Glycoproteins in prokaryotes

Isolation, structures and biological activities of medicinal glycoproteins from natural resources: A review

In recent years, natural resources have proven to be tremendous sources of glycoproteins. As biological macromolecules, glycoproteins are essential to the growth and development of organisms, and have attracted increasing attention around the world. This review summarized and discussed the development of glycoproteins from natural resources, including isolation methods, purification processes, structural features and biological activities. Generally, the vast majority of glycoproteins can be isolated by hot water extraction followed by purification through gel filtration chromatography. Combined with component analysis, the physicochemical properties of glycoproteins are studied by using several spectroscopic techniques such as ultraviolet-visible (UV-Visible), Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR). Moreover, natural glycoproteins possess various remarkable biological activities, including anti-tumor, anti-oxidant, anti-coagulant and anti-microbial activities. The content of this review will provide a theoretical basis for the research on related glycoproteins and give a perspective on the use of these medical resources.

Surface Glucan Structures in Aeromonas spp

Aeromonas spp. are generally found in aquatic environments, although they have also been isolated from both fresh and processed food. These Gram-negative, rod-shaped bacteria are mostly infective to poikilothermic animals, although they are also considered opportunistic pathogens of both aquatic and terrestrial homeotherms, and some species have been associated with gastrointestinal and extraintestinal septicemic infections in humans. Among the different pathogenic factors associated with virulence, several cell-surface glucans have been shown to contribute to colonization and survival of Aeromonas pathogenic strains, in different hosts. Lipopolysaccharide (LPS), capsule and α-glucan structures, for instance, have been shown to play important roles in bacterial–host interactions related to pathogenesis, such as adherence, biofilm formation, or immune evasion. In addition, glycosylation of both polar and lateral flagella has been shown to be mandatory for flagella production and motility in different Aeromonas strains, and has also been associated with increased bacterial adhesion, biofilm formation, and induction of the host proinflammatory response. The main aspects of these structures are covered in this review.

Glucuronic acid as a helix-inducing linker in short peptides

A new strategy is demonstrated for making peptides helical, using a carbohydrate to bridge between sidechains at each end of a pentapeptide. CD and NMR spectra establish that both an α-helix and a 3₁₀-helix structure can form depending upon the bridge.

The Plasmin-Sensitive Protein Pls in Methicillin-Resistant Staphylococcus aureus (MRSA) Is a Glycoprotein

Most bacterial glycoproteins identified to date are virulence factors of pathogenic bacteria, i.e. adhesins and invasins. However, the impact of protein glycosylation on the major human pathogen Staphylococcus aureus remains incompletely understood. To study protein glycosylation in staphylococci, we analyzed lysostaphin lysates of methicillin-resistant Staphylococcus aureus (MRSA) strains by SDS-PAGE and subsequent periodic acid-Schiff’s staining. We detected four (>300, ∼250, ∼165, and ∼120 kDa) and two (>300 and ∼175 kDa) glycosylated surface proteins with strain COL and strain 1061, respectively. The ∼250, ∼165, and ∼175 kDa proteins were identified as plasmin-sensitive protein (Pls) by mass spectrometry. Previously, Pls has been demonstrated to be a virulence factor in a mouse septic arthritis model. The pls gene is encoded by the staphylococcal cassette chromosome (SCC)mec type I in MRSA that also encodes the methicillin resistance-conferring mecA and further genes. In a search for glycosyltransferases, we identified two open reading frames encoded downstream of pls on the SCCmec element, which we termed gtfC and gtfD. Expression and deletion analysis revealed that both gtfC and gtfD mediate glycosylation of Pls. Additionally, the recently reported glycosyltransferases SdgA and SdgB are involved in Pls glycosylation. Glycosylation occurs at serine residues in the Pls SD-repeat region and modifying carbohydrates are N-acetylhexosaminyl residues. Functional characterization revealed that Pls can confer increased biofilm formation, which seems to involve two distinct mechanisms. The first mechanism depends on glycosylation of the SD-repeat region by GtfC/GtfD and probably also involves eDNA, while the second seems to be independent of glycosylation as well as eDNA and may involve the centrally located G5 domains. Other previously known Pls properties are not related to the sugar modifications. In conclusion, Pls is a glycoprotein and Pls glycosyl residues can stimulate biofilm formation. Thus, sugar modifications may represent promising new targets for novel therapeutic or prophylactic measures against life-threatening S. aureus infections.

Staphylococcus aureus is a serious pathogen that causes life-threatening infections due to its ability to attach to surfaces, form biofilms, and persist inside the host. One of previously identified virulence factors in S. aureus pathogenesis is the plasmin-sensitive surface protein Pls. We here identified Pls as a posttranslationally modified glycoprotein and characterized the domain within Pls that becomes glycosylated as well as the modifying sugars. Moreover, we found that the glycosyltransferases GtfC and GtfD carry out the glycosylation reactions. In a search for a role for the modifying sugars, we found that Pls can stimulate biofilm formation apparently via two distinct mechanisms, one being dependent on glycosylation by GtfC and GtfD the other being independent of glycosylation as well as eDNA. Moreover, we found that none of the already known Pls functions is mediated by the sugar moieties. Thus, we conclude that GtfC/GtfD-glycosylated Pls may contribute to MRSA pathogenicity via stimulation of biofilm formation and may serve as future target to combat or prevent infections with this serious pathogen.

Homologous expression of the Caldicellulosiruptor bescii CelA reveals that the extracellular protein is glycosylated

Members of the bacterial genus Caldicellulosiruptor are the most thermophilic cellulolytic microbes described with ability to digest lignocellulosic biomass without conventional pretreatment. The cellulolytic ability of different species varies dramatically and correlates with the presence of the multimodular cellulase CelA, which contains both a glycoside hydrolase family 9 endoglucanase and a glycoside hydrolase family 48 exoglucanase known to be synergistic in their activity, connected by three cellulose-binding domains via linker peptides. This architecture exploits the cellulose surface ablation driven by its general cellulase processivity as well as excavates cavities into the surface of the substrate, revealing a novel paradigm for cellulase activity. We recently reported that a deletion of celA in C. bescii had a significant effect on its ability to utilize complex biomass. To analyze the structure and function of CelA and its role in biomass deconstruction, we constructed a new expression vector for C. bescii and were able, for the first time, to express significant quantities of full-length protein in vivo in the native host. The protein, which contains a Histidine tag, was active and excreted from the cell. Expression of CelA protein with and without its signal sequence allowed comparison of protein retained intracellularly to protein transported extracellularly. Analysis of protein in culture supernatants revealed that the extracellular CelA protein is glycosylated whereas the intracellular CelA is not, suggesting that either protein transport is required for this post-translational modification or that glycosylation is required for protein export. The mechanism and role of protein glycosylation in bacteria is poorly understood and the ability to express CelA in vivo in C. bescii will allow the study of the mechanism of protein glycosylation in this thermophile. It will also allow the study of glycosylation of CelA itself and its role in the structure and function of this important enzyme in biomass deconstruction.

