Deconvoluting the functions of polypeptide N-alpha-acetylgalactosaminyltransferase family members by glycopeptide substrate profiling

An unusual dual sugar-binding lectin domain controls the substrate specificity of a mucin-type O-glycosyltransferase

N-acetylgalactosaminyl-transferases (GalNAc-Ts) initiate mucin-type O-glycosylation, an abundant and complex posttranslational modification that regulates host-microbe interactions, tissue development, and metabolism. GalNAc-Ts contain a lectin domain consisting of three homologous repeats (α, β, and γ), where α and β can potentially interact with O-GalNAc on substrates to enhance activity toward a nearby acceptor Thr/Ser. The ubiquitous isoenzyme GalNAc-T1 modulates heart development, immunity, and SARS-CoV-2 infectivity, but its substrates are largely unknown. Here, we show that both α and β in GalNAc-T1 uniquely orchestrate the O-glycosylation of various glycopeptide substrates. The α repeat directs O-glycosylation to acceptor sites carboxyl-terminal to an existing GalNAc, while the β repeat directs O-glycosylation to amino-terminal sites. In addition, GalNAc-T1 incorporates α and β into various substrate binding modes to cooperatively increase the specificity toward an acceptor site located between two existing O-glycans. Our studies highlight a unique mechanism by which dual lectin repeats expand substrate specificity and provide crucial information for identifying the biological substrates of GalNAc-T1.

Proteomics-based mass spectrometry profiling of SARS-CoV-2 infection from human nasopharyngeal samples

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the on-going global pandemic of coronavirus disease 2019 (COVID-19) that continues to pose a significant threat to public health worldwide. SARS-CoV-2 encodes four structural proteins namely membrane, nucleocapsid, spike, and envelope proteins that play essential roles in viral entry, fusion, and attachment to the host cell. Extensively glycosylated spike protein efficiently binds to the host angiotensin-converting enzyme 2 initiating viral entry and pathogenesis. Reverse transcriptase polymerase chain reaction on nasopharyngeal swab is the preferred method of sample collection and viral detection because it is a rapid, specific, and high-throughput technique. Alternate strategies such as proteomics and glycoproteomics-based mass spectrometry enable a more detailed and holistic view of the viral proteins and host-pathogen interactions and help in detection of potential disease markers. In this review, we highlight the use of mass spectrometry methods to profile the SARS-CoV-2 proteome from clinical nasopharyngeal swab samples. We also highlight the necessity for a comprehensive glycoproteomics mapping of SARS-CoV-2 from biological complex matrices to identify potential COVID-19 markers.

ISOGlyP: O-Glycosylation Site Prediction Using Peptide Sequences and GALNTs

Mucin-type O-glycosylation is one of the most common posttranslational modifications of proteins. The abnormal expression of various polypeptide GalNAc-transferases (GALNTs) which initiate and define sites of O-glycosylation is linked to many cancers and other diseases. Many current O-glycosylation prediction programs utilize O-glycoproteomics data obtained without using the transferase isoform(s) responsible for the glycosylation. With 20 different GALNTs in humans, having the ability to predict and interpret O-glycosylation sites in terms of specific GALNT isoforms is invaluable.To fill this gap, ISOGlyP (isoform-specific O-glycosylation prediction) has been developed. Using position-specific enhancement values generated based on GalNAc-T isoform-specific amino acid preferences, ISOGlyP predicts the propensity that a site would be glycosylated by a specific transferase. ISOGlyP gave an overall prediction accuracy of 70% against in vivo data, which is comparable to that of the NetOGlyc4.0 predictor. Additionally, ISOGlyP can identify the known effects of long- and short-range prior glycosylation and can generate potential peptide sequences selectively glycosylated by specific isoforms. ISOGlyP is freely available for use at https://ISOGlyP.utep.edu . The code is also available on GitHub ( https://github.com/jonmohl/ISOGlyP ).

Recent advances in the development of sialyltransferase inhibitors to control cancer metastasis: A comprehensive review

Metastasis accounts for the majority of cancer-associated mortalities, representing a huge health and economic burden. One of the mechanisms that enables metastasis is hypersialylation, characterized by an overabundance of sialylated glycans on the tumor surface, which leads to repulsion and detachment of cells from the original tumor. Once the tumor cells are mobilized, sialylated glycans hijack the natural killer T-cells through self-molecular mimicry and activatea downstream cascade of molecular events that result in inhibition of cytotoxicity and inflammatory responses against cancer cells, ultimately leading to immune evasion. Sialylation is mediated by a family of enzymes known as sialyltransferases (STs), which catalyse the transfer of sialic acid residue from the donor, CMP-sialic acid, onto the terminal end of an acceptor such as N-acetylgalactosamine on the cell-surface. Upregulation of STs increases tumor hypersialylation by up to 60% which is considered a distinctive hallmark of several types of cancers such as pancreatic, breast, and ovarian cancer. Therefore, inhibiting STs has emerged as a potential strategy to prevent metastasis. In this comprehensive review, we discuss the recent advances in designing novel sialyltransferase inhibitors using ligand-based drug design and high-throughput screening of natural and synthetic entities, emphasizing the most successful approaches. We analyse the limitations and challenges of designing selective, potent, and cell-permeable ST inhibitors that hindered further development of ST inhibitors into clinical trials. We conclude by analysing emerging opportunities, including advanced delivery methods which further increase the potential of these inhibitors to enrich the clinics with novel therapeutics to combat metastasis.

O-Linked Sialoglycans Modulate the Proteolysis of SARS-CoV-2 Spike and Likely Contribute to the Mutational Trajectory in Variants of Concern

The emergence of a polybasic cleavage motif for the protease furin in SARS-CoV-2 spike has been established as a major factor for human viral transmission. The region N-terminal to that motif is extensively mutated in variants of concern (VOCs). Besides furin, spikes from these variants appear to rely on other proteases for maturation, including TMPRSS2. Glycans near the cleavage site have raised questions about proteolytic processing and the consequences of variant-borne mutations. Here, we identify that sialic acid-containing O-linked glycans on Thr678 of SARS-CoV-2 spike influence furin and TMPRSS2 cleavage and posit O-linked glycosylation as a likely driving force for the emergence of VOC mutations. We provide direct evidence that the glycosyltransferase GalNAc-T1 primes glycosylation at Thr678 in the living cell, an event that is suppressed by mutations in the VOCs Alpha, Delta, and Omicron. We found that the sole incorporation of N-acetylgalactosamine did not impact furin activity in synthetic O-glycopeptides, but the presence of sialic acid reduced the furin rate by up to 65%. Similarly, O-glycosylation with a sialylated trisaccharide had a negative impact on TMPRSS2 cleavage. With a chemistry-centered approach, we substantiate O-glycosylation as a major determinant of spike maturation and propose disruption of O-glycosylation as a substantial driving force for VOC evolution.

Recombinant mucin biotechnology and engineering

Mucins represent a largely untapped class of polymeric building block for biomaterials, therapeutics, and other biotechnology. Because the mucin polymer backbone is genetically encoded, sequence-specific mucins with defined physical and biochemical properties can be fabricated using recombinant technologies. The pendent O-glycans of mucins are increasingly implicated in immunomodulation, suppression of pathogen virulence, and other biochemical activities. Recent advances in engineered cell production systems are enabling the scalable synthesis of recombinant mucins with precisely tuned glycan side chains, offering exciting possibilities to tune the biological functionality of mucin-based products. New metabolic and chemoenzymatic strategies enable further tuning and functionalization of mucin O-glycans, opening new possibilities to expand the chemical diversity and functionality of mucin building blocks. In this review, we discuss these advances, and the opportunities for engineered mucins in biomedical applications ranging from in vitro models to therapeutics.

