Glycopeptide N-acetylgalactosaminyltransferase specificities for O-glycosylated sites on MUC5AC mucin motif peptides

Targeting host O-linked glycan biosynthesis affects Ebola virus replication efficiency and reveals differential GalNAc-T acceptor site preferences on the Ebola virus glycoprotein

Ebola virus glycoprotein (EBOV GP) is one of the most heavily O-glycosylated viral glycoproteins, yet we still lack a fundamental understanding of the structure of its large O-glycosylated mucin-like domain and to what degree the host O-glycosylation capacity influences EBOV replication. Using tandem mass spectrometry, we identified 47 O-glycosites on EBOV GP and found similar glycosylation signatures on virus-like particle- and cell lysate-derived GP. Furthermore, we performed quantitative differential O-glycoproteomics on proteins produced in wild-type HEK293 cells and cell lines ablated for the three key initiators of O-linked glycosylation, GalNAc-T1, -T2, and -T3. The data show that 12 out of the 47 O-glycosylated sites were regulated, predominantly by GalNAc-T1. Using the glycoengineered cell lines for authentic EBOV propagation, we demonstrate the importance of O-linked glycan initiation and elongation for the production of viral particles and the titers of progeny virus. The mapped O-glycan positions and structures allowed to generate molecular dynamics simulations probing the largely unknown spatial arrangements of the mucin-like domain. The data highlight targeting GALNT1 or C1GALT1C1 as a possible way to modulate O-glycan density on EBOV GP for novel vaccine designs and tailored intervention approaches.IMPORTANCEEbola virus glycoprotein acquires its extensive glycan shield in the host cell, where it is decorated with N-linked glycans and mucin-type O-linked glycans. The latter is initiated by a family of polypeptide GalNAc-transferases that have different preferences for optimal peptide substrates resulting in a spectrum of both very selective and redundant substrates for each isoform. In this work, we map the exact locations of O-glycans on Ebola virus glycoprotein and identify subsets of sites preferentially initiated by one of the three key isoforms of GalNAc-Ts, demonstrating that each enzyme contributes to the glycan shield integrity. We further show that altering host O-glycosylation capacity has detrimental effects on Ebola virus replication, with both isoform-specific initiation and elongation playing a role. The combined structural and functional data highlight glycoengineered cell lines as useful tools for investigating molecular mechanisms imposed by specific glycans and for steering the immune responses in future vaccine designs.

High level in vivo mucin-type glycosylation in Escherichia coli

BACKGROUND

Increasing efforts have been made to assess the potential of Escherichia coli strains for the production of complex recombinant proteins. Since a considerable part of therapeutic proteins are glycoproteins, the lack of the post-translational attachment of sugar moieties in standard E. coli expression strains represents a major caveat, thus limiting the use of E. coli based cell factories. The establishment of an E. coli expression system capable of protein glycosylation could potentially facilitate the production of therapeutics with a putative concomitant reduction of production costs.

RESULTS

The previously established E. coli strain expressing the soluble form of the functional human-derived glycosyltransferase polypeptide N-acetylgalactosaminyltransferase 2 (GalNAc-T2) was further modified by co-expressing the UDP-GlcNAc 4-epimerase WbgU derived from Plesiomonas shigelloides. This enables the conversion of uridine 5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) to the sugar donor uridine 5'-diphospho-N-acetylgalactosamine (UDP-GalNAc) in the bacterial cytoplasm. Initially, the codon-optimised gene wbgU was inserted into a pET-derived vector and a Tobacco Etch Virus (TEV) protease cleavable polyhistidine-tag was translationally fused to the C- terminus of the amino acid sequence. The 4-epimerase was subsequently expressed and purified. Following the removal of the polyhistidine-tag, WbgU was analysed by circular dichroism spectroscopy to determine folding state and thermal transitions of the protein. The in vitro activity of WbgU was validated by employing a modified glycosyltransferase assay. The conversion of UDP-GlcNAc to UDP-GalNAc was shown by capillary electrophoresis analysis. Using a previously established chaperone pre-/co- expression platform, the in vivo activity of both glycosyltransferase GalNAc-T2 and 4-epimerase WbgU was assessed in E. coli, in combination with a mucin 10-derived target protein. Monitoring glycosylation by liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS), the results clearly indicated the in vivo glycosylation of the mucin-derived acceptor peptide.

