Purification and characterization of an active N-acetylglucosaminyltransferase enzyme complex from Streptococci

Genome analysis and 2'-fucosyllactose utilization characteristics of a new Akkermansia muciniphila strain isolated from mice feces

Akkermansia muciniphila is considered to be a next-generation probiotic, and closely related to host metabolism and immune response. Compared with other probiotics, little is known about its genomic analysis. Therefore, further researches about isolating more A. muciniphila strains and exploring functional genes are needed. In the present study, a new strain isolated from mice feces was identified as A. muciniphila (Muc^X). Whole-genome sequencing and annotation revealed that Muc^X possesses key genes necessary for human milk oligosaccharides (HMO) utilization, including α-L-fucosidases, β-galactosidases, exo-α-sialidases, and β-acetylhexosaminidases. The complete metabolic pathways for γ-aminobutyric acid and squalene and genes encoding functional proteins, such as the outer membrane protein Amuc_1100, were annotated in the Muc^X genome. Comparative genome analysis was used to identify functional genes unique to Muc^X compared to six other A. muciniphila strains. Results showed Muc^X genome possesses unique genes, including sugar transporters and transferases. Single-strain incubation revealed faster utilization of 2'-fucosyllactose (2'-FL), galacto-oligosaccharides, and lactose by Muc^X than by A. muciniphila DSM 22959. This study isolated and identified an A. muciniphila strain that can utilize 2'-FL, and expolored the genes related to HMO utilization and special metabolites, which provided a theoretical basis for the further excavation of A. muciniphila function and the compound application with fucosylated oligosaccharides.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Rapid screening of sugar-nucleotide donor specificities of putative glycosyltransferases

Determining the correct enzymatic activity of putative glycosyltransferases (GTs) can be challenging as these enzymes can utilize multiple donor and acceptor substrates. Upon initial determination of the donor-sugar nucleotide(s), a GT utilizes various acceptor molecules that can then be tested. Here, we describe a quick method to screen sugar-nucleotide donor specificities of GTs utilizing a sensitive, nonradioactive, commercially available bioluminescent uridine diphosphate detection kit. This in vitro method allowed us to validate the sugar-nucleotide donor-substrate specificities of recombinantly expressed human, bovine, bacterial and protozoan GTs. Our approach, which is less time consuming than many traditional assays that utilize radiolabeled sugars and chromatographic separations, should facilitate discovery of novel GTs that participate in diverse biological processes.

Unraveling the sequence of cytosolic reactions in the export of GspB adhesin from Streptococcus gordonii

Many pathogenic bacteria, including Streptococcus gordonii, possess a pathway for the cellular export of a single serine-rich-repeat protein that mediates the adhesion of bacteria to host cells and the extracellular matrix. This adhesin protein is O-glycosylated by several cytosolic glycosyltransferases and requires three accessory Sec proteins (Asp1-3) for export, but how the adhesin protein is processed for export is not well understood. Here, we report that the S. gordonii adhesin GspB is sequentially O-glycosylated by three enzymes (GtfA/B, Nss, and Gly) that attach N-acetylglucosamine and glucose to Ser/Thr residues. We also found that modified GspB is transferred from the last glycosyltransferase to the Asp1/2/3 complex. Crystal structures revealed that both Asp1 and Asp3 are related to carbohydrate-binding proteins, suggesting that they interact with carbohydrates and bind glycosylated adhesin, a notion that was supported by further analyses. We further observed that Asp1 also has an affinity for phospholipids, which is attenuated by Asp2. In summary, our findings support a model in which the GspB adhesin is sequentially glycosylated by GtfA/B, Nss, and Gly and then transferred to the Asp1/2/3 complex in which Asp1 mediates the interaction of the Asp1/2/3 complex with the lipid bilayer for targeting of matured GspB to the export machinery.