Cell-specific bioorthogonal tagging of glycoproteins

Altered glycoprotein expression is an undisputed corollary of cancer development. Understanding these alterations is paramount but hampered by limitations underlying cellular model systems. For instance, the intricate interactions between tumour and host cannot be adequately recapitulated in monoculture of tumour-derived cell lines. More complex co-culture models usually rely on sorting procedures for proteome analyses and rarely capture the details of protein glycosylation. Here, we report a strategy termed Bio-Orthogonal Cell line-specific Tagging of Glycoproteins (BOCTAG). Cells are equipped by transfection with an artificial biosynthetic pathway that transforms bioorthogonally tagged sugars into the corresponding nucleotide-sugars. Only transfected cells incorporate bioorthogonal tags into glycoproteins in the presence of non-transfected cells. We employ BOCTAG as an imaging technique and to annotate cell-specific glycosylation sites in mass spectrometry-glycoproteomics. We demonstrate application in co-culture and mouse models, allowing for profiling of the glycoproteome as an important modulator of cellular function.

Investigation of sequon engineering for improved O-glycosylation by the human polypeptide N-acetylgalactosaminyl transferase T2 isozyme and two orthologues

We have been developing bacterial expression systems for human mucin-type O-glycosylation on therapeutic proteins, which is initiated by the addition of α-linked GalNAc to serine or threonine residues by enzymes in the GT-27 family of glycosyltransferases. Substrate preference across different isoforms of this enzyme is influenced by isoform-specific amino acid sequences at the site of glycosylation, which we have exploited to engineer production of Core 1 glycan structures in bacteria on human therapeutic proteins. Using RP-HPLC with a novel phenyl bonded phase to resolve intact protein glycoforms, the effect of sequon mutation on O-glycosylation initiation was examined through in vitro modification of the naturally O-glycosylated human interferon α-2b, and a sequon engineered human growth hormone. As part of the development of our glycan engineering in the bacterial expression system we are surveying various orthologues of critical enzymes to ensure complete glycosylation. Here we present an in vitro enzyme kinetic profile of three related GT-27 orthologues on natural and engineered sequons in recombinant human interferon α2b and human growth hormone where we show a significant change in kinetic properties with the amino acid changes. It was found that optimizing the protein substrate amino acid sequence using Isoform Specific O-Glycosylation Prediction (ISOGlyP, http://isoglyp.utep.edu/index.php) resulted in a measurable increase in kcat/KM, thus improving glycosylation efficiency. We showed that the Drosophila orthologue showed superior activity with our human growth hormone designed sequons compared with the human enzyme.

GALNT3 suppresses lung cancer by inhibiting myeloid-derived suppressor cell infiltration and angiogenesis in a TNFR and c-MET pathway-dependent manner

The deregulation of polypeptide N-acetyl-galactosaminyltransferases (GALNTs) contributes to several cancers, but their roles in lung cancer remain unclear. In this study, we have identified a tumor-suppressing role of GALNT3 in lung cancer. We found that GALNT3 suppressed lung cancer development and progression in both xenograft and syngeneic mouse models. Specifically, GALNT3 suppressed lung cancer initiation by inhibiting the self-renewal of lung cancer cells. More importantly, GALNT3 attenuated lung cancer growth by preventing the creation of a favorable tumor microenvironment (TME), which was attributed to GALNT3's ability to inhibit myeloid-derived suppressor cell (MDSC) infiltration into tumor sites and subsequent angiogenesis. We also identified a GALNT3-regulated gene (GRG) signature and found that lung cancer patients whose tumors exhibit the GRG signature showed more favorable prognoses. Further investigation revealed that GALNT3 suppressed lung cancer cell self-renewal by reducing β-catenin levels, which led to reduced expression of the downstream targets of the WNT pathway. In addition, GALNT3 inhibited MDSC infiltration into tumor sites by suppressing both the TNFR1-NFκB and cMET-pAKT pathways. Specifically, GALNT3 inhibited the nuclear localization of NFκB and the c-MET-induced phosphorylation of AKT. This then led to reduced production of CXCL1, a chemokine required for MDSC recruitment. Finally, we confirmed that the GALNT3-induced inhibition of the TNFR1-NFκB and cMET-pAKT pathways involved the O-GalNAcylation of the TNFR1 and cMET receptors. In summary, we have identified GALNT3 as the first GALNT member capable of suppressing lung cancer and uncovered a novel mechanism by which GALNT3 regulates the TME.

Impact of Expressing Cells on Glycosylation and Glycan of the SARS-CoV-2 Spike Glycoprotein

The spike glycoprotein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the first point of contact for the virus to recognize and bind to host receptors, is the focus of biomedical research seeking to effectively prevent and treat coronavirus disease (COVID-19). The mass production of spike glycoproteins is usually carried out in different cell systems. Studies have been shown that different expression cell systems alter protein glycosylation of hemagglutinin and neuraminidase in the influenza virus. However, it is not clear whether the cellular system affects the spike protein glycosylation. In this work, we investigated the effect of an expression system on the glycosylation of the spike glycoprotein and its receptor-binding domain. We found that there are significant differences in the glycosylation and glycans attached at each glycosite of the spike glycoprotein obtained from different expression cells. Since glycosylation at the binding site and adjacent amino acids affects the interaction between the spike glycoprotein and the host cell receptor, we recognize that caution should be taken when selecting an expression system to develop inhibitors, antibodies, and vaccines.

Single-pot enzymatic synthesis of cancer-associated MUC16 O-glycopeptide libraries and multivalent protein glycoconjugates: a step towards cancer glycovaccines

Glycosyltransferases and nucleotide sugars are combined in single-pot settings to synthesize a library of cancer-associated MUC16 O-glycopeptides and multivalent protein glycoconjugates foreseeing future development of cancer glycovaccines.

ppGalNAc-T4-catalyzed O-Glycosylation of TGF-β type Ⅱ receptor regulates breast cancer cells metastasis potential

GalNAc-type O-glycosylation, initially catalyzed by polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts), is one of the most abundant and complex posttranslational modifications of proteins. Emerging evidence has proven that aberrant ppGalNAc-Ts are involved in malignant tumor transformation. However, the exact molecular functions of ppGalNAc-Ts are still unclear. Here, the role of one isoform, ppGalNAc-T4, in breast cancer cell lines was investigated. The expression of ppGalNAc-T4 was found to be negatively associated with migration of breast cancer cells. Loss-of-function studies revealed that ppGalNAc-T4 attenuated the migration and invasion of breast cancer cells by inhibiting the epithelial-mesenchymal transition (EMT) process. Correspondingly, transforming growth factor beta (TGF-β) signaling, which is the upstream pathway of EMT, was impaired by ppGalNAc-T4 expression. ppGalNAc-T4 knockout decreased O-GalNAc modification of TGF-β type Ⅰ and Ⅱ receptor (TβR Ⅰ and Ⅱ) and led to the elevation of TGF-β receptor dimerization and activity. Importantly, a peptide from TβR Ⅱ was identified as a naked peptide substrate of ppGalNAc-T4 with a higher affinity than ppGalNAc-T2. Further, Ser31, corresponding to the extracellular domain of TβR Ⅱ, was identified as the O-GalNAcylation site upon in vitro glycosylation by ppGalNAc-T4. The O-GalNAc-deficient S31 A mutation enhanced TGF-β signaling activity and EMT in breast cancer cells. Together, these results identified a novel mechanism of ppGalNAc-T4-catalyzed TGF-β receptors O-GalNAcylation that suppresses breast cancer cell migration and invasion via the EMT process. Targeting ppGalNAc-T4 may be a potential therapeutic strategy for breast cancer treatment.