Bacterial cell-envelope glycoconjugates

Prokaryotic glycosylation fulfills an important role in maintaining and protecting the structural integrity and function of the bacterial cell wall, as well as serving as a flexible adaption mechanism to evade environmental and host-induced pressure. The scope of bacterial and archaeal protein glycosylation has considerably expanded over the past decade(s), with numerous examples covering the glycosylation of flagella, pili, glycosylated enzymes, as well as surface-layer proteins. This article addresses structure, analysis, function, genetic basis, biosynthesis, and biomedical and biotechnological applications of cell-envelope glycoconjugates, S-layer glycoprotein glycans, and "nonclassical" secondary-cell wall polysaccharides. The latter group of polymers mediates the important attachment and regular orientation of the S-layer to the cell wall. The structures of these glycopolymers reveal an enormous diversity, resembling the structural variability of bacterial lipopolysaccharides and capsular polysaccharides. While most examples are presented for Gram-positive bacteria, the S-layer glycan of the Gram-negative pathogen Tannerella forsythia is also discussed. In addition, archaeal S-layer glycoproteins are briefly summarized.

GtfA and GtfB are both required for protein O-glycosylation in Lactobacillus plantarum

Acm2, the major autolysin of Lactobacillus plantarum WCFS1, was recently found to be O-glycosylated with N-acetylhexosamine, likely N-acetylglucosamine (GlcNAc). In this study, we set out to identify the glycosylation machinery by employing a comparative genomics approach to identify Gtf1 homologues, which are involved in fimbria-associated protein 1 (Fap1) glycosylation in Streptococcus parasanguinis. This in silico approach resulted in the identification of 6 candidate L. plantarum WCFS1 genes with significant homology to Gtf1, namely, tagE1 to tagE6. These candidate genes were targeted by systematic gene deletion, followed by assessment of the consequences on glycosylation of Acm2. We observed a changed mobility of Acm2 on SDS-PAGE in the tagE5E6 deletion strain, while deletion of other tagE genes resulted in Acm2 mobility comparable to that of the wild type. Subsequent mass spectrometry analysis of excised and in-gel-digested Acm2 confirmed the loss of glycosylation on Acm2 in the tagE5E6 deletion mutant, whereas a lectin blot using GlcNAc-specific succinylated wheat germ agglutinin (sWGA) revealed that besides Acm2, tagE5E6 deletion also abolished all but one other sWGA-reactive, protease-sensitive signal. Only complementation of both tagE5 and tagE6 restored those sWGA lectin signals, establishing that TagE5 and TagE6 are both required for the glycosylation of Acm2 as well as the vast majority of other sWGA-reactive proteins. Finally, sWGA lectin blotting experiments using a panel of 8 other L. plantarum strains revealed that protein glycosylation is a common feature in L. plantarum strains. With the establishment of these enzymes as protein glycosyltransferases, we propose to rename TagE5 and TagE6 as GtfA and GtfB, respectively.

Combining LC-MS/MS, PMF and N-terminal amino acid sequencing for multiplexed characterization of a bacterial surfactant glycoprotein biosynthesized by Acinetobacter radioresistens S13

The present study has highlighted the mechanisms of bacterial protein glycosylation. Experimental results underline that the consensus sequon can be different from the one found in Eukarya.

Role of N-glycosylation of human lysosomal phospholipase A2 for the formation of catalytically active enzyme

To understand the role of N-glycosylation of lysosomal phospholipase A2 (LPLA2), four potential N-glycosylation sites in human LPLA2 (hLPLA2) were individually modified replacing asparagine (Asn) with alanine by site-direct mutagenesis. COS-7 cells transiently transfected with wild-type (WT) hLPLA2 gene produced catalytically active LPLA2. A single mutation at 273-, 289-, or 398-Asn partially reduced production of active LPLA2. A single mutation at 99-Asn and quadruple mutations at all four Asn sites resulted in a marked reduction of active LPLA2 and loss of active LPLA2, respectively. Western blot analysis using anti-hLPLA2 antibody showed that the LPLA2 expression level was similar between all transfectants. N-glycosidase F digestion revealed that multiple forms of LPLA2 found in individual transfectants are due to different N-glycans linked to the core protein. The LPLA2 activity in individual transfectants was mostly recovered in the soluble fraction and correlated to the quantity of LPLA2 detected in the soluble fraction. LPLA2 mutated at 99-Asn was mostly retained in the membrane fraction. The WT transfectants treated with tunicamycin markedly lost LPLA2 activity. These data indicate that the 99-Asn is the most critical N-glycosylation site for formation of native hLPLA2 in vivo and that the N-glycosylation of LPLA2 is crucial for biosynthesis of catalytically active hLPLA2.

Expression level and glycan dynamics determine the net effects of TIMP-1 on cancer progression

Tissue inhibitor of metalloproteinases (TIMPs; TIMP-1, -2, -3 and -4) are endogenous inhibitor for matrix metalloproteinases (MMPs) that are responsible for remodeling the extracellular matrix (ECM) and involved in migration, invasion and metastasis of tumor cells. Unlike under normal conditions, the imbalance between MMPs and TIMPs is associated with various diseased states. Among TIMPs, TIMP-1, a 184-residue protein, is the only N-linked glycoprotein with glycosylation sites at N30 and N78. The structural analysis of the catalytic domain of human stromelysin-1 (MMP-3) and human TIMP-1 suggests new possibilities of the role of TIMP-1 glycan moieties as a tuner for the proteolytic activities by MMPs. Because the TIMP-1 glycosylation participate in the interaction, aberrant glycosylation of TIMP-1 presumably affects the interaction, thereby leading to pathogenic dysfunction in cancer cells. TIMP-1 has not only the cell proliferation activities but also anti-oncogenic properties. Cancer cells appear to utilize these bilateral aspects of TIMP-1 for cancer progression; an elevated TIMP-1 level exerts to cancer development via MMP-independent pathway during the early phase of tumor formation, whereas it is the aberrant glycosylation of TIMP-1 that overcome the high anti-proteolytic burden. The aberrant glycosylation of TIMP-1 can thus be used as staging and/or prognostic biomarker in colon cancer. [BMB Reports 2012; 45(11): 623-628]

