron-repressible integral OM transferrin-binding protein TbpA. Furthermore, the absence of periplasmic and cytoplasmic proteins in CFS after extraction was determined in immunoblot analyses with anti-bacterial alkaline phosphatase and anti-Hsp60 antisera, demonstrating that there was no rupture of the OMs or the plasma membranes due to the extraction procedure. Antigen preparations from A. pleuropneumoniae serotype 2 and 9 grown under iron restrictive conditions were combined, emulsified, and tested for their ability to confer protection in pigs. Pigs immunized with CFS from sodium deoxycholate extracted cultures developed a strong antibody response and, upon challenge with A. pleuropneumoniae serotype 2, the immunized pigs showed no or only mild clinical signs of disease and had a significantly lower degree of lung damage than the control pigs.

Immune response to an 18-kilodalton outer membrane antigen identifies lipoprotein 20 as a Helicobacter pylori vaccine candidate

Experiments were performed using the standardized murine model of Helicobacter pylori infection to determine the immunogenicity of H. pylori outer membrane vesicles in immune protection. These vesicles, which are naturally shed from the surface of the bacterium, induce a protective response when administered intragastrically to mice in the presence of cholera holotoxin, despite the absence of the urease enzyme and associated Hsp54 chaperonin. Immunoblotting identified a specific serum immunoglobulin G (IgG) response to an 18-kDa outer membrane protein in a significant number of immunized animals. This commonly expressed, immunodominant protein was subsequently identified as lipoprotein 20 (Lpp20). Hybridoma backpacks secreting an IgG1 subclass monoclonal antibody to Lpp20 were generated in H. pylori-infected mice and were found to significantly reduce bacterial numbers, providing evidence that this surface-exposed antigen is a true vaccine candidate and not merely an antigenic marker for successful, protective immunization.

Topology and transport of membrane lipids in bacteria

The last two decades have witnessed a break-through in identifying and understanding the functions of both the proteins and lipids of bacterial membranes. This development was parallelled by increasing insights into the biogenesis, topology, transport and sorting of membrane proteins. However, progress in research on the membrane distribution and transport of lipids in bacteria has been slow in that period. The development of novel biochemical in vitro approaches and recent genetic studies have increased our understanding of these subjects. The aim of this review is to present an overview of the current knowledge of the distribution and transport of lipids in both Gram-positive and Gram-negative bacteria. Special attention is paid to recently obtained results, which are expected to inspire further research to finally unravel these poorly understood phenomena.

Testing the '+2 rule' for lipoprotein sorting in the Escherichia coli cell envelope with a new genetic selection

We report a novel strategy for selecting mutations that mislocalize lipoproteins within the Escherichia coli cell envelope and describe the mutants obtained. A strain carrying a deletion of the chromosomal malE gene, coding for the periplasmic maltose-binding protein (MalE), cannot use maltose unless a wild-type copy of malE is present in trans. Replacement of the natural signal peptide of preMalE by the signal peptide and the first four amino acids of a cytoplasmic membrane-anchored lipoprotein resulted in N-terminal fatty acylation of MalE (lipoMalE) and anchoring to the periplasmic face of the cytoplasmic membrane, where it could still function. When the aspartate at position +2 of this protein was replaced by a serine, lipoMalE was sorted to the outer membrane, where it could not function. Chemical mutagenesis followed by selection for maltose-using mutants resulted in the identification of two classes of mutations. The single class I mutant carried a plasmid-borne mutation that replaced the serine at position +2 by phenylalanine. Systematic substitutions of the amino acid at position +2 revealed that, besides phenylalanine, tryptophan, tyrosine, glycine and proline could all replace classical cytoplasmic membrane lipoprotein sorting signal (aspartate +2). Analysis of known and putative lipoproteins encoded by the E. coli K-12 genome indicated that these amino acids are rarely found at position +2. In the class II mutants, a chromosomal mutation caused small and variable amounts of lipoMalE to remain associated with the cytoplasmic membrane. Similar amounts of another, endogenous outer membrane lipoprotein, NlpD, were also present in the cytoplasmic membrane in these mutants, indicating a minor, general defect in the sorting of outer membrane lipoproteins. Four representative class II mutants analysed were shown not to carry mutations in the lolA or lolB genes, known to be involved in the sorting of lipoproteins to the outer membrane.