CONCLUSION

In the present work, the previously established E. coli- based expression system was further optimized and the potential for in vivo O-glycosylation was shown by demonstrating the transfer of sugar moieties to a mucin-derived acceptor protein. The results offer the possibility to assess the practical use of the described expression platform for in vivo glycosylations of important biopharmaceutical compounds in E. coli.

UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases: completion of the family tree

The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). The human genome encodes 20 transferases; 17 of which have been characterized functionally. The complexity of the GalNAc-T family reflects the differential patterns of expression among the individual enzyme isoforms and the unique substrate specificities which are required to form the dense arrays of glycans that are essential for mucin function. We report the expression patterns and enzymatic activity of the remaining three members of the family and the further characterization of a recently reported isoform, GalNAc-T17. One isoform, GalNAcT-16 that is most homologous to GalNAc-T14, is widely expressed (abundantly in the heart) and has robust polypeptide transferase activity. The second isoform GalNAc-T18, most similar to GalNAc-T8, -T9 and -T19, completes a discrete subfamily of GalNAc-Ts. It is widely expressed and has low, albeit detectable, activity. The final isoform, GalNAc-T20, is most homologous to GalNAc-T11 but lacks a lectin domain and has no detectable transferase activity with the panel of substrates tested. We have also identified and characterized enzymatically active splice variants of GalNAc-T13 that differ in the sequence of their lectin domain. The variants differ in their affinities for glycopeptide substrates. Our findings provide a comprehensive view of the complexities of mucin-type O-glycan formation and provide insight into the underlying mechanisms employed to heavily decorate mucins and mucin-like domains with carbohydrate.

The catalytic and lectin domains of UDP-GalNAc:polypeptide alpha-N-Acetylgalactosaminyltransferase function in concert to direct glycosylation site selection

UDP-GalNAc:polypeptide alpha-N-Acetylgalactosaminyltransferases (ppGalNAcTs), a family (EC 2.4.1.41) of enzymes that initiate mucin-type O-glycosylation, are structurally composed of a catalytic domain and a lectin domain. Previous studies have suggested that the lectin domain modulates the glycosylation of glycopeptide substrates and may underlie the strict glycopeptide specificity of some isoforms (ppGalNAcT-7 and -10). Using a set of synthetic peptides and glycopeptides based upon the sequence of the mucin, MUC5AC, we have examined the activity and glycosylation site preference of lectin domain deletion and exchange constructs of the peptide/glycopeptide transferase ppGalNAcT-2 (hT2) and the glycopeptide transferase ppGalNAcT-10 (hT10). We demonstrate that the lectin domain of hT2 directs glycosylation site selection for glycopeptide substrates. Pre-steady-state kinetic measurements show that this effect is attributable to two mechanisms, either lectin domain-aided substrate binding or lectin domain-aided product release following glycosylation. We find that glycosylation of peptide substrates by hT10 requires binding of existing GalNAcs on the substrate to either its catalytic or lectin domain, thereby resulting in its apparent strict glycopeptide specificity. These results highlight the existence of two modes of site selection used by these ppGalNAcTs: local sequence recognition by the catalytic domain and the concerted recognition of distal sites of prior glycosylation together with local sequence binding mediated, respectively, by the lectin and catalytic domains. The latter mode may facilitate the glycosylation of serine or threonine residues, which occur in sequence contexts that would not be efficiently glycosylated by the catalytic domain alone. Local sequence recognition by the catalytic domain differs between hT2 and hT10 in that hT10 requires a pre-existing GalNAc residue while hT2 does not.