Bacterial size matters: Multiple mechanisms controlling septum cleavage and diplococcus formation are critical for the virulence of the opportunistic pathogen Enterococcus faecalis

Enterococcus faecalis is an opportunistic pathogen frequently isolated in clinical settings. This organism is intrinsically resistant to several clinically relevant antibiotics and can transfer resistance to other pathogens. Although E. faecalis has emerged as a major nosocomial pathogen, the mechanisms underlying the virulence of this organism remain elusive. We studied the regulation of daughter cell separation during growth and explored the impact of this process on pathogenesis. We demonstrate that the activity of the AtlA peptidoglycan hydrolase, an enzyme dedicated to septum cleavage, is controlled by several mechanisms, including glycosylation and recognition of the peptidoglycan substrate. We show that the long cell chains of E. faecalis mutants are more susceptible to phagocytosis and are no longer able to cause lethality in the zebrafish model of infection. Altogether, this work indicates that control of cell separation during division underpins the pathogenesis of E. faecalis infections and represents a novel enterococcal virulence factor. We propose that inhibition of septum cleavage during division represents an attractive therapeutic strategy to control infections.

Enterococcus faecalis is a commensal bacterium that colonizes the gastrointestinal tract of humans. This organism is an opportunistic pathogen that can cause a wide range of life-threatening infections in hospital settings. Despite the identification of several virulence factors, the mechanisms by which E. faecalis evades host immunity and causes infections remains poorly understood. Here, we explore how the formation of diplococci and short cell chains, a distinctive property of E. faecalis, contributes to pathogenesis. We describe several mechanisms that control the activity of AtlA, the enzyme dedicated to septum cleavage during division. Using a combination of in vitro assays and flow cytometry analyses of E. faecalis mutants, we show that AtlA activity is regulated by several mechanisms. We reveal that during pathogenesis, AtlA activity is critical for overcoming the host immune response. In the absence of AtlA, the long cell chains of E. faecalis mutants are more susceptible to phagocytosis and can no longer cause lethality in the zebrafish model of infection, thus indicating that control of cell chain length is a novel virulence factor in E. faecalis. This work highlights a link between cell division and pathogenesis and suggests that cell separation represents a step that can be targeted to control bacterial infections.

Engineering and Dissecting the Glycosylation Pathway of a Streptococcal Serine-rich Repeat Adhesin

Serine-rich repeat glycoproteins (SRRPs) are conserved in Gram-positive bacteria. They are crucial for modulating biofilm formation and bacterial-host interactions. Glycosylation of SRRPs plays a pivotal role in the process; thus understanding the glycosyltransferases involved is key to identifying new therapeutic drug targets. The glycosylation of Fap1, an SRRP of Streptococcus parasanguinis, is mediated by a gene cluster consisting of six genes: gtf1, gtf2, gly, gtf3, dGT1, and galT2. Mature Fap1 glycan possesses the sequence of Rha1–3Glc1-(Glc1–3GlcNAc1)-2,6-Glc1–6GlcNAc. Gtf12, Gtf3, and dGT1 are responsible for the first four steps of the Fap1 glycosylation, catalyzing the transfer of GlcNAc, Glc, Glc, and GlcNAc residues to the protein backbone sequentially. The role of GalT2 and Gly in the Fap1 glycosylation is unknown. In the present study, we synthesized the fully modified Fap1 glycan in Escherichia coli by incorporating all six genes from the cluster. This study represents the first reconstitution of an exogenous stepwise O-glycosylation synthetic pathway in E. coli. In addition, we have determined that GalT2 mediates the fifth step of the Fap1 glycosylation by adding a rhamnose residue, and Gly mediates the final glycosylation step by transferring glucosyl residues. Furthermore, inactivation of each glycosyltransferase gene resulted in differentially impaired biofilms of S. parasanguinis, demonstrating the importance of Fap1 glycosylation in the biofilm formation. The Fap1 glycosylation system offers an excellent model to engineer glycans using different permutations of glycosyltransferases and to investigate biosynthetic pathways of SRRPs because SRRP genetic loci are highly conserved.

Transcriptional organization of pneumococcal psrP-secY2A2 and impact of GtfA and GtfB deletion on PsrP-associated virulence properties