Identification and characterization of a new lipoprotein, NlpI, in Escherichia coli K-12

We report here the identification of a new lipoprotein, NlpI, in Escherichia coli K-12. The NlpI structural gene (nlpI) is located between the genes pnp (polynucleotide phosphorylase) and deaD (RNA helicase) at 71 min on the E. coli chromosome. The nlpI gene encodes a putative polypeptide of approximately 34 kDa, and multiple lines of evidence clearly demonstrate that NlpI is indeed a lipoprotein. An nlpI::cm mutation rendered growth of the cells osmotically sensitive, and incubation of the insertion mutant at an elevated temperature resulted in the formation of filaments. The altered phenotype of the mutant was a direct consequence of the mutation in nlpI, since it was complemented by the wild-type nlpI gene alone. Overexpression of the unaltered nlpI gene in wild-type cells resulted in the loss of the rod morphology and the formation of single prolate ellipsoids and pairs of prolate ellipsoids joined by partial constrictions. NlpI may be important for an as-yet-undefined step in the overall process of cell division.

Subunit II of the cytochrome bo3 ubiquinol oxidase from Escherichia coli is a lipoprotein

The purified Escherichia coli cytochrome bo3 ubiquinol oxidase contains four subunits that are each integral components of the cytoplasmic membrane. The molecular weight of each of the subunits has been determined by matrix-assisted laser desorption ionization mass spectrometry (MALDI). The observed molecular weight of subunit II (CyoA) is considerably less than the calculated value from the deduced amino acid sequence, indicating possible posttranslational processing. The similarity of a portion of the sequence near the N-terminus of CyoA with the sequences of known prokaryotic membrane-bound lipoproteins suggested that CyoA is proteolytically processed to generate an N-terminus at Cys25, and that Cys25 is covalently modified by the addition of lipids. This would be consistent with the observed molecular mass, and was confirmed by demonstrating the incorporation of radioactive palmitic acid into subunit II of the cytochrome bo3 oxidase. Site-directed mutagenesis replacing Cys25 by alanine prevents the processing, generating a precursor form of CyoA with a higher molecular mass. The C25A mutant of CyoA still assembles as an active quinol oxidase capable of supporting growth of the cells by aerobic respiration. Hence, this unusual processing of a cytoplasmic membrane protein, which is already anchored to the membrane by two transmembrane helices, is not essential for either assembly or function.

Lethality of the covalent linkage between mislocalized major outer membrane lipoprotein and the peptidoglycan of Escherichia coli

The major outer membrane lipoprotein (Lpp) of Escherichia coli possesses serine at position 2, which is thought to function as the outer membrane sorting signal, and lysine at the C terminus, through which Lpp covalently associates with peptidoglycan. Arginine (R) is present before the C-terminal lysine in the wild-type Lpp (LppSK). By replacing serine (S) at position 2 with aspartate (D), the putative inner membrane sorting signal, and by deleting lysine (K) at the C terminus, Lpp mutants with a different residue at either position 2 (LppDK) or the C terminus (LppSR) or both (LppDR) were constructed. Expression of LppSR and LppDR little affected the growth of E. coli. In contrast, the number of viable cells immediately decreased when LppDK was expressed. Prolonged expression of LppDK inhibited separation of the inner and outer membranes by sucrose density gradient centrifugation, whereas short-term expression did not. Pulse-labeled LppDK and LppDR were localized in the inner membrane, indicating that the amino acid residue at position 2 functions as a sorting signal for the membrane localization of Lpp. LppDK accumulated in the inner membrane covalently associated with the peptidoglycan and thus prevented the separation of the two membranes. Globomycin, an inhibitor of lipoprotein-specific signal peptidase II, was lethal for E. coli only when Lpp possessed the C-terminal lysine. Taken together, these results indicate that the inner membrane accumulation of Lpp per se is not lethal for E. coli. Instead, a covalent linkage between the inner membrane Lpp having the C-terminal lysine and the peptidoglycan is lethal for E. coli, presumably due to the disruption of the cell surface integrity.