Quantitative assessment of successive carbohydrate additions to the clustered O-glycosylation sites of IgA1 by glycosyltransferases

Mucin-type O-glycosylation occurs on many proteins that transit the Golgi apparatus. These glycans impact structure and function of many proteins and have important roles in cellular biosynthetic processes, signaling and differentiation. Although recent technological advances have enhanced our ability to profile glycosylation of glycoproteins, limitations in the understanding of the biosynthesis of these glycan structures remain. Some of these limitations stem from the difficulty to track the biosynthetic process of mucin-type O-glycosylation, especially when glycans occur in dense clusters in repeat regions of proteins, such as the mucins or immunoglobulin A1 (IgA1). Here, we describe a series of nano-liquid chromatography (LC)-mass spectrometry (MS) analyses that demonstrate the range of glycosyltransferase enzymatic activities involved in the biosynthesis of clustered O-glycans on IgA1. By utilizing nano-LC-MS relative quantitation of in vitro reaction products, our results provide unique insights into the biosynthesis of clustered IgA1 O-glycans. We have developed a workflow to determine glycoform-specific apparent rates of a human UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltrasnfersase (GalNAc-T EC 2.4.1.41) and demonstrated how pre-existing glycans affect subsequent activity of glycosyltransferases, such as core 1 galactosyltransferase and α2,3- and α2,6-specific sialyltransferases, in successive additions in the biosynthesis of clustered O-glycans. In the context of IgA1, these results have potential to provide insight into the molecular mechanisms implicated in the pathogenesis of IgA nephropathy, an autoimmune renal disease involving aberrant IgA1 O-glycosylation. In a broader sense, these methods and workflows are applicable to the studies of the concerted and competing functions of other glycosyltransferases that initiate and extend mucin-type core 1 clustered O-glycosylation.

Engineering Orthogonal Polypeptide GalNAc-Transferase and UDP-Sugar Pairs

O-Linked α-N-acetylgalactosamine (O-GalNAc) glycans constitute a major part of the human glycome. They are difficult to study because of the complex interplay of 20 distinct glycosyltransferase isoenzymes that initiate this form of glycosylation, the polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). Despite proven disease relevance, correlating the activity of individual GalNAc-Ts with biological function remains challenging due to a lack of tools to probe their substrate specificity in a complex biological environment. Here, we develop a "bump-hole" chemical reporter system for studying GalNAc-T activity in vitro. Individual GalNAc-Ts were rationally engineered to contain an enlarged active site (hole) and probed with a newly synthesized collection of 20 (bumped) uridine diphosphate N-acetylgalactosamine (UDP-GalNAc) analogs to identify enzyme-substrate pairs that retain peptide specificities but are otherwise completely orthogonal to native enzyme-substrate pairs. The approach was applicable to multiple GalNAc-T isoenzymes, including GalNAc-T1 and -T2 that prefer nonglycosylated peptide substrates and GalNAcT-10 that prefers a preglycosylated peptide substrate. A detailed investigation of enzyme kinetics and specificities revealed the robustness of the approach to faithfully report on GalNAc-T activity and paves the way for studying substrate specificities in living systems.

Evaluation of GALNT16 polymorphisms to breast cancer risk in Chinese population

BACKGROUND

Polypeptide N-acetylgalactosaminyltransferase 16 (GALNT16) is an N-acetylgalactosaminyltransferase gene that alters protein O-glycosylation, which plays a role in tumor development. This study aims to explore the association of eight GALNT16 polymorphisms with susceptibility to breast cancer (BC).

METHODS

This case-control study included 563 BC patients and 552 age-matched healthy controls from the Chinese Han population. The genotypes of GALNT16 polymorphisms were detected using the Agena MassARRAY. The relationship between GALNT16 polymorphisms and BC risk was evaluated using a chi-squared test with an odds ratio (OR) and 95% confidence intervals (CI) under five genetic models.

RESULTS

We observed that rs2105269 and rs72625676 were associated with higher BC risk in younger patients with age ≤51 (rs2105269, codominant: p = .006; recessive: p = .005 additive: p = .018; and allele: p = .012; rs72625676, codominant: p = .038; recessive: p = .037). For rs1275678 polymorphism, there was a significantly decreased risk of BC among elder patients (codominant: p = .017; dominant: p = .019; additive: p = .030; and allele: p = .029). Further analysis by clinical characteristics showed rs2105269 was associated with tumor size and lymph node metastasis.

CONCLUSION

Our study suggests that GALNT16 polymorphisms are associated with BC susceptibility in Chinese population.

IgA1 hinge-region clustered glycan fidelity is established early during semi-ordered glycosylation by GalNAc-T2

GalNAc-type O-glycans are often added to proteins post-translationally in a clustered manner in repeat regions of proteins, such as mucins and IgA1. Observed IgA1 glycosylation patterns show that glycans occur at similar sites with similar structures. It is not clear how the sites and number of glycans added to IgA1, or other proteins, can follow a conservative process. GalNAc-transferases initiate GalNAc-type glycosylation. In IgA nephropathy, an autoimmune disease, the sites and O-glycan structures of IgA1 hinge-region are altered, giving rise to a glycan autoantigen. To better understand how GalNAc-transferases determine sites and densities of clustered O-glycans, we used IgA1 hinge-region (HR) segment as a probe. Using LC-MS, we demonstrated a semi-ordered process of glycosylation by GalNAc-T2 towards the IgA1 HR. The catalytic domain was responsible for selection of four initial sites based on amino-acid sequence recognition. Both catalytic and lectin domains were involved in multiple second site-selections, each dependent on initial site-selection. Our data demonstrated that multiple start-sites and follow-up pathways were key to increasing the number of glycans added. The lectin domain predominately enhanced IgA1 HR glycan density by increasing synthesis pathway exploration by GalNAc-T2. Our data indicated a link between site-specific glycan addition and clustered glycan density that defines a mechanism of how conserved clustered O-glycosylation patterns and glycoform populations of IgA1 can be controlled by GalNAc-T2. Together, these findings characterized a correlation between glycosylation pathway diversity and glycosylation density, revealing mechanisms by which a single GalNAc-T isozyme can limit and define glycan heterogeneity in a disease-relevant context.

