Mammalian STT3A/B oligosaccharyltransferases segregate N-glycosylation at the translocon from lipid-linked oligosaccharide hydrolysis

Occurrence of free glycans in salmonid serum

Free N-glycans (FNGs) are oligosaccharides that are structurally related to N-linked glycans, and are widely found in nature. The mechanisms responsible for the formation and degradation of intracellular FNGs are well characterized in mammalian cells. More recent analysis in mammalian sera shows that there are various types of extracellular free glycans, including FNGs. However, it is unknown whether these free glycans are widely distributed in vertebrates. In this study, we investigated the occurrence of free glycans in salmonid serum. We found that it contained sialyl or neutral FNGs and sialyl lactose/N-acetyllactosamine (LacNAc)-type glycans, which was consistent with that found in mammalian sera. Many of the structures of FNGs matched those of N-glycans from serum glycoproteins. This study revealed that various types of free glycans are present in fish serum, demonstrating their wide occurrence among vertebrates.

Processing of N-glycans in the ER and Golgi influences the production of surface sialylated glycoRNA

Glycoconjugates, including glycans on proteins and lipids, have obtained significant attention due to their critical roles in both intracellular and intercellular biological functions and processes. Notably, recent discoveries have revealed the presence of glycosylated RNAs (glycoRNAs) on cell surfaces. Despite the well-characterized roles of RNA modifications, RNA glycosylation remains relatively unexplored. In this study, we investigate the relationship between N-glycosylation and RNA glycosylation. Using a recombinant Siglec11-Fc as a probe, we detected surface sialylated glycoRNAs in human cell lines and identified their dependency on the catalytic isoforms of the oligosaccharyltransferase (OST) complex, implicating STT3A-dependent protein glycosylation as a predominant contributor for affecting indirect generation of glycoRNAs. Additionally, perturbations in N-glycan biosynthesis pathways or changes in N-glycan structure impact surface sialylated glycoRNA levels, indicating a regulatory role of glycan metabolic pathways in RNA glycosylation. Together, our results underscore the intricate relationship between protein N-glycosylation and processing and RNA biology.

Brucella-driven host N-glycome remodeling controls infection

Many powerful methods have been employed to elucidate the global transcriptomic, proteomic, or metabolic responses to pathogen-infected host cells. However, the host glycome responses to bacterial infection remain largely unexplored, and hence, our understanding of the molecular mechanisms by which bacterial pathogens manipulate the host glycome to favor infection remains incomplete. Here, we address this gap by performing a systematic analysis of the host glycome during infection by the bacterial pathogen Brucella spp. that cause brucellosis. We discover, surprisingly, that a Brucella effector protein (EP) Rhg1 induces global reprogramming of the host cell N-glycome by interacting with components of the oligosaccharide transferase complex that controls N-linked protein glycosylation, and Rhg1 regulates Brucella replication and tissue colonization in a mouse model of brucellosis, demonstrating that Brucella exploits the EP Rhg1 to reprogram the host N-glycome and promote bacterial intracellular parasitism, thereby providing a paradigm for bacterial control of host cell infection.

Rat hepatocytes secrete free oligosaccharides

We recently established a method for the isolation of serum-free oligosaccharides, and characterized various features of their structures. However, the precise mechanism for how these glycans are formed still remains unclarified. To further investigate the mechanism responsible for these serum glycans, here, we utilized rat primary hepatocytes to examine whether they are able to secrete free glycans. Our findings indicated that a diverse array of free oligosaccharides such as sialyl/neutral free N-glycans (FNGs), as well as sialyl lactose/LacNAc-type glycans, were secreted into the culture medium by primary hepatocytes. The structural features of these free glycans in the medium were similar to those isolated from the sera of the same rat. Further evidence suggested that an oligosaccharyltransferase is involved in the release of the serum-free N-glycans. Our results indicate that the liver is indeed secreting various types of free glycans directly into the serum.

Hydrolytic activity of yeast oligosaccharyltransferase is enhanced when misfolded proteins accumulate in the endoplasmic reticulum

It is known that oligosaccharyltransferase (OST) has hydrolytic activity toward dolichol-linked oligosaccharides (DLO), which results in the formation of free N-glycans (FNGs), i.e. unconjugated oligosaccharides with structural features similar to N-glycans. The functional importance of this hydrolytic reaction, however, remains unknown. In this study, the hydrolytic activity of OST was characterized in yeast. It was shown that the hydrolytic activity of OST is enhanced in ubiquitin ligase mutants that are involved in endoplasmic reticulum-associated degradation. Interestingly, this enhanced hydrolysis activity is completely suppressed in asparagine-linked glycosylation (alg) mutants, bearing mutations related to the biosynthesis of DLO, indicating that the effect of ubiquitin ligase on OST-mediated hydrolysis is context-dependent. The enhanced hydrolysis activity in ubiquitin ligase mutants was also found to be canceled upon treatment of the cells with dithiothreitol, a reagent that potently induces protein unfolding in the endoplasmic reticulum (ER). Our results clearly suggest that the hydrolytic activity of OST is enhanced under conditions in which the formation of unfolded proteins is promoted in the ER in yeast. The possible role of FNGs on protein folding is discussed.