Pneumococcal serine-rich repeat protein (PsrP) is a glycoprotein that mediates Streptococcus pneumoniae attachment to lung cells and promotes biofilm formation. Herein, we investigated the transcriptional organization of psrP-secY2A2, the 37-kbp pathogenicity island encoding PsrP and its accessory genes. PCR amplification of cDNA and RNA-seq analysis found psrP-secY2A2 to be minimally composed of three operons: psrP-glyA, glyB, and glyC-asp5. Transcription of all three operons was greatest during biofilm growth and immunoblot analyses confirmed increased PsrP production by biofilm pneumococci. Using gas chromatography-mass spectrometry we identified monomeric N-acetylglucosamine as the primary glycoconjugate present on a recombinant intracellular version of PsrP, i.e. PsrP1-734. This finding was validated by immunoblot using lectins with known carbohydrate specificities. We subsequently deleted gtfA and gtfB, the GTFs thought to be responsible for addition of O-linked N-acetylglucosamine, and tested for PsrP and its associated virulence properties. These deletions negatively affected our ability to detect PsrP1-734 in bacterial whole cell lysates. Moreover, S. pneumoniae mutants lacking these genes pheno-copied the psrP mutant and were attenuated for: biofilm formation, adhesion to lung epithelial cells, and pneumonia in mice. Our studies identify the transcriptional organization of psrP-secY2A2 and show the indispensable role of GtfA and GtfB on PsrP-mediated pneumococcal virulence.

Defining the enzymatic pathway for polymorphic O-glycosylation of the pneumococcal serine-rich repeat protein PsrP

Protein O-glycosylation is an important post-translational modification in all organisms, but deciphering the specific functions of these glycans is difficult due to their structural complexity. Understanding the glycosylation of mucin-like proteins presents a particular challenge as they are modified numerous times with both the enzymes involved and the glycosylation patterns being poorly understood. Here we systematically explored the O-glycosylation pathway of a mucin-like serine-rich repeat protein PsrP from the human pathogen Streptococcus pneumoniae TIGR4. Previous works have assigned the function of 3 of the 10 glycosyltransferases thought to modify PsrP, GtfA/B, and Gtf3 as catalyzing the first two reactions to form a unified disaccharide core structure. We now use in vivo and in vitro glycosylation assays combined with hydrolytic activity assays to identify the glycosyltransferases capable of decorating this core structure in the third and fourth steps of glycosylation. Specifically, the full-length GlyE and GlyG proteins and the GlyD DUF1792 domain participate in both steps, whereas full-length GlyA and the GlyD GT8 domain catalyze only the fourth step. Incorporation of different sugars to the disaccharide core structure at multiple sites along the serine-rich repeats results in a highly polymorphic product. Furthermore, crystal structures of apo- and UDP-complexed GlyE combined with structural analyses reveal a novel Rossmann-fold "add-on" domain that we speculate to function as a universal module shared by GlyD, GlyE, and GlyA to forward the peptide acceptor from one enzyme to another. These findings define the complete glycosylation pathway of a bacterial glycoprotein and offer a testable hypothesis of how glycosyltransferase coordination facilitates glycan assembly.

Insights into bacterial protein glycosylation in human microbiota

The study of human microbiota is an emerging research topic. The past efforts have mainly centered on studying the composition and genomic landscape of bacterial species within the targeted communities. The interaction between bacteria and hosts is the pivotal event in the initiation and progression of infectious diseases. There is a great need to identify and characterize the molecules that mediate the bacteria-host interaction. Bacterial surface exposed proteins play an important role in the bacteria- host interaction. Numerous surface proteins are glycosylated, and the glycosylation is crucial for their function in mediating the bacterial interaction with hosts. Here we present an overview of surface glycoproteins from bacteria that inhabit three major mucosal environments across human body: oral, gut and skin. We describe the important enzymes involved in the process of protein glycosylation, and discuss how the process impacts the bacteria-host interaction. Emerging molecular details underlying glycosylation of bacterial surface proteins may lead to new opportunities for designing anti-infective small molecules, and developing novel vaccines in order to treat or prevent bacterial infection.

The sweet tooth of bacteria: common themes in bacterial glycoconjugates

Humans have been increasingly recognized as being superorganisms, living in close contact with a microbiota on all their mucosal surfaces. However, most studies on the human microbiota have focused on gaining comprehensive insights into the composition of the microbiota under different health conditions (e.g., enterotypes), while there is also a need for detailed knowledge of the different molecules that mediate interactions with the host. Glycoconjugates are an interesting class of molecules for detailed studies, as they form a strain-specific barcode on the surface of bacteria, mediating specific interactions with the host. Strikingly, most glycoconjugates are synthesized by similar biosynthesis mechanisms. Bacteria can produce their major glycoconjugates by using a sequential or an en bloc mechanism, with both mechanistic options coexisting in many species for different macromolecules. In this review, these common themes are conceptualized and illustrated for all major classes of known bacterial glycoconjugates, with a special focus on the rather recently emergent field of glycosylated proteins. We describe the biosynthesis and importance of glycoconjugates in both pathogenic and beneficial bacteria and in both Gram-positive and -negative organisms. The focus lies on microorganisms important for human physiology. In addition, the potential for a better knowledge of bacterial glycoconjugates in the emerging field of glycoengineering and other perspectives is discussed.