Polypeptide GalNAc-Ts: from redundancy to specificity

Mucin-type O-glycosylation is a post-translational modification (PTM) that is predicted to occur in more than the 80% of the proteins that pass through the Golgi apparatus. This PTM is initiated by a family of polypeptide GalNAc-transferases (GalNAc-Ts) that modify Ser and Thr residues of proteins through the addition of a GalNAc moiety. These enzymes are type II membrane proteins that consist of a Golgi luminal catalytic domain connected by a flexible linker to a ricin type lectin domain. Together, both domains account for the different glycosylation preferences observed among isoenzymes. Although it is well accepted that most of the family members share some degree of redundancy toward their protein and glycoprotein substrates, it has been recently found that several GalNAc-Ts also possess activity toward specific targets. Despite the high similarity between isoenzymes, structural differences have recently been reported that are key to understanding the molecular basis of both their redundancy and specificity. The present review focuses on the molecular aspects of the protein substrate recognition and the different glycosylation preferences of these enzymes, which in turn will serve as a roadmap to the rational design of specific modulators of mucin-type O-glycosylation.

Imaging specific cellular glycan structures using glycosyltransferases via click chemistry

Heparan sulfate (HS) is a polysaccharide fundamentally important for biologically activities. T/Tn antigens are universal carbohydrate cancer markers. Here, we report the specific imaging of these carbohydrates using a mesenchymal stem cell line and human umbilical vein endothelial cells (HUVEC). The staining specificities were demonstrated by comparing imaging of different glycans and validated by either removal of target glycans, which results in loss of signal, or installation of target glycans, which results in gain of signal. As controls, representative key glycans including O-GlcNAc, lactosaminyl glycans and hyaluronan were also imaged. HS staining revealed novel architectural features of the extracellular matrix (ECM) of HUVEC cells. Results from T/Tn antigen staining suggest that O-GalNAcylation is a rate-limiting step for O-glycan synthesis. Overall, these highly specific approaches for HS and T/Tn antigen imaging should greatly facilitate the detection and functional characterization of these biologically important glycans.

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.

Making Home Sweet and Sturdy: Toxoplasma gondii ppGalNAc-Ts Glycosylate in Hierarchical Order and Confer Cyst Wall Rigidity

The protozoan intracellular parasite Toxoplasma gondii forms latent cysts in the central nervous system (CNS) and persists for the lifetime of the host. This cyst is cloaked with a glycosylated structure called the cyst wall. Previously, we demonstrated that a mucin-like glycoprotein, CST1, localizes to the cyst wall and confers structural rigidity on brain cysts in a mucin-like domain-dependent manner. The mucin-like domain of CST1 is composed of 20 units of threonine-rich tandem repeats that are O-GalNAc glycosylated. A family of enzymes termed polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts) initiates O-GalNAc glycosylation. To identify which isoforms of ppGalNAc-Ts are responsible for the glycosylation of the CST1 mucin-like domain and to evaluate the function of each ppGalNAc-T in the overall glycosylation of the cyst wall, all five ppGalNAc-T isoforms were deleted individually from the T. gondii genome. The ppGalNAc-T2 and -T3 deletion mutants produced various glycosylation defects on the cyst wall, implying that many cyst wall glycoproteins are glycosylated by T2 and T3. Both T2 and T3 glycosylate the CST1 mucin-like domain, and this glycosylation is necessary for CST1 to confer structural rigidity on the cyst wall. We established that T2 is required for the initial glycosylation of the mucin-like domain and that T3 is responsible for the sequential glycosylation on neighboring acceptor sites, demonstrating hierarchical glycosylation by two distinct initiating and filling-in ppGalNAc-Ts in an intact organism.

IMPORTANCE

Toxoplasma gondii is an obligate intracellular parasite that infects a third of the world's population. It can cause severe congenital disease and devastating encephalitis in immunocompromised individuals. We identified two glycosyltransferases, ppGalNAc-T2 and -T3, which are responsible for glycosylating cyst wall proteins in a hierarchical fashion. This glycosylation confers structural rigidity on the brain cyst. Our studies provide new insights into the mechanisms of O-GalNAc glycosylation in T. gondii.

GALNT14 promotes lung-specific breast cancer metastasis by modulating self-renewal and interaction with the lung microenvironment

Some polypeptide N-acetyl-galactosaminyltransferases (GALNTs) are associated with cancer, but their function in organ-specific metastasis remains unclear. Here, we report that GALNT14 promotes breast cancer metastasis to the lung by enhancing the initiation of metastatic colonies as well as their subsequent growth into overt metastases. Our results suggest that GALNT14 augments the self-renewal properties of breast cancer cells (BCCs). Furthermore, GALNT14 overcomes the inhibitory effect of lung-derived bone morphogenetic proteins (BMPs) on self-renewal and therefore facilitates metastasis initiation within the lung microenvironment. In addition, GALNT14 supports continuous growth of BCCs in the lung by not only inducing macrophage infiltration but also exploiting macrophage-derived fibroblast growth factors (FGFs). Finally, we identify KRAS-PI3K-c-JUN signalling as an upstream pathway that accounts for the elevated expression of GALNT14 in lung-metastatic BCCs. Collectively, our findings uncover an unprecedented role for GALNT14 in the pulmonary metastasis of breast cancer and elucidate the underlying molecular mechanisms.

Effects of the multiple O-glycosylation states on antibody recognition of the immunodominant motif in MUC1 extracellular tandem repeats

The conformational impact of the clusteredO-glycans strongly influences recognition by antibodies of the cancer-relevant epitope in the MUC1 extracellular tandem repeat domain.

Mucin-type O-glycosylation is controlled by short- and long-range glycopeptide substrate recognition that varies among members of the polypeptide GalNAc transferase family

A large family of UDP-GalNAc:polypeptide GalNAc transferases (ppGalNAc-Ts) initiates and defines sites of mucin-type Ser/Thr-O-GalNAc glycosylation. Family members have been classified into peptide- and glycopeptide-preferring subfamilies, although both families possess variable activities against glycopeptide substrates. All but one isoform contains a C-terminal carbohydrate-binding lectin domain whose roles in modulating glycopeptide specificity is just being understood. We have previously shown for several peptide-preferring isoforms that the presence of a remote Thr-O-GalNAc, 6-17 residues from a Ser/Thr acceptor site, may enhance overall catalytic activity in an N- or C-terminal direction. This enhancement varies with isoform and is attributed to Thr-O-GalNAc interactions at the lectin domain. We now report on the glycopeptide substrate utilization of a series of glycopeptide (human-ppGalNAc-T4, T7, T10, T12 and fly PGANT7) and peptide-preferring transferases (T2, T3 and T5) by exploiting a series of random glycopeptide substrates designed to probe the functions of their catalytic and lectin domains. Glycosylation was observed at the -3, -1 and +1 residues relative to a neighboring Thr-O-GalNAc, depending on isoform, which we attribute to specific Thr-O-GalNAc binding at the catalytic domain. Additionally, these glycopeptide-preferring isoforms show remote lectin domain-assisted Thr-O-GalNAc enhancements that vary from modest to none. We conclude that the glycopeptide specificity of the glycopeptide-preferring isoforms predominantly resides in their catalytic domain but may be further modulated by remote lectin domain interactions. These studies further demonstrate that both domains of the ppGalNAc-Ts have specialized and unique functions that work in concert to control and order mucin-type O-glycosylation.