Influence of glycosylation on the immunogenicity and antigenicity of viral immunogens

A key aspect of successful viral vaccine design is the elicitation of neutralizing antibodies targeting viral attachment and fusion glycoproteins that embellish viral particles. This observation has catalyzed the development of numerous viral glycoprotein mimetics as vaccines. Glycans can dominate the surface of viral glycoproteins and as such, the viral glycome can influence the antigenicity and immunogenicity of a candidate vaccine. In one extreme, glycans can form an integral part of epitopes targeted by neutralizing antibodies and are therefore considered to be an important feature of key immunogens within an immunization regimen. In the other extreme, the existence of peptide and bacterially expressed protein vaccines shows that viral glycosylation can be dispensable in some cases. However, native-like glycosylation can indicate native-like protein folding and the presence of conformational epitopes. Furthermore, going beyond native glycan mimicry, in either occupancy of glycosylation sites or the glycan processing state, may offer opportunities for enhancing the immunogenicity and associated protection elicited by an immunogen. Here, we review key determinants of viral glycosylation and how recombinant immunogens can recapitulate these signatures across a range of enveloped viruses, including HIV-1, Ebola virus, SARS-CoV-2, Influenza and Lassa virus. The emerging understanding of immunogen glycosylation and its control will help guide the development of future vaccines in both recombinant protein- and nucleic acid-based vaccine technologies.

Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications

Protein posttranslational modifications (PTMs) refer to the breaking or generation of covalent bonds on the backbones or amino acid side chains of proteins and expand the diversity of proteins, which provides the basis for the emergence of organismal complexity. To date, more than 650 types of protein modifications, such as the most well-known phosphorylation, ubiquitination, glycosylation, methylation, SUMOylation, short-chain and long-chain acylation modifications, redox modifications, and irreversible modifications, have been described, and the inventory is still increasing. By changing the protein conformation, localization, activity, stability, charges, and interactions with other biomolecules, PTMs ultimately alter the phenotypes and biological processes of cells. The homeostasis of protein modifications is important to human health. Abnormal PTMs may cause changes in protein properties and loss of protein functions, which are closely related to the occurrence and development of various diseases. In this review, we systematically introduce the characteristics, regulatory mechanisms, and functions of various PTMs in health and diseases. In addition, the therapeutic prospects in various diseases by targeting PTMs and associated regulatory enzymes are also summarized. This work will deepen the understanding of protein modifications in health and diseases and promote the discovery of diagnostic and prognostic markers and drug targets for diseases.

Characterization of PcSTT3B as a Key Oligosaccharyltransferase Subunit Involved in N-glycosylation and Its Role in Development and Pathogenicity of Phytophthora capsici

Asparagine (Asn, N)-linked glycosylation is a conserved process and an essential post-translational modification that occurs on the NXT/S motif of the nascent polypeptides in endoplasmic reticulum (ER). The mechanism of N-glycosylation and biological functions of key catalytic enzymes involved in this process are rarely documented for oomycetes. In this study, an N-glycosylation inhibitor tunicamycin (TM) hampered the mycelial growth, sporangial release, and zoospore production of Phytophthora capsici, indicating that N-glycosylation was crucial for oomycete growth development. Among the key catalytic enzymes involved in N-glycosylation, the PcSTT3B gene was characterized by its functions in P. capsici. As a core subunit of the oligosaccharyltransferase (OST) complex, the staurosporine and temperature sensive 3B (STT3B) subunit were critical for the catalytic activity of OST. The PcSTT3B gene has catalytic activity and is highly conservative in P. capsici. By using a CRISPR/Cas9-mediated gene replacement system to delete the PcSTT3B gene, the transformants impaired mycelial growth, sporangial release, zoospore production, and virulence. The PcSTT3B-deleted transformants were more sensitive to an ER stress inducer TM and display low glycoprotein content in the mycelia, suggesting that PcSTT3B was associated with ER stress responses and N-glycosylation. Therefore, PcSTT3B was involved in the development, pathogenicity, and N-glycosylation of P. capsici.

Glycomic, Glycoproteomic, and Proteomic Profiling of Philippine Lung Cancer and Peritumoral Tissues: Case Series Study of Patients Stages I-III

Lung cancer is the leading cause of cancer death and non-small cell lung carcinoma (NSCLC) accounting for majority of lung cancers. Thus, it is important to find potential biomarkers, such as glycans and glycoproteins, which can be used as diagnostic tools against NSCLC. Here, the N-glycome, proteome, and N-glycosylation distribution maps of tumor and peritumoral tissues of Filipino lung cancer patients (n = 5) were characterized. We present several case studies with varying stages of cancer development (I-III), mutation status (EGFR, ALK), and biomarker expression based on a three-gene panel (CD133, KRT19, and MUC1). Although the profiles of each patient were unique, specific trends arose that correlated with the role of aberrant glycosylation in cancer progression. Specifically, we observed a general increase in the relative abundance of high-mannose and sialofucosylated N-glycans in tumor samples. Analysis of the glycan distribution per glycosite revealed that these sialofucosylated N-glycans were specifically attached to glycoproteins involved in key cellular processes, including metabolism, cell adhesion, and regulatory pathways. Protein expression profiles showed significant enrichment of dysregulated proteins involved in metabolism, adhesion, cell-ECM interactions, and N-linked glycosylation, supporting the protein glycosylation results. The present case series study provides the first demonstration of a multi-platform mass-spectrometric analysis specifically for Filipino lung cancer patients.