Glycosyltransferase-mediated Sweet Modification in Oral Streptococci

Bacterial glycosyltransferases play important roles in bacterial fitness and virulence. Oral streptococci have evolved diverse strategies to survive and thrive in the carbohydrate-rich oral cavity. In this review, we discuss 2 important biological processes mediated by 2 distinct groups of glycosyltransferases in oral streptococci that are important for bacterial colonization and virulence. The first process is the glycosylation of highly conserved serine-rich repeat adhesins by a series of glycosyltransferases. Using Streptococcus parasanguinis as a model, we highlight new features of several glycosyltransferases that sequentially modify the serine-rich glycoprotein Fap1. Distinct features of a novel glycosyltransferase fold from a domain of unknown function 1792 are contrasted with common properties of canonical glycosyltransferases. The second biological process we cover is involved in building sticky glucan matrix to establish cariogenic biofilms by an important opportunistic pathogen Streptococcus mutans through the action of a family of 3 glucosyltransferases. We focus on discussing the structural feature of this family as a glycoside hydrolase family of enzymes. While the 2 processes are distinct, they all produce carbohydrate-coated biomolecules, which enable bacteria to stick better in the complex oral microbiome. Understanding the making of the sweet modification presents a unique opportunity to develop novel antiadhesion and antibiofilm strategies to fight infections by oral streptococci and beyond.

Structure of a novel O-linked N-acetyl-D-glucosamine (O-GlcNAc) transferase, GtfA, reveals insights into the glycosylation of pneumococcal serine-rich repeat adhesins

Protein glycosylation catalyzed by the O-GlcNAc transferase (OGT) plays a critical role in various biological processes. In Streptococcus pneumoniae, the core enzyme GtfA and co-activator GtfB form an OGT complex to glycosylate the serine-rich repeat (SRR) of adhesin PsrP (pneumococcal serine-rich repeat protein), which is involved in the infection and pathogenesis. Here we report the 2.0 Å crystal structure of GtfA, revealing a β-meander add-on domain beyond the catalytic domain. It represents a novel add-on domain, which is distinct from the all-α-tetratricopeptide repeats in the only two structure-known OGTs. Structural analyses combined with binding assays indicate that this add-on domain contributes to forming an active GtfA-GtfB complex and recognizing the acceptor protein. In addition, the in vitro glycosylation system enables us to map the O-linkages to the serine residues within the first SRR of PsrP. These findings suggest that fusion with an add-on domain might be a universal mechanism for diverse OGTs that recognize varying acceptor proteins/peptides.

The highly conserved domain of unknown function 1792 has a distinct glycosyltransferase fold

More than 33,000 glycosyltransferases have been identified. Structural studies, however, have only revealed two distinct glycosyltransferase (GT) folds, GT-A and GT-B. Here we report a 1.34-Å resolution X-ray crystallographic structure of a previously uncharacterized 'domain of unknown function' 1792 (DUF1792) and show that the domain adopts a new fold and is required for glycosylation of a family of serine-rich repeat streptococcal adhesins. Biochemical studies reveal that the domain is a glucosyltransferase, and it catalyses the transfer of glucose to the branch point of the hexasaccharide O-linked to the serine-rich repeat of the bacterial adhesin, Fap1 of Streptococcus parasanguinis. DUF1792 homologues from both Gram-positive and Gram-negative bacteria also exhibit the activity. Thus, DUF1792 represents a new family of glycosyltransferases; therefore, we designate it as a GT-D glycosyltransferase fold. As the domain is highly conserved in bacteria and not found in eukaryotes, it can be explored as a new antibacterial target.