Recent insights into the biological roles of mucin-type O-glycosylation

In this special issue of the Glycoconjugate Journal focusing on glycosciences and development, we summarize recent advances in our understanding of the role of mucin-type O-glycans in development and disease. The presence of this widespread protein modification has been known for decades, yet identification of its biological functions has been hampered by the redundancy and complexity of the enzyme family controlling the initiation of O-glycosylation, as well as the diversity of extensions of the core sugar. Recent studies in organisms as diverse as mammals and Drosophila have yielded insights into the function of this highly abundant and evolutionarily-conserved protein modification. Gaining an understanding of mucin-type O-glycans in these diverse systems will elucidate crucial conserved processes underlying many aspects of development and homeostasis.

Interleukin-4 induces specific pp-GalNAc-T expression and alterations in mucin O-glycosylation in colonic epithelial cells

Mucus hypersecretion occurs as a consequence of the Th2 immune response in epithelia, yet it was not previously known whether the degree of O-glycosylation was modulated under such conditions. A colonic carcinoma cell line LS174T was used to assess the effect of interleukin (IL)-4 on the mRNA levels of eight pp-GalNAc-Ts. A three- to four-fold increase in pp-GalNAc-T1, T4, and T7 levels was observed. Lysates of untreated or IL-4-treated cells were examined for their ability to transfer GalNAc residues onto a peptide corresponding to the tandem repeat portion of human MUC2. The number of incorporated GalNAc residues was greater after incubation with lysates of IL-4-treated cells than with lysates of untreated cells. Mucin-like large glycoproteins secreted by IL-4-treated cells had higher binding capacity to PNA and VVA-B(4) than those secreted by untreated cells. The results indicated that IL-4-treated LS174T cells are able to produce mucins with a higher degree of O-glycosylation than untreated counterparts.

COUP-TFII mediates progesterone regulation of uterine implantation by controlling ER activity

Progesterone and estrogen are critical regulators of uterine receptivity. To facilitate uterine remodeling for embryo attachment, estrogen activity in the uterine epithelia is attenuated by progesterone; however, the molecular mechanism by which this occurs is poorly defined. COUP-TFII (chicken ovalbumin upstream promoter transcription factor II; also known as NR2F2), a member of the nuclear receptor superfamily, is highly expressed in the uterine stroma and its expression is regulated by the progesterone–Indian hedgehog–Patched signaling axis that emanates from the epithelium. To further assess COUP-TFII uterine function, a conditional COUP-TFII knockout mouse was generated. This mutant mouse is infertile due to implantation failure, in which both embryo attachment and uterine decidualization are impaired. Using this animal model, we have identified a novel genetic pathway in which BMP2 lies downstream of COUP-TFII. Epithelial progesterone-induced Indian hedgehog regulates stromal COUP-TFII, which in turn controls BMP2 to allow decidualization to manifest in vivo. Interestingly, enhanced epithelial estrogen activity, which impedes maturation of the receptive uterus, was clearly observed in the absence of stromal-derived COUP-TFII. This finding is consistent with the notion that progesterone exerts its control of implantation through uterine epithelial-stromal cross-talk and reveals that stromal-derived COUP-TFII is an essential mediator of this complex cross-communication pathway. This finding also provides a new signaling paradigm for steroid hormone regulation in female reproductive biology, with attendant implications for furthering our understanding of the molecular mechanisms that underlie dysregulation of hormonal signaling in such human reproductive disorders as endometriosis and endometrial cancer.

Pregnancy is established and maintained through a series of precisely choreographed cellular and molecular events that are controlled by two sex hormones, estrogen and progesterone. Both hormones exert their actions through their distinct nuclear receptors. During the peri-implantation period, estrogen activity is attenuated by progesterone to facilitate epithelial remodeling and embryo attachment, but the detailed molecular mechanism of how this process is achieved remains largely undefined. COUP-TFII (chicken ovalbumin upstream promoter transcription factor II; also known as NR2F2), a member of the nuclear receptor superfamily, is highly expressed in the uterine stroma, and its expression is controlled by progesterone–Indian hedgehog–Patched signaling from the epithelium to the stroma. To assess the uterine function of COUP-TFII, uterine-specific COUP-TFII knockout mice were generated. These mutant mice are infertile due to failure of implantation. We identified a novel genetic pathway in which the epithelial Ihh regulates the stroma COUP-TFII to control BMP2 and regulates decidualization. Interestingly, enhanced epithelial estrogen activity, which impedes the maturation of receptive uterus, was clearly noted in the absence of COUP-TFII. This finding reveals that COUP-TFII plays a critical role in maintaining the balance between estrogen and progesterone activities to establish proper implantation. This finding also provides new insights into women's health care associated with uncontrolled estrogen activity, such as breast cancer and endometriosis.