Characterization of braun's lipoprotein and determination of its attachment sites to peptidoglycan by (252)Cf-PD and MALDI time-of-flight mass spectrometry

A strategy for the characterization of bacterial lipoprotein-in this case Braun's lipoprotein (an outer membrane 7-ku lipoprotein) isolated from Escherichia coli-is described by time-of-flight mass spectrometric (TOF/MS) techniques [(252)Cf plasma desorption (PD) TOF/MS and matrix-assisted laser desorption-ionization (MALDI) TOF/MS]. Covalent linkage of lipid at the N-terminal cysteine (posttranslationally modified to a S-[2,3-bis(acyloxy)-propyl]-N-acylcysteine) and, therefore, strict insolubility in aqueous solution constitute common features for this class of proteins. Relative molecular mass determination of the major molecular species of Braun's lipoprotein was obtained by selection of an appropriate mixture of organic solvents compatible with matrix/support materials useful for the mass spectrometric techniques applied. Minor components of this lipoprotein that differ only in the fatty acid composition of the lipid anchor were detected by PD TOF/MS after enzymatic release of the extremely hydrophobic N-terminal amino acid followed by selective extraction with chloroform. Part of the primary sequence of this lipoprotein was confirmed based on peptide fragment ions observed in the positive ion PD mass spectra of cyanogen bromide-generated peptide fragments that had been isolated previously by reverse phase high-performance liquid chromatography (HPLC). Peptidoglycan fragments that represent the attachment sites of lipoprotein to peptidoglycan were enzymatically released, separated by reverse phase HPLC, and finally characterized by time-of-flight mass spectrometric techniques ((252)Cf-PD TOF/MS, MALDI TOF/MS). The results obtained with both techniques differed only in the better sensitivity obtained with MALDI TOF/MS, which consumed a factor of 100 to 1000 less material than with PD TOF/MS.

The nlpD gene is located in an operon with rpoS on the Escherichia coli chromosome and encodes a novel lipoprotein with a potential function in cell wall formation

rpoS is the structural gene for sigma s, which is a second vegetative sigma subunit of RNA polymerase in Escherichia coli and is involved in the expression of many stationary phase-induced genes. Upstream of rpoS is an open reading frame (ORF) whose function and regulation have not been studied. Strong overproduction of its gene product using the IPTG-inducible tac promoter leads to the formation of bulges at the cell septum and the cell poles, and in rapidly growing cells brings about cell lysis, indicating that the gene product has a hydrolytic function in cell wall formation or maintenance. This is corroborated by sequence homology to lysostaphin, a cell wall lytic exoenzyme synthesized by two Staphylococcus strains. Using globomycin, a specific inhibitor of signal peptidase II, we demonstrate that the product of the ORF is a novel lipoprotein (NlpD). Two transcriptional start sites for nlpD have been localized. In contrast to rpoS, nlpD is not induced during entry into stationary phase. Growth-phase-regulated transcription of rpoS is initiated at additional sites within the nlpD ORF, but the nlpD promoters contribute substantially to the basal level of rpoS expression in exponentially growing cells, indicating that nlpD and rpoS form an operon.

Characterization of the carbon starvation-inducible and stationary phase-inducible gene slp encoding an outer membrane lipoprotein in Escherichia coli