O-glycosylation as a novel control mechanism of peptidoglycan hydrolase activity

Acm2, the major autolysin of Lactobacillus plantarum, is a tripartite protein. Its catalytic domain is surrounded by an O-glycosylated N-terminal region rich in Ala, Ser, and Thr (AST domain), which is of low complexity and unknown function, and a C-terminal region composed of five SH3b peptidoglycan (PG) binding domains. Here, we investigate the contribution of these two accessory domains and of O-glycosylation to Acm2 functionality. We demonstrate that Acm2 is an N-acetylglucosaminidase and identify the pattern of O-glycosylation (21 mono-N-acetylglucosamines) of its AST domain. The O-glycosylation process is species-specific as Acm2 purified from Lactococcus lactis is not glycosylated. We therefore explored the functional role of O-glycosylation by purifying different truncated versions of Acm2 that were either glycosylated or non-glycosylated. We show that SH3b domains are able to bind PG and are responsible for Acm2 targeting to the septum of dividing cells, whereas the AST domain and its O-glycosylation are not involved in this process. Notably, our data reveal that the lack of O-glycosylation of the AST domain significantly increases Acm2 enzymatic activity, whereas removal of SH3b PG binding domains dramatically reduces this activity. Based on this antagonistic role, we propose a model in which access of the Acm2 catalytic domain to its substrate may be hindered by the AST domain where O-glycosylation changes its conformation and/or mediates interdomain interactions. To the best of our knowledge, this is the first time that O-glycosylation is shown to control the activity of a bacterial enzyme.

Global identification of prokaryotic glycoproteins based on an Escherichia coli proteome microarray

Glycosylation is one of the most abundant protein posttranslational modifications. Protein glycosylation plays important roles not only in eukaryotes but also in prokaryotes. To further understand the roles of protein glycosylation in prokaryotes, we developed a lectin binding assay to screen glycoproteins on an Escherichia coli proteome microarray containing 4,256 affinity-purified E.coli proteins. Twenty-three E.coli proteins that bound Wheat-Germ Agglutinin (WGA) were identified. PANTHER protein classification analysis showed that these glycoprotein candidates were highly enriched in metabolic process and catalytic activity classes. One sub-network centered on deoxyribonuclease I (sbcB) was identified. Bioinformatics analysis suggests that prokaryotic protein glycosylation may play roles in nucleotide and nucleic acid metabolism. Fifteen of the 23 glycoprotein candidates were validated by lectin (WGA) staining, thereby increasing the number of validated E. coli glycoproteins from 3 to 18. By cataloguing glycoproteins in E.coli, our study greatly extends our understanding of protein glycosylation in prokaryotes.

The main Aeromonas pathogenic factors

The members of the Aeromonas genus are ubiquitous, water-borne bacteria. They have been isolated from marine waters, rivers, lakes, swamps, sediments, chlorine water, water distribution systems, drinking water and residual waters; different types of food, such as meat, fish, seafood, vegetables, and processed foods. Aeromonas strains are predominantly pathogenic to poikilothermic animals, and the mesophilic strains are emerging as important pathogens in humans, causing a variety of extraintestinal and systemic infections as well as gastrointestinal infections. The most commonly described disease caused by Aeromonas is the gastroenteritis; however, no adequate animal model is available to reproduce this illness caused by Aeromonas. The main pathogenic factors associated with Aeromonas are: surface polysaccharides (capsule, lipopolysaccharide, and glucan), S-layers, iron-binding systems, exotoxins and extracellular enzymes, secretion systems, fimbriae and other nonfilamentous adhesins, motility and flagella.

Site-specific N-glycosylation identification of recombinant human lectin-like oxidized low density lipoprotein receptor-1 (LOX-1)

Human LOX-1/OLR 1 plays a key role in atherogenesis and endothelial dysfunction. The N-glycosylation of LOX-1 has been shown to affect its biological functions in vivo and modulate the pathogenesis of atherosclerosis. However, the N-glycosylation pattern of LOX-1 has not been described yet. The present study was aimed at elucidating the N-glycosylation of recombinant human LOX-1 with regard to N-glycan profile and N-glycosylation sites. Here, an approach using nonspecific protease (Pronase E) digestion followed by MALDI-QIT-TOF MS and multistage MS (MS(3)) analysis is explored to obtain site-specific N-glycosylation information of recombinant human LOX-1, in combination with glycan structure confirmation through characterizing released glycans using tandem MS. The results reveal that N-glycans structures as well as their corresponding attached site of LOX-1 can be identified simultaneously by direct MS analysis of glycopeptides from non-specific protease digestion. With this approach, one potential glycosylation site of recombinant human LOX-1 on Asn(139) is readily identified and found to carry heterogeneous complex type N-glycans. In addition, manual annotation of multistage MS data utilizing diagnostic ions, which were found to be particularly useful in defining the structure of glycopeptides and glycans was addressed for proper spectra interpretation. The findings described herein will shed new light on further research of the structure-function relationships of LOX-1 N-glycan.

ProGlycProt: a repository of experimentally characterized prokaryotic glycoproteins

ProGlycProt (http://www.proglycprot.org/) is an open access, manually curated, comprehensive repository of bacterial and archaeal glycoproteins with at least one experimentally validated glycosite (glycosylated residue). To facilitate maximum information at one point, the database is arranged under two sections: (i) ProCGP-the main data section consisting of 95 entries with experimentally characterized glycosites and (ii) ProUGP-a supplementary data section containing 245 entries with experimentally identified glycosylation but uncharacterized glycosites. Every entry in the database is fully cross-referenced and enriched with available published information about source organism, coding gene, protein, glycosites, glycosylation type, attached glycan, associated oligosaccharyl/glycosyl transferases (OSTs/GTs), supporting references, and applicable additional information. Interestingly, ProGlycProt contains as many as 174 entries for which information is unavailable or the characterized glycosites are unannotated in Swiss-Prot release 2011_07. The website supports a dedicated structure gallery of homology models and crystal structures of characterized glycoproteins in addition to two new tools developed in view of emerging information about prokaryotic sequons (conserved sequences of amino acids around glycosites) that are never or rarely seen in eukaryotic glycoproteins. ProGlycProt provides an extensive compilation of experimentally identified glycosites (334) and glycoproteins (340) of prokaryotes that could serve as an information resource for research and technology applications in glycobiology.