Biological and biochemical properties of two Xenopus laevis N-acetylgalactosaminyltransferases with contrasting roles in embryogenesis

The biosynthesis of mucin-type O-linked glycans in animals is initiated by members of the large family of polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts), which play important roles in embryogenesis, organogenesis, adult tissue homeostasis and carcinogenesis. Until now, the mammalian forms of these enzymes have been the best characterized. However, two N-acetylgalactosaminyltransferases (xGalNAc-T6 and xGalNAc-T16) from the African clawed frog (Xenopus laevis), which are most homologous to those encoded by the human GALNT6 and GALNT16 (GALNTL1) genes, were shown to have contrasting roles in TGF-β/BMP signaling in embryogenesis. In this study we have examined these two enzymes further and show differences in their in vivo function during X. laevis embyrogenesis as evidenced by in situ hybridization and overexpression experiments. In terms of enzymatic activity, both enzymes were found to be active towards the EA2 peptide, but display differential activity towards a peptide based on the sequence of ActR-IIB, a receptor relevant to TGF-β/BMP signaling. In summary, these data demonstrate that these two enzymes from different branches of the N-acetylgalactosaminyltransferase do not only display differential substrate specificities, but also specific and distinct expression pattern and biological activities in vivo.

Expression of the functional recombinant human glycosyltransferase GalNAcT2 in Escherichia coli

BACKGROUND

Recombinant protein-based therapeutics have become indispensable for the treatment of many diseases. They are produced using well-established expression systems based on bacteria, yeast, insect and mammalian cells. The majority of therapeutic proteins are glycoproteins and therefore the post-translational attachment of sugar residues is required. The development of an engineered Escherichia coli-based expression system for production of human glycoproteins could potentially lead to increased yields, as well as significant decreases in processing time and costs.

RESULTS

This work describes the expression of functional human-derived glycosyltransferase UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 2 (GalNAcT2) in a recombinant E. coli strain. For expression, a codon-optimised gene encoding amino acids 52-571 of GalNAcT2 lacking the transmembrane N-terminal domain was inserted into a pET-23 derived vector encoding a polyhistidine-tag which was translationally fused to the N-terminus of the glycosyltransferase (HisDapGalNAcT2). The glycosyltransferase was produced in E. coli using a recently published expression system. Soluble HisDapGalNAcT2 produced in SHuffle® T7 host cells was purified using nickel affinity chromatography and was subsequently analysed by size exclusion chromatography coupled to multi-angle light scattering (SEC-MALS) and circular dichroism spectroscopy to determine molecular mass, folding state and thermal transitions of the protein. The activity of purified HisDapGalNAcT2 was monitored using a colorimetric assay based on the release of phosphate during transfer of glycosyl residues to a model acceptor peptide or, alternatively, to the granulocyte-colony stimulating growth factor (G-CSF). Modifications were assessed by Matrix Assisted Laser Desorption Ionization Time-of-flight Mass Spectrometry analysis (MALDI-TOF-MS) and Electrospray Mass Spectrometry analysis (ESI-MS). The results clearly indicate the glycosylation of the acceptor peptide and of G-CSF.

CONCLUSION

In the present work, we isolated a human-derived glycosyltransferase by expressing soluble HisDapGalNAcT2 in E. coli. The functional activity of the enzyme was shown in vitro. Further investigations are needed to assess the potential of in vivo glycosylation in E. coli.

O-linked glycosylation of the mucin domain of the herpes simplex virus type 1-specific glycoprotein gC-1 is temporally regulated in a seed-and-spread manner

The herpes simplex virus type 1 (HSV-1) glycoprotein gC-1, participating in viral receptor interactions and immunity interference, harbors a mucin-like domain with multiple clustered O-linked glycans. Using HSV-1-infected diploid human fibroblasts, an authentic target for HSV-1 infection, and a protein immunoaffinity procedure, we enriched fully glycosylated gC-1 and a series of its biosynthetic intermediates. This fraction was subjected to trypsin digestion and a LC-MS/MS glycoproteomics approach. In parallel, we characterized the expression patterns of the 20 isoforms of human GalNAc transferases responsible for initiation of O-linked glycosylation. The gC-1 O-glycosylation was regulated in an orderly manner initiated by synchronous addition of one GalNAc unit each to Thr-87 and Thr-91 and one GalNAc unit to either Thr-99 or Thr-101, forming a core glycopeptide for subsequent additions of in all 11 GalNAc residues to selected Ser and Thr residues of the Thr-76-Lys-107 stretch of the mucin domain. The expression patterns of GalNAc transferases in the infected cells suggested that initial additions of GalNAc were carried out by initiating GalNAc transferases, in particular GalNAc-T2, whereas subsequent GalNAc additions were carried out by followup transferases, in particular GalNAc-T10. Essentially all of the susceptible Ser or Thr residues had to acquire their GalNAc units before any elongation to longer O-linked glycans of the gC-1-associated GalNAc units was permitted. Because the GalNAc occupancy pattern is of relevance for receptor binding of gC-1, the data provide a model to delineate biosynthetic steps of O-linked glycosylation of the gC-1 mucin domain in HSV-1-infected target cells.

UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyl-transferase from the snail Biomphalaria glabrata - substrate specificity and preference of glycosylation sites

O-glycosylation is a widely occurring posttranslational modification of proteins. The glycosylation status of a specific site may influence the location, activity and function of a protein. The initiating enzyme of mucin-type O-glycosylation is UDP-GalNAc:polypeptide GalNAc transferase (ppGalNAcT; EC 2.4.1.41). Using electron-transfer dissociation mass spectrometry, ppGalNAcT from the snail Biomphalaria glabrata was characterized regarding its ability to glycosylate threonine and serine residues in different peptide sequence environments. The preferences of the snail enzyme for flanking amino acids of the potential glycosylation site were very similar to vertebrate and insect members of the family. Acceptor sites with adjacent proline residues were highly preferred, while other residues caused less pronounced effects. No specific O-glycosylation consensus sequence was found. The results obtained from synthetic peptides were in good correlation with the observed glycosylation patterns of native peptides and with the order of attachment in a multi-glycosylated peptide. The snail enzyme clearly preferred threonine over serine in the in vitro assays. No significant differences of transfer speed or efficiency could be detected using a mutant of the enzyme lacking the lectin domain. This is the first characterisation of the substrate specificity of a member of the ppGalNAcT family from mollusc origin.

Chemical reporters for biological discovery

Functional tools are needed to understand complex biological systems. Here we review how chemical reporters in conjunction with bioorthogonal labeling methods can be used to image and retrieve nucleic acids, proteins, glycans, lipids and other metabolites in vitro, in cells as well as in whole organisms. By tagging these biomolecules, researchers can now monitor their dynamics in living systems and discover specific substrates of cellular pathways. These advances in chemical biology are thus providing important tools to characterize biological pathways and are poised to facilitate our understanding of human diseases.