Structural basis of carbohydrate transfer activity by human UDP-GalNAc: polypeptide alpha-N-acetylgalactosaminyltransferase (pp-GalNAc-T10)

Mucin-type O-glycans are important carbohydrate chains involved in differentiation and malignant transformation. Biosynthesis of the O-glycan is initiated by the transfer of N-acetylgalactosamine (GalNAc) which is catalyzed by UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases (pp-GalNAc-Ts). Here we present crystal structures of the pp-GalNAc-T10 isozyme, which has specificity for glycosylated peptides, in complex with the hydrolyzed donor substrate UDP-GalNAc and in complex with GalNAc-serine. A structural comparison with uncomplexed pp-GalNAc-T1 suggests that substantial conformational changes occur in two loops near the catalytic center upon donor substrate binding, and that a distinct interdomain arrangement between the catalytic and lectin domains forms a narrow cleft for acceptor substrates. The distance between the catalytic center and the carbohydrate-binding site on the lectin beta sub-domain influences the position of GalNAc glycosylation on GalNAc-glycosylated peptide substrates. A chimeric enzyme in which the two domains of pp-GalNAc-T10 are connected by a linker from pp-GalNAc-T1 acquires activity toward non-glycosylated acceptors, identifying a potential mechanism for generating the various acceptor specificities in different isozymes to produce a wide range of O-glycans.

The beginnings of mucin biosynthesis: the crystal structure of UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferase-T1

UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases (ppGaNTases) initiate the formation of mucin-type, O-linked glycans by catalyzing the transfer of alpha-N-acetylgalactosamine from UDP-GalNAc to Ser or Thr residues of core proteins to form the Tn antigen (GalNAc-alpha-1-O-Ser/Thr). ppGaNTases are unique among glycosyltransferases in containing a C-terminal lectin domain. We present the x-ray crystal structure of a ppGaNTase, murine ppGaNTase-T1, and show that it folds to form distinct catalytic and lectin domains. The association of the two domains forms a large cleft in the surface of the enzyme that contains a Mn2+ ion complexed by invariant D209 and H211 of the "DXH" motif and by invariant H344. Each of the three potential lectin domain carbohydrate-binding sites (alpha, beta, and gamma) is located on the active-site face of the enzyme, suggesting a mechanism by which the transferase may accommodate multiple conformations of glycosylated acceptor substrates. A model of a mucin 1 glycopeptide substrate bound to the enzyme shows that the spatial separation between the lectin alpha site and a modeled active site UDP-GalNAc is consistent with the in vitro pattern of glycosylation observed for this peptide catalyzed by ppGaNTase-T1. The structure also provides a template for the larger ppGaNTase family, and homology models of several ppGaNTase isoforms predict dramatically different surface chemistries consistent with isoform-selective acceptor substrate recognition.

The expression of human FUT1 in HT-29/M3 colon cancer cells instructs the glycosylation of MUC1 and MUC5AC apomucins

Recently, we have reported that in normal gastric epithelium, the expression of gastric apomucins MUC5AC and MUC6 is associated with the specific expression of type 1 and type 2 Lewis antigens, and FUT2 and FUT1 fucosyltransferases, respectively. Until now, there are no data demonstrating the direct implication of specific glycosyltransferases in the specific patterns of apomucin glycosylation. HT29/M3 colon cancer cell line express MUC1, MUC5AC, type 1 Lewis antigens and FUT2 but not type 2 structures and FUT1, as it occurs in the epithelial cells of the gastric superficial epithelium. These cells were transfected with the cDNA of human FUT1, the alpha-1,2-fucosyltransferase responsible for the synthesis of type 2 Lewis antigens, to assess the implication of FUT1 in the glycosylation of MUC1 and MUC5AC. The M3-FUT1 clones obtained express high levels of type 2 Lewis antigens: H type 2 and Ley antigens. Immunoprecipitation of MUC1 and MUC5AC apomucins gives the direct evidence that FUT1 catalyses the addition of alpha-1,2-fucose to these apomucins, supporting the hypothesis that the pattern of apomucin glycosylation is not only instructed by the mucin primary sequence but also by the set of glycosyltransferases expressed in each specific cell type.