Glycan Reader: automated sugar identification and simulation preparation for carbohydrates and glycoproteins

Understanding how glycosylation affects protein structure, dynamics, and function is an emerging and challenging problem in biology. As a first step toward glycan modeling in the context of structural glycobiology, we have developed Glycan Reader and integrated it into the CHARMM-GUI, http://www.charmm-gui.org/input/glycan. Glycan Reader greatly simplifies the reading of PDB structure files containing glycans through (i) detection of carbohydrate molecules, (ii) automatic annotation of carbohydrates based on their three-dimensional structures, (iii) recognition of glycosidic linkages between carbohydrates as well as N-/O-glycosidic linkages to proteins, and (iv) generation of inputs for the biomolecular simulation program CHARMM with the proper glycosidic linkage setup. In addition, Glycan Reader is linked to other functional modules in CHARMM-GUI, allowing users to easily generate carbohydrate or glycoprotein molecular simulation systems in solution or membrane environments and visualize the electrostatic potential on glycoprotein surfaces. These tools are useful for studying the impact of glycosylation on protein structure and dynamics.

Cell lines expressing mutant FX proteins to generate proteins with reduced rate of fucosylation: WO2010/141478

BACKGROUND

The application (WO 2010/141478) is in the field of glycobiology and deals with the synthesis of recombinant proteins with a reduced rate of fucosylation.

OBJECTIVE

It aims at generating mammalian cell lines that express mutant forms of GDP-4-keto-deoxy-mannose-3,5,-epimerase-4-reductase (FX) and cells that conditionally fucosylate proteins.

METHODS

Mutant forms of FX were synthesized, and mammalian cell lines genetically engineered to express mutant FX proteins and the protein of interest. Cell lines that conditionally fucosylate proteins were generated from a mutant FX form that has reduced ability to fucosylate glycoproteins at 37°C and not at 34°C.

RESULTS

Cells genetically engineered to express mutant forms of FX protein show reduced ability to fucosylate proteins, particularly antibodies, with rates as low as 5 - 0.5% fucosylation, compared to cells expressing wild-type FX.

CONCLUSION

Cells genetically engineered to express the mutant forms of FX protein provide a means to synthesize and express proteins with a reduced rate of fucosylation. The paradigm may be used to synthesize antibodies that mediate antibody-dependent cell-mediated cytotoxicity more efficiently. The application uses mammalian cell lines, genetically engineered to express mutant FX proteins, to synthesize and produce proteins with a reduced rate of fucosylation. The application claims the conditional control of protein fucosylation by FX mutant proteins.

Modulation of GDP-fucose level for generating proteins with reduced rate of fucosylation (WO2010141855)

BACKGROUND

The application (WO2010141855) is in the field of glycobiology, and involves the control of the rate of fucosylation of proteins by exogenous factors.

OBJECTIVE

It aims at controlling the rate of protein fucosylation with inhibitors (drugs or nucleic acid antagonists) of enzymes involved in the synthesis of GDP-fucose.

METHODS

Mammalian cell lines were cultured in the presence of inhibitors, for example, siRNA. The rates of GDP-fucose in cells and during protein fucosylation were characterized.

RESULTS

The level of protein fucosylation decreases rapidly in response to a decrease in GDP-fucose level.

CONCLUSION

The relationship between the rate of fucosylation of proteins and the level of GDP-fucose in a cell is non-linear. Reduction in the rate of protein fucosylation can be achieved with a minimal reduction of the level of GDP-fucose in cells. The paradigm may be used to synthesize proteins and antibodies, with a reduced rate of fucosylation. The application claims that the use of drugs or nucleic acid antagonists that inhibit the enzymes involved in GDP-fucose biosynthesis optimizes the level of GDP-fucose present in cells, and reduces the rate of fucosylation of glycoproteins.

Comparative genomics analysis of completely sequenced microbial genomes reveals the ubiquity of N-linked glycosylation in prokaryotes

Glycosylation of proteins in prokaryotes has been known for the last few decades. Glycan structures and/or the glycosylation pathways have been experimentally characterized in only a small number of prokaryotes. Even this has become possible only during the last decade or so, primarily due to technological and methodological developments. Glycosylated proteins are diverse in their function and localization. Glycosylation has been shown to be associated with a wide range of biological phenomena. Characterization of the various types of glycans and the glycosylation machinery is critical to understand such processes. Such studies can help in the identification of novel targets for designing drugs, diagnostics, and engineering of therapeutic proteins. In view of this, the experimentally characterized pgl system of Campylobacter jejuni, responsible for N-linked glycosylation, has been used in this study to identify glycosylation loci in 865 prokaryotes whose genomes have been completely sequenced. Results from the present study show that only a small number of organisms have homologs for all the pgl enzymes and a few others have homologs for none of the pgl enzymes. Most of the organisms have homologs for only a subset of the pgl enzymes. There is no specific pattern for the presence or absence of pgl homologs vis-à-vis the 16S rRNA sequence-based phylogenetic tree. This may be due to differences in the glycan structures, high sequence divergence, horizontal gene transfer or non-orthologous gene displacement. Overall, the presence of homologs for pgl enzymes in a large number of organisms irrespective of their habitat, pathogenicity, energy generation mechanism, etc., hints towards the ubiquity of N-linked glycosylation in prokaryotes.

Improving N-glycosylation efficiency in Escherichia coli using shotgun proteomics, metabolic network analysis, and selective reaction monitoring

Recently, the prospect of using Escherichia coli as a host for human glycoprotein production has increased due to detailed characterization of the prokaryotic N-glycosylation process and the ability to transfer the system into this bacterium. Although functionality of the native Campylobacter jejuni N-glycosylation system in E. coli has been demonstrated, the efficiency of the process using the well-characterized C. jejuni glycoprotein AcrA, was found to be low at 13.4±0.9% of total extracted protein. A combined approach using isobaric labeling of peptides and probability-based network analysis of metabolic changes was applied to forward engineer E. coli to improve glycosylation efficiency of AcrA. Enhancing flux through the glyoxylate cycle was identified as a potential metabolic manipulation to improve modification efficiency and was achieved by increasing the expression of isocitrate lyase. While the overall recombinant protein titre did not change significantly, the amount of glycosylated protein increased by approximately 300%.