Escherichia coli induces the expression of more than 50 proteins in response to starvation for a carbon source. Strains MC7 (csi7::phoA) and MC19 (csi19::phoA) contain fusions of a signal peptide-deficient phoA reporter sequence to a csi (carbon starvation-inducible) gene. PhoA expression increased when these strains were deprived of a carbon source or entered stationary phase but did not when the cells were deprived of a nitrogen source or subjected to osmotic, oxidative or thermal stress. Mapping and sequence analysis of the cloned phoA fusions in strains MC7 and MC19 indicated that they had occurred in different locations within the same previously unidentified gene. The wild-type allele of this gene was cloned and the encoded protein was found to be a new lipoprotein. Therefore we propose to call this locus slp (starvation lipoprotein). The 22 kDa Slp protein is associated with the outer membrane fraction. The slp gene was located at 78.6 centisomes on the E. coli genetic map. The -10 and -35 regions upstream of the mRNA start site were characteristic of a sigma 70 promoter. The major transcript from this promoter was sufficiently large to contain slp sequences but not the downstream open reading frame. Induction of beta-galactosidase activity from a slp::lacZ translational fusion during carbon starvation or stationary phase was independent of cAMP, RpoS (KatF) and DnaK, all of which are known to affect the expression of certain starvation-inducible or stationary phase-inducible proteins.

A gene at 59 minutes on the Escherichia coli chromosome encodes a lipoprotein with unusual amino acid repeat sequences

We report a 1.432-kb DNA sequence at 59 min on the Escherichia coli chromosome that connects the published sequences of the pcm gene for the isoaspartyl protein methyltransferase and that of the katF or rpoS (katF/rpoS) gene for a sigma factor involved in stationary-phase gene expression. Analysis of the DNA sequence reveals an open reading frame potentially encoding a polypeptide of 379 amino acids. The polypeptide sequence includes a consensus bacterial lipidation sequence present at residues 23 to 26 (Leu-Ala-Gly-Cys), four octapeptide proline- and glutamine-rich repeats of consensus sequence QQPQIQPV, and four heptapeptide threonine- and serine-rich repeats of consensus sequence PTA(S,T)TTE. The deduced amino acid sequence, especially in the C-terminal region, is similar to that of the Haemophilus somnus LppB lipoprotein outer membrane antigen (40% overall sequence identity; 77% identity in last 95 residues). The LppB lipoprotein binds Congo red dye and has been proposed to be a virulence determinant in H. somnus. Utilizing a plasmid construct with the E. coli gene under the control of a phage T7 promoter, we demonstrate the lipidation of this gene product by the incorporation of [3H]palmitic acid into a 42-kDa polypeptide. We also show that treatment of E. coli cells with globomycin, an inhibitor of the lipoprotein signal peptidase, results in the accumulation of a 46-kDa precursor. We thus designate the protein NlpD (new lipoprotein D). E. coli cells overexpressing NlpD bind Congo red dye, suggesting a common function with the H. somnus LppB protein. Disruption of the chromosomal E. coli nlpD gene by insertional mutagenesis results in decreased stationary-phase survival after 7 days.

The role of the lipoprotein sorting signal (aspartate +2) in pullulanase secretion

The analyses of hybrid proteins and of deletion and insertion mutations reveal that the only amino acid at the amino-proximal end of the cell surface lipoprotein pullulanase that is specifically required for its extracellular secretion is an aspartate at position +2, immediately after the fatty acylated amino-terminal cysteine. To see whether the requirement for this amino acid is related to its proposed role as a cytoplasmic membrane lipoprotein sorting signal, we used sucrose gradient floatation analysis to determine the subcellular location of pullulanase variants (with or without the aspartate residue) that accumulated in cells lacking the pullulanase-specific secretion genes. A non-secretable pullulanase variant with a serine at position +2 cofractionated mainly with the major peak of outer membrane porin. In contrast, most (55%) of a pullulanase variant with an aspartate at position +2 cofractionated with slightly lighter fractions that contained small proportions of both outer membrane porin and the cytoplasmic membrane marker NADH oxidase. Only 5% of this pullulanase variant cofractionated with the major NADH oxidase peak, while the rest (c. 40%) remained at the bottom of the gradient in fractions totally devoid of porin and NADH oxidase. When analysed by sedimentation through sucrose gradients, however, a large proportion of this variant was recovered from fractions near the top of the gradient that also contained the major NADH oxidase peak. When this peak fraction was applied to a floatation gradient the pullulanase activity remained at the bottom while the NADH oxidase floated to the top.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of a protective outer membrane lipoprotein (OmlA) from Actinobacillus pleuropneumoniae serotype 1