O-Glycosylation of the N-terminal region of the serine-rich adhesin Srr1 of Streptococcus agalactiae explored by mass spectrometry

Serine-rich (Srr) proteins exposed at the surface of Gram-positive bacteria are a family of adhesins that contribute to the virulence of pathogenic staphylococci and streptococci. Lectin-binding experiments have previously shown that Srr proteins are heavily glycosylated. We report here the first mass-spectrometry analysis of the glycosylation of Streptococcus agalactiae Srr1. After Srr1 enrichment and trypsin digestion, potential glycopeptides were identified in collision induced dissociation spectra using X! Tandem. The approach was then refined using higher energy collisional dissociation fragmentation which led to the simultaneous loss of sugar residues, production of diagnostic oxonium ions and backbone fragmentation for glycopeptides. This feature was exploited in a new open source software tool (SpectrumFinder) developed for this work. By combining these approaches, 27 glycopeptides corresponding to six different segments of the N-terminal region of Srr1 [93-639] were identified. Our data unambiguously indicate that the same protein residue can be modified with different glycan combinations including N-acetylhexosamine, hexose, and a novel modification that was identified as O-acetylated-N-acetylhexosamine. Lectin binding and monosaccharide composition analysis strongly suggested that HexNAc and Hex correspond to N-acetylglucosamine and glucose, respectively. The same protein segment can be modified with a variety of glycans generating a wide structural diversity of Srr1. Electron transfer dissociation was used to assign glycosylation sites leading to the unambiguous identification of six serines and one threonine residues. Analysis of purified Srr1 produced in mutant strains lacking accessory glycosyltransferase encoding genes demonstrates that O-GlcNAcylation is an initial step in Srr1 glycosylation that is likely required for subsequent decoration with Hex. In summary, our data obtained by a combination of fragmentation mass spectrometry techniques associated to a new software tool, demonstrate glycosylation heterogeneity of Srr1, characterize a new protein modification, and identify six glycosylation sites located in the N-terminal region of the protein.

Structural and biochemical analysis of a bacterial glycosyltransferase

Glycosyltransferases (GTs) are a large family of enzymes that specifically transfer sugar moieties to a diverse range of substrates. The process of bacterial glycosylation (such as biosynthesis of glycolipids, glycoproteins, and polysaccharides) has been studied extensively, yet the majority of GTs involved remains poorly characterized. Besides predicting enzymatic parameters of GTs, the resolution of three-dimensional structures of GTs can help to determine activity, donor sugar binding, and acceptor substrate binding sites. It also facilitates amino acid sequence-based structural modeling and biochemical characterization of their homologues. Here we describe a general procedure to accomplish expression and purification of soluble and active recombinant GTs. Enzymatic characterization, and crystallization of GTs, and data refinement for structural analysis are also covered in this protocol.

Insight into the composition of the intercellular matrix of Streptococcus pneumoniae biofilms

Biofilm matrices consist of a mixture of extracellular polymeric substances synthesized in large part by the biofilm-producing microorganisms themselves. These matrices are responsible for the cohesion and three-dimensional architecture of biofilms. The present study demonstrates the existence of a matrix composed of extracellular DNA, proteins and polysaccharides in the biofilm formed by the human pathogen Streptococcus pneumoniae. Extracellular DNA, visualized by fluorescent labelling, was an important component of this matrix. The existence of DNA-protein complexes associated with bacterial aggregates and other polymers was hypothesized based on the unexpected DNA binding activity of lysozyme LytC, a novel moonlighting protein. Actually, a 25-amino-acid-long peptide derived from LytC (positions 408 and 432 of the mature LytC) was also capable of efficiently binding to DNA. Moreover, the presence of intercellular DNA-LytC protein complexes in pneumococcal biofilms was demonstrated by confocal laser scanning microscopy. Evidence of extracellular polysaccharide different from the capsule was obtained by staining with Calcofluor dye and four types of lectin conjugated to Alexa fluorophores, and by incubation with glycoside hydrolases. The presence of residues of Glcp(1→4) and GlcNAc(1→4) (in its deacetylated form) in the pneumococcal biofilm was confirmed by GC-MS techniques.