A novel glucosyltransferase is required for glycosylation of a serine-rich adhesin and biofilm formation by Streptococcus parasanguinis

Fap1-like serine-rich glycoproteins are conserved in streptococci, staphylococci, and lactobacilli, and are required for bacterial biofilm formation and pathogenesis. Glycosylation of Fap1 is mediated by a gene cluster flanking the fap1 locus. The key enzymes responsible for the first step of Fap1 glycosylation are glycosyltransferases Gtf1 and Gtf2. They form a functional enzyme complex that catalyzes the transfer of N-acetylglucosamine (GlcNAc) residues to the Fap1 polypeptide. However, until now nothing was known about the subsequent step in Fap1 glycosylation. Here, we show that the second step in Fap1 glycosylation is catalyzed by nucleotide-sugar synthetase-like (Nss) protein. The nss gene located upstream of fap1 is also highly conserved in streptococci and lactobacilli. Nss-deficient mutants failed to catalyze the second step of Fap1 glycosylation in vivo in Streptococcus parasanguinis and in a recombinant Fap1 glycosylation system. Nss catalyzed the direct transfer of the glucosyl residue to the GlcNAc-modified Fap1 substrate in vitro, demonstrating that Nss is a glucosyltransferase. Thus we renamed Nss as glucosyltransferase 3 (Gtf3). A gtf3 mutant exhibited a biofilm defect. Taken together, we conclude that this new glucosyltransferase mediates the second step of Fap1 glycosylation and is required for biofilm formation.

Glycosylation and biogenesis of a family of serine-rich bacterial adhesins

Glycosylation of bacterial proteins is an important process for bacterial physiology and pathophysiology. Both O- and N-linked glycan moieties have been identified in bacterial glycoproteins. The N-linked glycosylation pathways are well established in Gram-negative bacteria. However, the O-linked glycosylation pathways are not well defined due to the complex nature of known O-linked glycoproteins in bacteria. In this review, we examine a new family of serine-rich O-linked glycoproteins which are represented by fimbriae-associated adhesin Fap1 of Streptococcus parasanguinis and human platelet-binding protein GspB of Streptococcus gordonii. This family of glycoproteins is conserved in streptococcal and staphylococcal species. A gene cluster coding for glycosyltransferases and accessory Sec proteins has been implicated in the protein glycosylation. A two-step glycosylation model is proposed. Two glycosyltransferases interact with each other and catalyse the first step of the protein glycosylation in the cytoplasm; the cross-talk between glycosylation-associated proteins and accessory Sec components mediates the second step of the protein glycosylation, an emerging mechanism for bacterial O-linked protein glycosylation. Dissecting the molecular mechanism of this conserved biosynthetic pathway offers opportunities to develop new therapeutic strategies targeting this previously unrecognized pathway, as serine-rich glycoproteins have been shown to play a role in bacterial pathogenesis.

Infrared multiphoton dissociation mass spectrometry for structural elucidation of oligosaccharides

The structural elucidation of oligosaccharides remains a major challenge. Mass spectrometry provides a rapid and convenient method for structural elucidation on the basis of tandem mass spectrometry. Ions are commonly selected and subjected to collision-induced dissociation (CID) to obtain structural information. However, a disadvantage of CID is the decrease in both the degree and efficiency of dissociation with increasing mass. In this chapter, we illustrate the use of infrared multiphoton dissociation (IRMPD) to obtain structural information for O- and N-linked oligosaccharides. The IRMPD and CID behaviors of oligosaccharides are compared.

Molecular characterization of Ehrlichia interactions with tick cells and macrophages

Several tick-transmitted Anaplasmataceae family rickettsiales of the genera Ehrlichia and Anaplasma have been discovered in recent years. Some species are classified as pathogens causing emerging diseases with growing health concern for people. They include human monocytic ehrlichiosis, human granulocytic ewingii ehrlichiosis and human granulocytic anaplasmosis which are caused by Ehrlichia chaffeensis, E. ewingii and Anaplasma phagocytophilum, respectively. Despite the complex cellular environments and defense systems of arthropod and vertebrate hosts, rickettsials have evolved strategies to evade host clearance and persist in both vertebrate and tick host environments. For example, E. chaffeensis growing in vertebrate macrophages has distinct patterns of global host cell-specific protein expression and differs considerably in morphology compared with its growth in tick cells. Immunological studies suggest that host cell-specific differences in Ehrlichia gene expression aid the pathogen, extending its survival. Bacteria from tick cells persist longer when injected into mice compared with mammalian macrophage-grown bacteria, and the host response is also significantly different. This review presents the current understanding of tick-Ehrlichia interactions and implications for future.

Glycosylation of the phosphate binding protein, PstS, in Streptomyces coelicolor by a pathway that resembles protein O-mannosylation in eukaryotes

Previously mutations in a putative protein O-mannosyltransferase (SCO3154, Pmt) and a polyprenol phosphate mannose synthase (SCO1423, Ppm1) were found to cause resistance to phage, phiC31, in the antibiotic producing bacteria Streptomyces coelicolor A3(2). It was proposed that these two enzymes were part of a protein O-glycosylation pathway that was necessary for synthesis of the phage receptor. Here we provide the evidence that Pmt and Ppm1 are indeed both required for protein O-glycosylation. The phosphate binding protein PstS was found to be glycosylated with a trihexose in the S. coelicolor parent strain, J1929, but not in the pmt(-) derivative, DT1025. Ppm1 was necessary for the transfer of mannose to endogenous polyprenol phosphate in membrane preparations of S. coelicolor. A mutation in ppm1 that conferred an E218V substitution in Ppm1 abolished mannose transfer and glycosylation of PstS. Mass spectrometry analysis of extracted lipids showed the presence of a glycosylated polyprenol phosphate (PP) containing nine repeated isoprenyl units (C(45)-PP). S. coelicolor membranes were also able to catalyse the transfer of mannose to peptides derived from PstS, indicating that these could be targets for Pmt in vivo.