Distinct orders of GalNAc incorporation into a peptide with consecutive threonines

Mucin O-glycosylation is initiated by a transfer of N-acetyl-d-galactosamine (GalNAc) to Ser and Thr residues in polypeptides with a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (pp-GalNAc-Ts). In this paper, four human pp-GalNAc-Ts (pp-GalNAc-T1, T2, T3, and T4) were tested for their preferential orders of GalNAc incorporation into FITC-PTTTPITTTTK, a portion of the tandem repeat of human MUC2. The products were separated by reverse-phase HPLC and characterized by MALDI-TOF MS and peptide sequencing. pp-GalNAc-T1 showed preference for acceptor sites, but the order of the incorporation into these sites seemed to be random. In contrast, the GalNAc incorporation by pp-GalNAc-T2, T3, or T4 was not only site-specific but also according to the specific orders. Furthermore, pp-GalNAc-T2, T3, or T4 had distinct maximum numbers of GalNAc incorporations into this peptide.

Human airway mucin glycosylation: a combinatory of carbohydrate determinants which vary in cystic fibrosis

Human airway mucins represent a very broad family of polydisperse high molecular mass glycoproteins, which are part of the airway innate immunity. Apomucins, which correspond to their peptide part, are encoded by at least 6 different mucin genes (MUC1, MUC2, MUC4, MUC5B, MUC5AC and MUC7). The expression of some of these genes (at least MUC2 and MUC5AC) is induced by bacterial products, tobacco smoke and different cytokines. Human airway mucins are highly glycosylated (70-80% per weight). They contain from one single to several hundred carbohydrate chains. The carbohydrate chains that cover the apomucins are extremely diverse, adding to the complexity of these molecules. Structural information is available for more than 150 different O-glycan chains corresponding to the shortest chains (less than 12 sugars). The biosynthesis of these carbohydrate chains is a stepwise process involving many glycosyl- or sulfo-transferases. The only structural element shared by all mucin O-glycan chains is a GalNAc residue linked to a serine or threonine residue of the apomucin. There is growing evidence that the apomucin sequences influence the first glycosylation reactions. The elongation of the chains leads to various linear or branched extensions. Their non-reducing end, which corresponds to the termination of the chains, may bear different carbohydrate structures, such as histo-blood groups A or B determinants, H and sulfated H determinants, Lewis a, Lewis b, Lewis x or Lewis y epitopes, as well as sialyl- or sulfo- (sometimes sialyl- and sulfo-) Lewis a or Lewis x determinants. The synthesis of these different terminal determinants involves three different pathways with a whole set of glycosyl- and sulfo-transferases. Due to their wide structural diversity forming a combinatory of carbohydrate determinants as well as their location at the surface of the airways, mucins are involved in multiple interactions with microorganisms and are very important in the protection of the underlying airway mucosa. Airway mucins are oversulfated in cystic fibrosis and this feature has been considered as being linked to a primary defect of the disease. However, a similar pattern is observed in mucins from patients suffering from chronic bronchitis when they are severely infected. Airway mucins from severely infected patients suffering either from cystic fibrosis or from chronic bronchitis are also highly sialylated, and highly express sialylated and sulfated Lewis x determinants, a feature which may reflect severe mucosal inflammation or infection. These determinants are potential sites of attachment for Pseudomonas aeruginosa, the pathogen responsible for most of the morbidity and mortality in cystic fibrosis, and the expression of the sulfo- and glycosyl-transferases involved in their biosynthesis is increased by TNFalpha. In summary, airway inflammation may simultaneously induce the expression of mucin genes (MUC2 and MUC5AC) and the expression of several glycosyl- and sulfo-transferases, therefore modifying the combinatory glycosylation of these molecules.