Gap1 functions as a molecular chaperone to stabilize its interactive partner Gap3 during biogenesis of serine-rich repeat bacterial adhesin

Serine-rich repeat glycoproteins (SRRPs) are important bacterial adhesins that are conserved in streptococci and staphylococci. Fimbriae-associated protein (Fap1) from Streptococcus parasanguinis, was the first SRRP identified; it plays an important role in bacterial biofilm formation. A gene cluster encoding glycosyltransferases and accessory secretion components is required for Fap1 biogenesis. Two glycosylation-associated proteins, Gap1 and Gap3 within the cluster, interact with each other and function in concert in Fap1 biogenesis. Here we report the new molecular events underlying contribution of the interaction to Fap1 biogenesis. The Gap1-deficient mutant rendered Gap3 unstable and degraded in vitro and in vivo. Inactivation of a gene encoding protease ClpP reversed the phenotype of the gap1 mutant, suggesting that ClpP is responsible for degradation of Gap3. Molecular chaperone GroEL was co-purified with Gap3 only when Gap1 was absent and also reacted with Gap1 monoclonal antibody, suggesting that Gap1 functions as a specific chaperone for Gap3. The N-terminal interacting domains of Gap1 mediated the Gap3 stability and Fap1 biogenesis. Gap1 homologues from Streptococcus agalactiae and Staphylococcus aureus also interacted with and stabilized corresponding Gap3 homologues, suggesting that the chaperone activity of the Gap1 homologues is common in biogenesis of SRRPs.

A molecular chaperone mediates a two-protein enzyme complex and glycosylation of serine-rich streptococcal adhesins

Serine-rich repeat glycoproteins identified from streptococci and staphylococci are important for bacterial adhesion and biofilm formation. Two putative glycosyltransferases, Gtf1 and Gtf2, from Streptococcus parasanguinis form a two-protein enzyme complex that is required for glycosylation of a serine-rich repeat adhesin, Fap1. Gtf1 is a glycosyltransferase; however, the function of Gtf2 is unknown. Here, we demonstrate that Gtf2 enhances the enzymatic activity of Gtf1 by its chaperone-like property. Gtf2 interacted with Gtf1, mediated the subcellular localization of Gtf1, and stabilized Gtf1. Deletion of invariable amino acid residues in a conserved domain of unknown function (DUF1975) at the N terminus of Gtf2 had a greater impact on Fap1 glycosylation than deletion of the C-terminal non-DUF1975 residues. The DUF1975 deletions concurrently reduced the interaction between Gtf1 and Gtf2, altered the subcellular localization of Gtf1, and destabilized Gtf1, suggesting that DUF1975 is crucial for the chaperone activity of Gtf2. Homologous GtfA and GtfB from Streptococcus agalactiae rescued the glycosylation defect in the gtf1gtf2 mutant; like Gtf2, GtfB also possesses chaperone-like activity. Taken together, our studies suggest that Gtf2 and its homologs possess the conserved molecular chaperone activity that mediates protein glycosylation of bacterial adhesins.

Similarities and differences in the glycosylation mechanisms in prokaryotes and eukaryotes

Recent years have witnessed a rapid growth in the number and diversity of prokaryotic proteins shown to carry N- and/or O-glycans, with protein glycosylation now considered as fundamental to the biology of these organisms as it is in eukaryotic systems. This article overviews the major glycosylation pathways that are known to exist in eukarya, bacteria and archaea. These are (i) oligosaccharyltransferase (OST)-mediated N-glycosylation which is abundant in eukarya and archaea, but is restricted to a limited range of bacteria; (ii) stepwise cytoplasmic N-glycosylation that has so far only been confirmed in the bacterial domain; (iii) OST-mediated O-glycosylation which appears to be characteristic of bacteria; and (iv) stepwise O-glycosylation which is common in eukarya and bacteria. A key aim of the review is to integrate information from the three domains of life in order to highlight commonalities in glycosylation processes. We show how the OST-mediated N- and O-glycosylation pathways share cytoplasmic assembly of lipid-linked oligosaccharides, flipping across the ER/periplasmic/cytoplasmic membranes, and transferring “en bloc” to the protein acceptor. Moreover these hallmarks are mirrored in lipopolysaccharide biosynthesis. Like in eukaryotes, stepwise O-glycosylation occurs on diverse bacterial proteins including flagellins, adhesins, autotransporters and lipoproteins, with O-glycosylation chain extension often coupled with secretory mechanisms.