Comparison of HPLC/ESI-FTICR MS versus MALDI-TOF/TOF MS for glycopeptide analysis of a highly glycosylated HIV envelope glycoprotein

Defining the structures and locations of the glycans attached on secreted proteins and virus envelope proteins is important in understanding how glycosylation affects their biological properties. Glycopeptide mass spectrometry (MS)-based analysis is a very powerful, emerging approach to characterize glycoproteins, in which glycosylation sites and the corresponding glycan structures are elucidated in a single MS experiment. However, to date there is not a consensus regarding which mass spectrometric platform provides the best glycosylation coverage information. Herein, we employ two of the most widely used MS approaches, online high performance liquid chromatography-electrospray ionization mass spectrometry (HPLC/ESI-MS) and offline HPLC followed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), to determine which of the two approaches provides the best glycosylation coverage information of a complex glycoprotein, the group M consensus HIV-1 envelope, CON-S gp140DeltaCFI, which has 31 potential glycosylation sites. Our results highlight differences in the informational content obtained between the two methods such as the overall number of glycosylation sites detected, the numbers of N-linked glycans present at each site, and the type of confirmatory information obtained about the glycopeptide using MS/MS experiments. The two approaches are quite complementary, both in their coverage of glycopeptides and in the information they provide in MS/MS experiments. The information in this study contributes to the field of mass spectrometry by demonstrating the strengths and limitations of two widely used MS platforms in glycoprotein analysis.

A thermostable dolichol phosphoryl mannose synthase responsible for glycoconjugate synthesis of the hyperthermophilic archaeon Pyrococcus horikoshii

Dolichol phosphoryl mannose synthase (DPM synthase) is an essential enzyme in the synthesis of N- and O-linked glycoproteins and the glycosylphosphatidyl-inositol anchor. An open reading frame, PH0051, from the hyperthermophilic archaeon Pyrococcus horikoshii encodes a DPM synthase ortholog, PH0051p. A full-length version of PH0051p was produced using an E. coli in vitro translation system and its thermostable activity was confirmed with a DPM synthesis assay, although the in vitro productivity was not sufficient for further characterization. Then, a yeast expression vector coding for the N-terminal catalytic domain of PH0051p was constructed. The N-terminal domain, named DPM(1-237), was successfully expressed, and turned out to be a membrane-bound form in Saccharomyces cerevisiae cells, even without its hydrophobic C-terminal domain. The membrane-bound DPM(1-237) was solubilized with a detergent and purified to homogeneity. The purified DPM(1-237) showed thermostability at up to 75 degrees C and an optimum temperature of 60 degrees C. The truncated mutant DPM(1-237) required Mg(2+) and Mn(2+) ions as cofactors the same as eukaryotic DPM synthases. By site-directed mutagenesis, Asp(89) and Asp(91) located at the most conserved motif, DXD, were confirmed as the catalytic residues, the latter probably bound to a cofactor, Mg(2+). DPM(1-237) was able to utilize both acceptor lipids, dolichol phosphate and the prokaryotic carrier lipid C(55)-undecaprenyl phosphate, with Km values of 1.17 and 0.59 microM, respectively. The DPM synthase PH0051p seems to be a key component of the pathway supplying various lipid-linked phosphate sugars, since P. horikoshii could synthesize glycoproteins as well as the membrane-associated PH0051p in vivo.

Gingipain-dependent interactions with the host are important for survival of Porphyromonas gingivalis

Porphyromonas gingivalis, a major periodontal pathogen, must acquire nutrients from host derived substrates, overcome oxidative stress and subvert the immune system. These activities can be coordinated via the gingipains which represent the most significant virulence factor produced by this organism. In the context of our contribution to this field, we will review the current understanding of gingipain biogenesis, glycosylation, and regulation, as well as discuss their role in oxidative stress resistance and apoptosis. We can postulate a model, in which gingipains may be part of the mechanism for P. gingivalis virulence.

Anthrose biosynthetic operon of Bacillus anthracis

The exosporium of Bacillus anthracis spores consists of a basal layer and an external hair-like nap. The nap is composed primarily of the glycoprotein BclA, which contains a collagen-like region with multiple copies of a pentasaccharide side chain. This oligosaccharide possesses an unusual terminal sugar called anthrose, followed by three rhamnose residues and a protein-bound N-acetylgalactosamine. Based on the structure of anthrose, we proposed an enzymatic pathway for its biosynthesis. Examination of the B. anthracis genome revealed six contiguous genes that could encode the predicted anthrose biosynthetic enzymes. These genes are transcribed in the same direction and appear to form two operons. We introduced mutations into the B. anthracis chromosome that either delete the promoter of the putative upstream, four-gene operon or delete selected genes in both putative operons. Spores produced by strains carrying mutations in the upstream operon completely lacked or contained much less anthrose, indicating that this operon is required for anthrose biosynthesis. In contrast, inactivation of the downstream, two-gene operon did not alter anthrose content. Additional experiments confirmed the organization of the anthrose operon and indicated that it is transcribed from a sigma(E)-specific promoter. Finally, we demonstrated that anthrose biosynthesis is not restricted to B. anthracis as previously suggested.

Evidence for glycosylation on a DNA-binding protein of Salmonella enterica

BACKGROUND

All organisms living under aerobic atmosphere have powerful mechanisms that confer their macromolecules protection against oxygen reactive species. Microorganisms have developed biomolecule-protecting systems in response to starvation and/or oxidative stress, such as DNA biocrystallization with Dps (DNA-binding protein from starved cells). Dps is a protein that is produced in large amounts when the bacterial cell faces harm, which results in DNA protection. In this work, we evaluated the glycosylation in the Dps extracted from Salmonella enterica serovar Typhimurium. This Dps was purified from the crude extract as an 18-kDa protein, by means of affinity chromatography on an immobilized jacalin column.

RESULTS

The N-terminal sequencing of the jacalin-bound protein revealed 100% identity with the Dps of S. enterica serovar Typhimurium. Methyl-alpha-galactopyranoside inhibited the binding of Dps to jacalin in an enzyme-linked lectin assay, suggesting that the carbohydrate recognition domain (CRD) of jacalin is involved in the interaction with Dps. Furthermore, monosaccharide compositional analysis showed that Dps contained mannose, glucose, and an unknown sugar residue. Finally, jacalin-binding Dps was detected in larger amounts during the bacterial earlier growth periods, whereas high detection of total Dps was verified throughout the bacterial growth period.

CONCLUSION

Taken together, these results indicate that Dps undergoes post-translational modifications in the pre- and early stationary phases of bacterial growth. There is also evidence that a small mannose-containing oligosaccharide is linked to this bacterial protein.