Inhibition of mucin-type O-glycosylation through metabolic processing and incorporation of N-thioglycolyl-D-galactosamine peracetate (Ac5GalNTGc)

Mucin-type O-glycans form one of the most abundant and complex post-translational modifications (PTM) on cell surface proteins that govern adhesion, migration, and trafficking of hematopoietic cells. Development of targeted approaches to probe functions of O-glycans is at an early stage. Among several approaches, small molecules with unique chemical functional groups that could modulate glycan biosynthesis form a critical tool. Herein, we show that metabolism of peracetyl N-acyl-D-galactosamine derivatives carrying an N-thioglycolyl (Ac5GalNTGc, 1) moiety-but not N-glycolyl (Ac5GalNGc, 2) and N-acetyl (Ac4GalNAc, 3)-through the N-acetyl-D-galactosamine (GalNAc) salvage pathway induced abrogation of MAL-II and PNA epitopes in Jurkat cells. Mass spectrometry of permethylated O-glycans from Jurkat cells confirmed the presence of significant amounts of elaborated O-glycans (sialyl-T and disialyl-T) which were inhibited upon treatment with 1. O-Glycosylation of CD43, a cell surface antigen rich in O-glycans, was drastically reduced by 1 in a thiol-dependent manner. By contrast, only mild effects were observed for CD45 glycoforms. Direct metabolic incorporation of 1 was confirmed by thiol-selective Michael addition reaction of immunoprecipitated CD43-myc/FLAG. Mechanistically, CD43 glycoforms were unperturbed by peracetylated N-(3-acetylthiopropanoyl) (4), N-(4-acetylthiobutanoyl) (5), and N-methylthioacetyl (6) galactosamine derivatives, N-thioglycolyl-D-glucosamine (7, C-4 epimer of 1), and α-O-benzyl 2-acetamido-2-deoxy-D-galactopyranoside (8), confirming the critical requirement of both free sulfhydryl and galactosamine moieties for inhibition of mucin-type O-glycans. Similar, yet differential, effects of 1 were observed for CD43 glycoforms in multiple hematopoietic cells. Development of small molecules that could alter glycan patterns in an antigen-selective and cell-type selective manner might provide avenues for understanding biological functions of glycans.

Suppression of core 1 Gal-transferase is associated with reduction of TF and reciprocal increase of Tn, sialyl-Tn and Core 3 glycans in human colon cancer cells

It has long been presumed, though with surprisingly little evidence, a competition between Core 1 Gal-transferase (C1GalT), Core 3 GlcNAc-transferase (C3GnT) and sialyl-transferase (ST6GalNAc-T) for elongation of O-linked mucin-type glycans initiated with GalNAcα-Ser/Thr. This study tested this presumption by selective suppression of one of these glycosyltransferases and then analysed the expressions of the enzymatic products of the other three glycosyltransferases. It was found that siRNA suppression of C1GalT markedly reduced the expression of Galβ1,3GalNAcα- (Core 1) and in the meantime increased the expressions of sialyl-GalNAcα- (sialyl-Tn), GalNAcα- (Tn) and GlcNAcβ1,3GalNAcα- (Core 3)-associated glycans in human colon cancer HT29 and SW620 cells. This supports a competitive modification of the GalNAcα-Ser/Thr between C1GalT, C3GnT and ST6GalNAc-T in O-glycan biosynthesis. As Tn, TF and sialyl-Tn are oncofetal antigens and are over-expressed in most human cancers, this information is useful for the development of glycosyltransferase-targeted therapeutic strategies for cancer treatment.

Site-specific protein O-glycosylation modulates proprotein processing - deciphering specific functions of the large polypeptide GalNAc-transferase gene family

BACKGROUND

Posttranslational modifications (PTMs) greatly expand the function and regulation of proteins, and glycosylation is the most abundant and diverse PTM. Of the many different types of protein glycosylation, one is quite unique; GalNAc-type (or mucin-type) O-glycosylation, where biosynthesis is initiated in the Golgi by up to twenty distinct UDP-N-acetyl-α-d-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). These GalNAc-Ts are differentially expressed in cells and have different (although partly overlapping) substrate specificities, which provide for both unique functions and considerable redundancy. Recently we have begun to uncover human diseases associated with deficiencies in GalNAc-T genes (GALNTs). Thus deficiencies in individual GALNTs produce cell and protein specific effects and subtle distinct phenotypes such as hyperphosphatemia with hyperostosis (GALNT3) and dysregulated lipid metabolism (GALNT2). These phenotypes appear to be caused by deficient site-specific O-glycosylation that co-regulates proprotein convertase (PC) processing of FGF23 and ANGPTL3, respectively.

SCOPE OF REVIEW

Here we summarize recent progress in uncovering the interplay between human O-glycosylation and protease regulated processing and describes other important functions of site-specific O-glycosylation in health and disease.

MAJOR CONCLUSIONS

Site-specific O-glycosylation modifies pro-protein processing and other proteolytic events such as ADAM processing and thus emerges as an important co-regulator of limited proteolytic processing events.

GENERAL SIGNIFICANCE

Our appreciation of this function may have been hampered by our sparse knowledge of the O-glycoproteome and in particular sites of O-glycosylation. New strategies for identification of O-glycoproteins have emerged and recently the concept of SimpleCells, i.e. human cell lines made deficient in O-glycan extension by zinc finger nuclease gene targeting, was introduced for broad O-glycoproteome analysis.

GALNT9 gene expression is a prognostic marker in neuroblastoma patients

BACKGROUND

The enzymes encoded by the GALNT [UDP-N-acetyl-α-d-galactosamine:polypeptide N-acetylgalactosaminyltransferase (GALNAC-T)] gene family catalyze the first step of O-glycosylation. Little is known about the link between expression of the genes encoding GALNAC-T enzymes and tumor progression in neuroblastoma, a pediatric cancer that can be classified as either low or high risk. We assessed the expression of genes in the GALNT family in a large cohort of neuroblastoma patients and characterized members of this family that might be used as new prognostic markers.

METHODS

Reverse-transcription PCR analysis of 14 GALNT genes with a panel of neuroblastoma cell lines identified the GALNT9 gene as playing a potential role in disease progression. We used the log-rank test and the multivariable Cox proportional hazards model with a cohort of 122 neuroblastoma patients to analyze the relationship between GALNT9 expression and overall survival or disease-free survival.

RESULTS

In the high-risk neuroblastoma experimental model IGR-N-91, GALNT9 expression was present in neuroblasts derived from primary tumors but not in neuroblasts from metastatic bone marrow. Moreover, GALNT9 in neuroblastoma cell lines was expressed in substrate adherent (S)-type cell lines but not in neuronal (N)-type lines. In the tumor cohort, GALNT9 expression was associated with high overall survival, independent of the standard risk-stratification covariates. GALNT9 expression was significantly associated with disease-free survival for patients currently classified as at low risk (P < 0.0007).

CONCLUSIONS

GALNT9 expression correlates with both improved overall survival in low- and high-risk groups and an improved clinical outcome (overall and disease-free survival) in low-risk patients. Thus, the GALNT9 expression may be a prognostic marker for personalized therapy.

Carbohydrate synthesis and biosynthesis technologies for cracking of the glycan code: recent advances

The glycan code of glycoproteins can be conceptually defined at molecular level by the sequence of well characterized glycans attached to evolutionarily predetermined amino acids along the polypeptide chain. Functional consequences of protein glycosylation are numerous, and include a hierarchy of properties from general physicochemical characteristics such as solubility, stability and protection of the polypeptide from the environment up to specific glycan interactions. Definition of the glycan code for glycoproteins has been so far hampered by the lack of chemically defined glycoprotein glycoforms that proved to be extremely difficult to purify from natural sources, and the total chemical synthesis of which has been hitherto possible only for very small molecular species. This review summarizes the recent progress in chemical and chemoenzymatic synthesis of complex glycans and their protein conjugates. Progress in our understanding of the ways in which a particular glycoprotein glycoform gives rise to a unique set of functional properties is now having far reaching implications for the biotechnology of important glycodrugs such as therapeutical monoclonal antibodies, glycoprotein hormones, carbohydrate conjugates used for vaccination and other practically important protein-carbohydrate conjugates.