An expression library was constructed from an Actinobacillus pleuropneumoniae serotype 1 clinical isolate using a plasmid vector. The library was screened with serum raised against the culture supernatant of this strain. One Escherichia coli transformant which also reacted with convalescent serum was isolated and found to express a protein with an electrophoretic mobility of approximately 50,000. The A. pleuropneumoniae-derived DNA encoding the protein was localized and characterized by nucleotide sequence analysis and primer extension mapping. One open reading frame of 1,095 bases was detected and confirmed by TnphoA insertion mutagenesis. It encoded a protein with a calculated molecular mass of 40 kDa which was lipid modified and present in the outer membrane and in membrane blebs of A. pleuropneumoniae. This protein was designated as outer membrane lipoprotein A (OmlA), and the encoding gene as omlA. Southern blotting under low-stringency conditions revealed the presence of hybridizing sequences in all A. pleuropneumoniae type strains, and a specific serum detected a homologous protein in serotypes 2, 8, 9, 11, and 12 type strains. Pigs immunized with this recombinant protein preparation were protected from death in an aerosol challenge experiment with an A. pleuropneumoniae serotype 1 isolate.

Interactions of Escherichia coli membrane lipoproteins with the murein sacculus

Bifunctional cross-linking reagents were used to identify cell envelope proteins that interacted with the murein sacculus. This revealed that a number of [3H]leucine-labeled proteins and [3H]palmitate-labeled lipoproteins were reproducibly cross-linked to the sacculus in plasmolyzed cells. The results suggested that most of the cell envelope lipoproteins, and not only the murein lipoprotein, mediate interactions between the murein sacculus and the inner and/or outer membrane of the cell.

Characterization of two genes encoding distinct transferrin-binding proteins in different Actinobacillus pleuropneumoniae isolates

The gene encoding the Actinobacillus pleuropneumoniae serotype 1 transferrin-binding protein (tfbA) was cloned, and the carboxy-terminal 70% of the protein was expressed as an aggregate protein in Escherichia coli. The nucleotide sequences of the tfbA genes from A. pleuropneumoniae serotypes 7 (G.-F. Gerlach, C. Anderson, A. A. Potter, S. Klashinsky, and P. J. Willson, Infect. Immun. 60:892-898, 1992) and 1 were determined, and a comparison revealed that they had 65% sequence identity. The deduced amino acid sequences showed a sequence agreement of 55%, and both proteins possessed a lipoprotein-like signal sequence. The serotype 1 TfbA protein had a predicted molecular mass of 65 kDa, compared with 60 kDa for the serotype 7 TfbA protein, and both proteins were immunologically distinct as assessed in a competitive enzyme-linked immunosorbent assay. Southern hybridization and Western blot (immunoblot) analysis of the 13 A. pleuropneumoniae type strains revealed that serotypes 2, 3, 4, 8, 9, 10, and 11 encode and express a TfbA protein highly homologous to that of A. pleuropneumoniae serotype 7 whereas the TfbA proteins and the encoding genes of serotypes 6 and 12 were highly homologous to that found in A. pleuropneumoniae serotype 1. The tfbA genes of A. pleuropneumoniae serotypes 5A and 5B were recognized, under medium-stringency hybridization conditions, by the A. pleuropneumoniae serotype 1-derived tfbA probe, and the respective proteins were weakly reactive with the antibody raised against the A. pleuropneumoniae serotype 7 TfbA protein.