Rapid and individual-specific glycoprofiling of the low abundance N-glycosylated protein tissue inhibitor of metalloproteinases-1

A gel-based method for a mass spectrometric site-specific glycoanalysis was developed using a recombinant glycoprotein expressed in two different cell lines. Hydrophilic interaction liquid chromatography at nanoscale level was used to enrich for glycopeptides prior to MS. The glycoprofiling was performed using matrix-assisted laser desorption/ionization MS and MS/MS. The method proved to be fast and sensitive and furthermore yielded a comprehensive site-specific glycan analysis, allowing a differentiation of the glycoprofiles of the two sources of recombinant protein, both comprising N-glycans of a highly heterogeneous nature. To test the potential of the method, tissue inhibitor of metalloproteinases-1 (TIMP-1), a secreted low abundance N-glycosylated protein and a cancer marker, was purified in an individual-specific manner from plasma of five healthy individuals using IgG depletion and immunoaffinity chromatography. The corresponding TIMP-1 glycoprofiles were determined to be highly similar, comprising mainly bi- and triantennary complex oligosaccharides. Additionally it was shown that platelet-derived TIMP-1 displayed a similar glycoprofile. This is the first study to investigate the glycosylation of naturally occurring human TIMP-1, and the high similarity of the glycoprofiles showed that individual-specific glycosylation variations of TIMP-1 are minimal. In addition, the results showed that TIMP-1 derived from platelets and plasma is similarly glycosylated. This comprehensive and rapid glycoprofiling of a low abundance glycoprotein performed in an individual-specific manner allows for future studies of glycosylated biomarkers for person-specific detection of altered glycosylation and may thus allow early detection and monitoring of diseases.

Archaeal flagella, bacterial flagella and type IV pili: a comparison of genes and posttranslational modifications

The archaeal flagellum is a unique motility organelle. While superficially similar to the bacterial flagellum, several similarities have been reported between the archaeal flagellum and the bacterial type IV pilus system. These include the multiflagellin nature of the flagellar filament, N-terminal sequence similarities between archaeal flagellins and bacterial type IV pilins, as well as the presence of homologous proteins in the two systems. Recent advances in archaeal flagella research add to the growing list of similarities. First, the preflagellin peptidase that is responsible for processing the N-terminal signal peptide in preflagellins has been identified. The preflagellin peptidase is a membrane-bound enzyme topologically similar to its counterpart in the type IV pilus system (prepilin peptidase); the two enzymes are demonstrated to utilize the same catalytic mechanism. Second, it has been suggested that the archaeal flagellum and the bacterial type IV pilus share a similar mode of assembly. While bacterial flagellins and type IV pilins can be modified with O-linked glycans, N-linked glycans have recently been reported on archaeal flagellins. This mode of glycosylation, as well as the observation that the archaeal flagellum lacks a central channel, are both consistent with the proposed assembly model. On the other hand, the failure to identify other genes involved in archaeal flagellation by homology searches likely implies a novel aspect of the archaeal flagellar system. These interesting features remain to be deciphered through continued research. Such knowledge would be invaluable to motility and protein export studies in the Archaea.

Investigating the heterogeneity of cell growth in microbial colonies by FTIR microspectroscopy

Microorganisms rarely occur as individual cells in nature and are, instead, organized in complex multicellular communities such as colonies, fruiting bodies, or biofilms. Interest in the natural microbial life-style has increased during the last decade and a whole plethora of techniques has been used to gain insight into the development, structure and composition of diverse microbial communities. We have developed a technique for investigating the spatial heterogeneity of microbial growth in macro-colonies which essentially entails excision of the colonies with the underlying agar, freezing and subsequent cryotoming of the colonies, then FTIR microspectroscopic mapping of the cryosections. Colonies from Legionella, Bacillus, and Candida strains were chosen as model systems of multi-cellular communities to evaluate the technique. The results obtained indicate pronounced cell population heterogeneity even in relatively young colonies cultivated under laboratory conditions. Spectral data obtained from different positions within, e.g., a colony of Legionella bozemanii 120 h old indicated that levels of the storage material poly-beta-hydroxybutyric acid were significantly higher in cells at the surface of the colonies than in those growing at the bottom next to the agar surface. Similarly, in a 24-h-old macro-colony of Bacillus megaterium significantly more of the capsular compound polyglutamic acid was detected in upper layers than in deeper layers of the colony. Results demonstrate that FTIR microspectroscopy can be an useful tool for investigation of the spatial heterogeneity of cell growth within microbial macro-colonies. It is suggested that the method also can be adapted to the analysis of more complex multicellular communities, for example fruiting bodies, biofilms, or colonies growing under natural conditions.

The use of mass spectrometry for the proteomic analysis of glycosylation

Of all protein PTMs, glycosylation is by far the most common, and is a target for proteomic research. Glycosylation plays key roles in controlling various cellular processes and the modifications of the glycan structures in diseases highlight the clinical importance of this PTM. Glycosylation analysis remains a difficult task. MS, in combination with modern separation methodologies, is one of the most powerful and versatile techniques for the structural analysis of glycoconjugates. This review describes methodologies based on MS for detailed characterization of glycoconjugates in complex biological samples at the sensitivity required for proteomic work.

Identification, subcellular localization and functional interactions of PilMNOWQ and PilA4 involved in transformation competency and pilus biogenesis in the thermophilic bacterium Thermus thermophilus HB27

The natural transformation system of the thermophilic bacterium Thermus thermophilus HB27 comprises at least 16 distinct competence proteins encoded by seven distinct loci. In this article, we present for the first time biochemical analyses of the Thermus thermophilus competence proteins PilMNOWQ and PilA4, and demonstrate that the pilMNOWQ genes are each essential for natural transformation. We identified three different forms of PilA4, one with an apparent molecular mass of 14 kDa, which correlates with that of the deduced protein, an 18-kDa form and a 23-kDa form; the last was found to be glycosylated. We demonstrate that PilM, PilN and PilO are located in the inner membrane, whereas PilW, PilQ and PilA4 are located in the inner and outer membranes. These data show that PilMNOWQ and PilA4 are components of a DNA translocator structure that spans the inner and outer membranes. We further show that PilA4 and PilQ both copurify with pilus structures. Possible functions of PilQ and PilA4 in DNA translocation and in pilus biogenesis are discussed. Comparative mutant studies revealed that mutations in either pilW or pilQ significantly affect the location of the other protein in the outer membrane. Furthermore, no PilA4 was present in the outer membranes of these mutants. From these findings, we conclude that the abilities of PilW, PilQ and PilA4 to stably localize or accumulate in the outer membrane fraction are strongly dependent on one another, which is in accord with an outer membrane DNA translocator complex comprising PilW, PilQ, and PilA4.