Elucidation of the sugar recognition ability of the lectin domain of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 3 by using unnatural glycopeptide substrates

Mucin-type glycosylation [α-N-acetyl-D-galactosamine (α-GalNAc)-O-Ser/Thr] on proteins is initiated biosynthetically by 16 homologous isoforms of GalNAc-Ts (uridine diphosphate-GalNAc:polypeptide N-acetylgalactosaminyltransferases). All the GalNAc-Ts consist of a catalytic domain and a lectin domain. Previous reports of GalNAc-T assays toward peptides and α-GalNAc glycopeptides showed that the lectin domain recognized the sugar on the substrates and affected the reaction; however, the details are not clear. Here, we report a new strategy to give insight on the sugar recognition ability and the function of the GalNAc-T3 lectin domain using chemically synthesized natural-type (α-GalNAc-O-Thr) and unnatural-type [β-GalNAc-O-Thr, α-Fuc-O-Thr and β-GlcNAc-O-Thr] MUC5AC glycopeptides. GalNAc-T3 is one of isoforms expressed in various organs, its substrate specificity extensively characterized and its anomalous expression has been identified in several types of cancer (e.g. pancreas and stomach). The glycopeptides used in this study were designed based on a preliminary peptide assay with a sequence derived from the MUC5AC tandem repeat. Through GalNAc-T3 and lectin-inactivated GalNAc-T3, competition assays between the glycopeptide substrates and product analyses (MALDI-TOF MS, RP-HPLC and ETD-MS/MS), we show that the lectin domain strictly recognized GalNAc on the substrate and this specificity controlled the glycosylation pathway.

O-glycoprotein biosynthesis: site localization by Edman degradation and site prediction based on random peptide substrates

The characterization of mucin-type O-glycosylation is fraught with extreme difficulty at almost every level of analysis: from difficulties in obtaining glycopeptides suitable for study, their structural heterogeneity, lack of broad acting glycosidase tools capable of simplifying the glycans, and finally the vast complexity of performing analysis on multiply glycosylated glycopeptides. This, along with a lack of known peptide sequence motif(s) for the transferases that initiate mucin-type O-glycosylation, significantly hinders our understanding of mucin-type O-glycosylation at almost every level from their biosynthesis to their biological and biophysical properties. In this chapter, the use of partial chemical deglycosylation coupled with Edman amino acid sequencing is described to quantify sites of O-glycosylation. In addition, the use of oriented random peptide substrates is described for providing the specificities of the polypeptide α-N-acetylgalactosaminyltransferases, which can be used to estimate transferase-specific sites of O-glycosylation.

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.

Toward automated glycan analysis

As drastic structural changes in cell-surface glycans of glycoproteins and glycosphingolipids, as well as serum glycoproteins, are often observed during cell differentiation and cancer progression, it is considered that glycans can be potential candidates for novel diagnostic and therapeutic biomarkers. Although there have been substantial advances in our understanding of the effects of glycosylation on some biological systems, we still do not fully understand the significance and mechanism of glycoform alteration that is widely observed in many human diseases. This is due to the highly complicated structures of the glycans and the extremely tedious and time-consuming processes required for their separation from complex mixtures and their subsequent analysis. As a result, with a few notable exceptions, the therapeutic potential of complex glycans has not been well exploited. This article is focused on the state of the art and current advances in glycomics, and efforts for the development of automated glycan analysis, which should greatly accelerate functional glycobiology and its medical/pharmaceutical applications. The "glycoblotting method" is the only method currently available that allows rapid and large-scale clinical glycomics of human whole-serum glycoproteins, because it requires very little material and, when combined with an automated system "SweetBlot," takes only ∼14h to complete whole glycan profiling by mass spectrometry. The upcoming goal is to combine glycoblotting methods and various MS-based platforms for the development of a fully automated glycan analytical system and accelerating research to discover highly sensitive and clinically important biomarker molecules.

Site-specific modification of recombinant proteins: a novel platform for modifying glycoproteins expressed in E. coli

The site-specific modification of proteins is expected to be an important capability for the synthesis of bioconjugates in the future. However, the traditional repertoire of reactions available for the direct modification of proteins suffers from lack of specificity, necessitating costly downstream processing to isolate the specific species of interest. (1) Here, we use a well-established, glycan-specific chemistry to PEGylate model glycoproteins, each containing a unique reactive GalNAc attached to a specifically engineered threonine residue. By engineering E. coli to execute the initial steps of human, mucin-type O-glycosylation, we were able to obtain homogeneous site-specifically modified glycoproteins with fully human glycan linkages. Two mucin-based reporters as well as several fusion proteins containing eight-amino-acid GalNAc-T recognition sequences were glycosylated in this engineered glycocompetent strain of E. coli. The use of one sequence in particular, PPPTSGPT, resulted in site-specific glycan occupancy of approximately 69% at the engineered threonine. The GalNAc present on the purified glycoprotein was oxidized by galactose oxidase and then coupled to hydroxylamine functionalized 20 kDa PEG in the presence of aniline. The glycoprotein could be converted to the PEGylated product at approximately 85% yield and >98% purity as determined by comparison to the products of control reactions.

Location, location, location: new insights into O-GalNAc protein glycosylation

O-GalNAc glycosylation of proteins confers essential structural, protective and signaling roles in eumetazoans. Addition of O-glycans onto proteins is an extremely complex process that regulates both sites of attachment and the types of oligosaccharides added. Twenty distinct polypeptide GalNAc-transferases (GalNAc-Ts) initiate O-glycosylation and fine-tuning their expression provides a mechanism for regulating this action. Recently, a new mode of regulation has emerged where activation of Src kinase selectively redistributes Golgi-localized GalNAc-Ts to the ER. This relocalization results in a strong increase in the density of O-glycan decoration. In this review, we discuss how different mechanisms can regulate the number and the types of O-glycans decorating proteins. In addition, we speculate how Src-dependent relocation of GalNAc-Ts could play an important role in cancerous cellular transformation.

Emerging paradigms for the initiation of mucin-type protein O-glycosylation by the polypeptide GalNAc transferase family of glycosyltransferases

Mammalian mucin-type O-glycosylation is initiated by a large family of ∼20 UDP-GalNAc:polypeptide α-N-acetylgalactosaminyltransferases (ppGalNAc Ts) that transfer α-GalNAc from UDP-GalNAc to Ser and Thr residues of polypeptide acceptors. Characterizing the peptide substrate specificity of each isoform is critical to understanding their properties, biological roles, and significance. Presently, only the specificities of ppGalNAc T1, T2, and T10 and the fly orthologues of T1 and T2 have been systematically characterized utilizing random peptide substrates. We now extend these studies to ppGalNAc T3, T5, and T12, transferases variously associated with human disease. Our results reveal several common features; the most striking is the similar pattern of enhancements for the three residues C-terminal to the site of glycosylation for those transferases that contain a common conserved Trp. In contrast, residues N-terminal to the site of glycosylation show a wide range of isoform-specific enhancements, with elevated preferences for Pro, Val, and Tyr being the most common at the -1 position. Further analysis reveals that the ratio of positive (Arg, Lys, and His) to negative (Asp and Glu) charged residue enhancements varied among transferases, thus further modulating substrate preference in an isoform-specific manner. By utilizing the obtained transferase-specific preferences, the glycosylation patterns of the ppGalNAc Ts against a series of peptide substrates could roughly be reproduced, demonstrating the potential for predicting isoform-specific glycosylation. We conclude that each ppGalNAc T isoform may be uniquely sensitive to peptide sequence and overall charge, which together dictates the substrate sites that will be glycosylated.