A fusion plasmid for the synthesis of lipopeptide-antigen chimeras in Escherichia coli

Lipopeptides are potential vaccine candidates with a built-in adjuvant property. To circumvent the present chemical route of synthesis for lipopeptide-antigen conjugates, the lipoprotein property of the pColE2-P9-encoded lysis protein, CelB, was used to create the bacterial fusion plasmid, pKLY3, to produce lipopeptide-antigen chimeras in Escherichia coli. Plasmid pKLY3 is a derivative of pKK233-2 with the origin of replication of the single-stranded DNA phage, fl. Under control of the promoter, ptrc, is the 5' end of the celB gene coding for a lipoprotein signal peptide and the first five amino acids (aa) (CQANY) of the mature lysis protein. As model systems for the synthesis of small and large lipopeptide-antigens, DNA sequences coding for the P2 peptide and E. coli alkaline phosphatase (PhoA) were fused in frame to the region of celB coding for a lipoprotein signal peptide and CQANY. P2 is a 12-aa peptide including a tyrosine phosphorylation site of the epidermal growth factor receptor (EGF-R). Inducible expression of stable lipohexapeptide CQANYV, lipo-CQANY-P2, and lipo-CQANYA-PhoA, was demonstrated. Similar expression was obtained for lipo-CIEGR-P2 and lipo-CIEGRA-PhoA in which IEGR is a cleavage recognition site for the blood coagulation factor, Xa. Like QANY, IEGR is predicted to form a beta-turn structure. The presence of a lipid moiety on the products was confirmed by demonstrating the incorporation of radioactive palmitic acid and inhibition of processing by globomycin. The lipid-modified peptides were also identified by incorporation of radioactive tyrosine, and the nature of the P2 peptide was verified immunologically.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

A gene for a new lipoprotein in the dapA-purC interval of the Escherichia coli chromosome

Cloning and sequence analysis of the region located downstream of the dapA gene of Escherichia coli has revealed the presence of an open reading frame that is cotranscribed with dapA. This gene codes for a 344-amino-acid polypeptide with a potential signal sequence characteristic of a lipoprotein. When this gene, called nlpB, is expressed from a multicopy plasmid in bacteria grown in the presence of [3H]palmitate, a labeled 37-kDa protein is produced. A slightly larger precursor molecule is detected when minicells expressing nlpB are treated with globomycin, a specific inhibitor of lipoprotein signal peptidase. Therefore, the nlpB gene encodes a new lipoprotein, designated NlpB. This lipoprotein is detected in outer membrane vesicles prepared from osmotically lysed spheroplasts and appears to be nonessential, since a strain in which the nlpB gene is disrupted by insertion of a chloramphenicol resistance gene is still able to grow and shows no discernible NlpB phenotype. The putative transcription termination signals of the dapA-nlpB operon overlap the promoter of the adjacent purC gene.

The T-cell-stimulating 17-kilodalton protein of Francisella tularensis LVS is a lipoprotein

A T-cell-stimulating, membrane-located 17-kDa protein of the live vaccine strain Francisella tularensis LVS has previously been cloned and sequenced. In the present study, it is shown to be a lipoprotein. When F. tularensis was grown in the presence of [3H]palmitate, several proteins of the organism, including a 17-kDa protein, were radiolabeled. The labeled 17-kDa protein was found by Western blot (immunoblot) analysis to be identical to the cloned protein. It was located in the detergent phase after partitioning with the nonionic detergent Triton X-114, thereby behaving like a hydrophobic integral membrane protein. The protein was predominantly hydrophilic and contained no putative transmembrane domain. The presence of fatty acids is therefore the probable explanation of the membrane location of the 17-kDa protein. The amino acid sequence of the 17-kDa protein contains the tetrapeptide Leu-Ala-Ser-Cys, which is a recognition sequence of the lipoprotein signal peptidase. Globomycin, a specific inhibitor of the peptidase, inhibited maturation of the 17-kDa lipoprotein. The protein incorporated [3H]palmitate also when expressed by Escherichia coli. The 17-kDa lipoprotein was recognized not only by T cells but also by serum antibodies of F. tularensis-primed individuals.