Identification and characterization of the genes encoding a unique surface (S-) layer of Tannerella forsythia

A newly emerged periodontopathic pathogen Tannerella forsythia (formerly Bacteroides forsythus), a Gram-negative, filament-shaped, strict anaerobic, non-pigmented oral bacterium, possesses a surface (S-) layer. In our previous studies, the S-layer has been isolated, and shown to mediate hemagglutination, adhesion/invasion of epithelial cell, and murine subcutaneous abscess formation. In the present study, biochemical and molecular genetic characterization of the S-layer are reported. Amino acid sequencing and mass spectrometry indicated that the S-layer is composed of two different proteins, termed 200 and 210 kDa proteins. It was also shown that these proteins are glycosylated. The genes encoding the core proteins of these glycoproteins, designated as tfsA and tfsB, have been identified in silico, cloned, and their sequences have been determined. The tfsA (3.5 kb) and tfsB (4.1 kb) genes are located in tandem, and encode for 135 and 152 kDa proteins, respectively. An apparent discrepancy in molecular weights, 135 vs. 200 kDa and 152 vs. 210 kDa, is accounted for carbohydrate residues attached to the core proteins. Amino acid sequence comparison exhibited a 24% similarity between the 200 and 210 kDa proteins. Further sequence analyses showed that TfsA and TfsB possess putative signal peptide sequences with cleavage sites at alanine residues, and transmembrane domains on the C-terminal region. Northern blot and RT-PCR analyses confirmed an operon structure of tfsAB, suggesting co-regulation of these genes in producing the S-layer. Putative promoter sequences and transcription termination sequences for this operon have also been identified. Comparison with database indicates that the S-layer of T. forsythia has a unique structure exhibiting no homology to other known S-layers of prokaryotic organisms. The present study shows that the T. forsythia S-layer is very unique, since it appears to be composed of two large glycoproteins, and it does not reveal any homology to other known S-layer proteins or glycoproteins of prokaryotic organisms.

Myxoxanthophyll is required for normal cell wall structure and thylakoid organization in the cyanobacterium Synechocystis sp. strain PCC 6803

Myxoxanthophyll is a carotenoid glycoside in cyanobacteria that is of unknown biological significance. The sugar moiety of myxoxanthophyll in Synechocystis sp. strain PCC 6803 was identified as dimethyl fucose. The open reading frame sll1213 encoding a fucose synthetase orthologue was deleted to probe the role of fucose and to determine the biological significance of myxoxanthophyll in Synechocystis sp. strain PCC 6803. Upon deletion of sll1213, a pleiotropic phenotype was obtained: when propagated at 0.5 micromol photons m(-2) s(-1), photomixotrophic growth of cells lacking sll1213 was poor. When grown at 40 micromol photons m(-2) s(-1), growth was comparable to that of the wild type, but cells showed a severe reduction in or loss of the glycocalyx (S-layer). As a consequence, cells aggregated in liquid as well as on plates. At both light intensities, new carotenoid glycosides accumulated, but myxoxanthophyll was absent. New carotenoid glycosides may be a consequence of less-specific glycosylation reactions that gained prominence upon the disappearance of the native sugar moiety (fucose) of myxoxanthophyll. In the mutant, the N-storage compound cyanophycin accumulated, and the organization of thylakoid membranes was altered. Altered cell wall structure and thylakoid membrane organization and increased cyanophycin accumulation were also observed for deltaslr0940K, a strain lacking zeta-carotene desaturase and thereby all carotenoids but retaining fucose. Therefore, lack of myxoxanthophyll and not simply of fucose results in most of the phenotypic effects described here. It is concluded that myxoxanthophyll contributes significantly to the vigor of cyanobacteria, as it stabilizes thylakoid membranes and is critical for S-layer formation.

Autotransporter Proteins

This review focuses on the function of the Escherichia coli and Salmonella autotransporters for which a considerable amount of literature is available. Members of the serine protease autotransporters of the Enterobacteriaceae (SPATEs) family are proteins from E. coli and Shigella spp., which, like the Neisseria and Haemophilus influenzae IgA1 proteases and Hap, possess a consensus serine protease motif. The largest subfamily of autotransporters is defined by the AidA conserved domain COG3468 and consists of members from a diverse range of animal and plant pathogens including E. coli, S. enterica, Yersinia pestis. This subfamily, which is composed of more than 55 proteins, possesses some of the best-characterized autotransporter proteins including the S. flexneri mediator of motility IcsA, the major phase-variable E. coli outer membrane protein antigen 43 (Ag43) and the diffuse adhering E. coli (DAEC) adhesin AIDA-I, from which this subfamily derives its name. Another member of the AIDA-I family, and one of the most studied autotransporter proteins, is IcsA. The autotransporter pathway is emerging as the most common mechanism of protein translocation across the gram-negative outer membrane.

Posttranslational protein modification in Archaea

One of the first hurdles to be negotiated in the postgenomic era involves the description of the entire protein content of the cell, the proteome. Such efforts are presently complicated by the various posttranslational modifications that proteins can experience, including glycosylation, lipid attachment, phosphorylation, methylation, disulfide bond formation, and proteolytic cleavage. Whereas these and other posttranslational protein modifications have been well characterized in Eucarya and Bacteria, posttranslational modification in Archaea has received far less attention. Although archaeal proteins can undergo posttranslational modifications reminiscent of what their eucaryal and bacterial counterparts experience, examination of archaeal posttranslational modification often reveals aspects not previously observed in the other two domains of life. In some cases, posttranslational modification allows a protein to survive the extreme conditions often encountered by Archaea. The various posttranslational modifications experienced by archaeal proteins, the molecular steps leading to these modifications, and the role played by posttranslational modification in Archaea form the focus of this review.

Ehrlichia chaffeensis expresses macrophage- and tick cell-specific 28-kilodalton outer membrane proteins

Ehrlichia chaffeensis, a tick-transmitted rickettsial agent, causes human monocyte/macrophage-tropic ehrlichiosis. In this study, proteomic approaches were used to demonstrate host cell-specific antigenic expression by E. chaffeensis. The differentially expressed antigens include those from the 28-kDa outer membrane protein (p28-Omp) multigene locus. The proteins expressed in infected macrophages are the products of p28-Omp19 and p28-Omp20 genes, whereas in tick cells, the protein expressed is the p28-Omp14 gene product. The differentially expressed proteins are posttranslationally modified by phosphorylation and glycosylation to generate multiple expressed forms. Host cell-specific protein expression is not influenced by growth temperatures and is reversible. Host cell-specific protein expression coupled with posttranslational modifications may be a hallmark for the pathogen's adaptation to a dual-host life cycle and its persistence.

Type V protein secretion pathway: the autotransporter story

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function.