The Tn antigen-structural simplicity and biological complexity

Glycoproteins in animal cells contain a variety of glycan structures that are added co- and/or posttranslationally to proteins. Of over 20 different types of sugar-amino acid linkages known, the two major types are N-glycans (Asn-linked) and O-glycans (Ser/Thr-linked). An abnormal mucin-type O-glycan whose expression is associated with cancer and several human disorders is the Tn antigen. It has a relatively simple structure composed of N-acetyl-D-galactosamine with a glycosidic α linkage to serine/threonine residues in glycoproteins (GalNAcα1-O-Ser/Thr), and was one of the first glycoconjugates to be chemically synthesized. The Tn antigen is normally modified by a specific galactosyltransferase (T-synthase) in the Golgi apparatus of cells. Expression of active T-synthase is uniquely dependent on the molecular chaperone Cosmc, which is encoded by a gene on the X chromosome. Expression of the Tn antigen can arise as a consequence of mutations in the genes for T-synthase or Cosmc, or genes affecting other steps of O-glycosylation pathways. Because of the association of the Tn antigen with disease, there is much interest in the development of Tn-based vaccines and other therapeutic approaches based on Tn expression.

Simian immunodeficiency virus from the sooty mangabey and rhesus macaque is modified with O-linked carbohydrate

Although stretches of serine and threonine are sometimes sites for O-linked carbohydrate attachment, specific sequence and structural determinants for O-linked attachment remain ill defined. The gp120 envelope protein of SIVmac239 contains a serine-threonine-rich stretch of amino acids at positions 128 to 139. Here we show that lectin protein from jackfruit seed (jacalin), which binds to non- and monosialylated core 1 O-linked carbohydrate, potently inhibited the replication of SIVmac239. Selection of a jacalin-resistant SIVmac239 variant population resulted in virus with specific substitutions within amino acids 128 to 139. Cloned simian immunodeficiency virus (SIV) variants with substitutions in the 128-to-139 region had infectivities equivalent to, or within 1 log unit of, that of SIVmac239 and were resistant to the inhibitory effects of jacalin. Characterization of the SIVmac239 gp120 O-linked glycome showed the presence of core 1 and core 2 O-linked carbohydrate; a 128-to-139-substituted variant gp120 from jacalin-resistant SIV lacked O-linked carbohydrate. Unlike that of SIVmac239, the replication of HIV-1 strain NL4-3 was resistant to inhibition by jacalin. Purified gp120s from four SIVmac and SIVsm strains bound jacalin strongly in an enzyme-linked immunosorbent assay, while nine different HIV-1 gp120s, two SIVcpz gp120s, and 128-to-139-substituted SIVmac239 gp120 did not bind jacalin. The ability or inability to bind jacalin thus correlated with the presence of the serine-threonine-rich stretch in the SIVmac and SIVsm gp120s and the absence of such stretches in the SIVcpz and HIV-1 gp120s. Consistent with sequence predictions, two HIV-2 gp120s bound jacalin, while one did not. These data demonstrate the presence of non- and monosialylated core 1 O-linked carbohydrate on the gp120s of SIVmac and SIVsm and the lack of these modifications on HIV-1 and SIVcpz gp120s.

An efficient approach to the discovery of potent inhibitors against glycosyltransferases

We describe a standardized approach for searching potent and selective inhibitors of glycosyltransferases by high throughput quantitative MALDI-TOFMS-based screening of focused compound libraries constructed by 1,3-dipolar cycloaddition of the desired azidosugar nucleotides with various alkynes. An aminooxy-functionalized reagent with a stable isotope was conjugated with oligosaccharides to afford glycopeptides as acceptor substrates with improved ion sensitivity. Enhanced ionization potency of new substrates allowed for MALDI-TOFMS-based facile and quantitative analysis of enzymatic glycosylation in the presence of glycosyl donor substrates. A non-natural synthetic sugar nucleotide was identified to be the first highly specific inhibitor for rat recombinant alpha2,3-(N)-sialyltransferase (alpha2,3ST, IC(50) = 8.2 microM), while this compound was proved to become a favorable substrate for rat recombinant alpha2,6-(N)-sialyltransferase (alpha2,6ST, K(m) = 125 microM). Versatility of this strategy was demonstrated by identification of two selective inhibitors for human recombinant alpha1,3-fucosyltransferase V (alpha1,3-FucT, K(i) = 293 nM) and alpha1,6-fucosyltransferase VIII (alpha1,6-FucT, K(i) = 13.8 microM).

Unexpected tolerance of glycosylation by UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferase revealed by electron capture dissociation mass spectrometry: carbohydrate as potential protective groups

UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases (ppGalNAcTs, EC 2.4.1.41), a family of key enzymes that initiate posttranslational modification with O-glycans in mucin synthesis by introduction of alpha-GalNAc residues, are structurally composed of a catalytic domain and a lectin domain. It has been known that multiple Ser/Thr residues are assigned in common mucin glycoproteins as potential O-glycosylation sites and more than 20 distinct isoforms of this enzyme family contribute to produce densely O-glycosylated mucin glycoproteins. However, it seems that the functional role of the lectin domain of ppGalNAcTs remains unclear. We considered that electron capture dissociation mass spectrometry (ECD-MS), a promising method for highly selective fragmentation at peptide linkages of glycopeptides to generate unique c and z series of ions, should allow for precise structural characterization to uncover the mechanism in O-glycosylation of mucin peptides by ppGalNAcTs. In the present study, it was demonstrated that a system composed of an electrospray source, a linear RFQ ion trap that isolates precursor ions, the ECD device, and a TOF mass spectrometer is a nice tool to identify the preferential O-glycosylation sites without any decomposition of the carbohydrate moiety. It should be noted that electrons used for ECD are accelerated within a range from 1.75 to 9.75 eV depending on the structures of glycopeptides of interest. We revealed for the first time that additional installation of a alpha-GalNAc residue at potential glycosylation sites by ppGalNAcT2 proceeds smoothly in various unnatural glycopeptides having alpha-Man, alpha-Fuc, and beta-Gal residues as well as alpha-GalNAc residues. The results may suggest that ppGalNAcT2 did not differentiate totally presubstituted sugar residues in terms of configuration of functional groups, d-, l-configuration, and even alpha-, beta-stereochemistry at an anomeric carbon atom when relatively short synthetic peptides were employed for the acceptor substrates. Unexpected characteristics of ppGalNAcT2 motivated us to challenge site-directed installation of alpha-GalNAc residues at desired position(s) by protecting some hydroxyl groups of Thr/Ser residues with selectively removable sugars, notably a novel concept as "carbohydrate as protective groups", toward a goal of the systematic chemical and enzymatic synthesis of biologically important mucin glycopeptides.

The role of mucin-type O-glycans in eukaryotic development

Newly emerging genetic studies have revealed that a subset of the family of glycosyltransferases responsible for the formation of mucin-type O glycans is essential for normal development. As additional genetic, biochemical and physical tools are developed to interrogate the complex structure and surface location of this under-studied class of carbohydrate, no doubt additional roles will be elucidated.