The variable antigens Vmp7 and Vmp21 of the relapsing fever bacterium Borrelia hermsii are structurally analogous to the VSG proteins of the African trypanosome

The relapsing fever agent Borrelia hermsii avoids the host's immune response by the strategy of multiphasic antigenic variation. A given Borrelia cell can express one of a number of alleles for polymorphic outer-membrane proteins, known as Vmp proteins. The genes for the variant-specific Vmp proteins of serotypes 7 and 21 of B. hermsii strain HS1 were sequenced. The genes, which were designated vmp7 and vmp21, were obtained from populations of borreliae before and after a switch in serotypes from 7 to 21. The analysis showed that vmp7 and vmp21 are 77% identical in terms of their coding sequence. The deduced translation products of vmp7 and vmp21 are polypeptides of 369 (37.2 kD) and 364 amino acids (37.1 kD), respectively. Vmp7 and Vmp21 have sequence features of prokaryotic lipoproteins and are processed as such during expression in E. coli. The secondary structure predictions of the Vmp proteins reveals analogous structures to the VSG proteins of the African trypanosome.

Lipoproteins in bacteria

Covalent modification of membrane proteins with lipids appears to be ubiquitous in all living cells. The major outer membrane (Braun's) lipoprotein of E. coli, the prototype of bacterial lipoproteins, is first synthesized as a precursor protein. Analysis of signal sequences of 26 distinct lipoprotein precursors has revealed a consensus sequence of lipoprotein modification/processing site of Leu-(Ala, Ser)-(Gly, Ala)-Cys at -3 to +1 positions which would represent the cleavage region of about three-fourth of all lipoprotein signal sequences in bacteria. Unmodified prolipoprotein with the putative consensus sequence undergoes sequential modification and processing reactions catalyzed by glyceryl transferase, O-acyl transferase(s), prolipoprotein signal peptidase (signal peptidase II), and N-acyl transferase to form mature lipoprotein. Like all exported proteins, the export of lipoprotein requires functional SecA, SecY, and SecD proteins. Thus all precursor proteins are exported through a common pathway accessible to both signal peptidase I and signal peptidase II. The rapidly increasing list of lipid-modified proteins in both prokaryotic as well as eukaryotic cells indicates that lipoproteins comprise a diverse group of structurally and functionally distinct proteins. They share a common structural feature which is derived from a common biosynthetic pathway.

A lipopeptide-encoding sequence upstream from the lysA gene of Pseudomonas aeruginosa

An open reading frame (ORF) of 141 bp was observed upstream from the Pseudomonas aeruginosa lysA gene. The translation product of this ORF contains a signal peptide with a lipoprotein box, Ile-Ala-Ala-Cys, at the predicted signal peptidase cleavage site. The Escherichia coli phoA gene without its signal sequence was fused in frame to this ORF in a broad host-range plasmid. The resulting construct expressed a hybrid protein exhibiting alkaline phosphatase activity in phoA mutants of both E. coli and P. aeruginosa. This indicates that the ORF encodes a peptide, part of which acts as an export signal. The hybrid peptide was identified by immunoblotting with alkaline phosphatase antiserum. The accumulation of a precursor form was observed when P. aeruginosa cells carrying this gene fusion on a plasmid were treated with globomycin. Moreover, the mature form could be labelled with 2-[3H]-glycerol, indicating that lipidic residues may be linked to the hybrid protein. Taken together, these results strongly suggest that the ORF encodes a lipopeptide. We propose that the gene is called IppL.

Human metallothionein-II is synthesized as a stable membrane-localized fusion protein in Escherichia coli

A synthetic gene encoding human metallothionein-II (HMT) was cloned into the specially constructed high-copy-number expression vector, pUA7, and expressed in Escherichia coli. The plasmid construct includes the promoter/operator and regulatory sequences of the Salmonella typhimurium ara operon and part of the 5'-coding and all of the 3'-noncoding regions of the E. coli lpp. Upon induction with arabinose, the resulting Lpp::HMT fusion protein was produced 75,000-fold over uninduced cells, with a relatively stable mRNA (T1/2 of 8.3 min) and a completely stable protein. In addition, over 95% of the final fusion protein was localized in the outer membrane and was capable of binding heavy metals (especially cadmium) in vitro. Cells producing Lpp::HMT bioaccumulated heavy metals (e.g., cadmium) 66-fold over nonproducing cells.