B7 family protein glycosylation: Promising novel targets in tumor treatment

Cancer immunotherapy, including the inhibition of immune checkpoints, improves the tumor immune microenvironment and is an effective tool for cancer therapy. More effective and alternative inhibitory targets are critical for successful immune checkpoint blockade therapy. The interaction of the immunomodulatory ligand B7 family with corresponding receptors induces or inhibits T cell responses by sending co-stimulatory and co-inhibitory signals respectively. Blocking the glycosylation of the B7 family members PD-L1, PD-L2, B7-H3, and B7-H4 inhibited the self-stability and receptor binding of these immune checkpoint proteins, leading to immunosuppression and rapid tumor progression. Therefore, regulation of glycosylation may be the "golden key" to relieve tumor immunosuppression. The exploration of a more precise glycosylation regulation mechanism and glycan structure of B7 family proteins is conducive to the discovery and clinical application of antibodies and small molecule inhibitors.

The N-glycosylation-related genes as potential targets for RNAi-mediated pest control of the Colorado potato beetle (Leptinotarsa decemlineata)

BACKGROUND

N-glycosylation is one of the most common and important post-translational modifications in the eukaryotic cell. The study of protein N-glycosylation in several model insects confirmed the importance of this process in insect development, immunity, survival and fertility. The Colorado potato beetle (Leptinotarsa decemlineata) (CPB) is a common pest of Solanaceae crops. With the infamous title of champion of insecticide resistance, novel pest control strategies for this insect are needed. Luckily this pest insect is reported as very sensitive for the post-genomic technology of RNA interference (RNAi).

RESULTS

In this project, we investigated the importance of N-glycosylation in the survival and development of CPB using RNAi-mediated gene silencing of N-glycosylation-related genes (NGRGs) during the different transition steps from the larva, through the pupa to the adult stage. High mortality was observed in the larval stage with the silencing of early NGRGs, as STT3a, DAD1 and GCS1. With dsRNA against middle NGRGs, abnormal phenotypes at the ecdysis process and adult formation were observed, while the silencing of late NGRGs did not cause mortality.

CONCLUSION

The lethal phenotypes observed on silencing of the genes involved in the early processing steps of the N-glycosylation pathway suggest these genes are good candidates for RNAi-mediated control of CPB. Next to the gene-specific mechanism of RNAi for biosafety and possible implementation in integrated pest management, we believe these early NGRGs provide a possible alternative to the well-known target genes Snf7 and vacuolar ATPases that are now used in the first commercial RNAi-based products and thus they may be useful in the context of proactive resistance management. © 2021 Society of Chemical Industry.

Analysis of Tumor Glycosylation Characteristics and Implications for Immune Checkpoint Inhibitor’s Efficacy for Breast Cancer

The alterations of glycosylation, which is a common post-translational modification of proteins, have been acknowledged as key events in breast cancer (BC) oncogenesis and progression. The aberrant expression of glycosyltransferases leads to aberrant glycosylation patterns, posing the diagnostic potential in BC outcomes. The present study aims to establish a glycosyltransferase-based signature to predict BC prognosis and response to immune checkpoint inhibitors. We firstly screened 9 glycosyltransferase genes from The Cancer Genome Atlas (TCGA) database and accordingly established a glyco-signature for predicting the prognosis in BC patients. Patients with BC were successfully divided into high-risk and low-risk groups based on the median cutoff point for risk scores in this signature. Next, the combinational analyses of univariate and multivariate Cox regression, Kaplan–Meier, and receiver operating characteristic (ROC) curves were used to prove that this glyco-signature possessed excellent predictive performance for prognosis of BC patients, as the high-risk group possessed worse outcomes, in comparison to the low-risk group. Additionally, the Gene Set Enrichment Analysis (GSEA) and immunologic infiltration analysis were adopted and indicated that there was a more immunosuppressive state in the high-risk group than that in the low-risk group. The clinical sample validation verified that glycosyltransferase genes were differentially expressed in patients in the low- and high-risk groups, while the biomarkers of antitumor M1 macrophages were increased and N-glycosyltransferase STT3A decreased in the low-risk group. The final in vitro assay showed that the silencing of STT3A suppressed the proliferation and migration of BC cells. Collectively, our well-constructed glyco-signature is able to distinguish the high- and low-risk groups and accordingly predict BC prognosis, which will synergistically promote the prognosis evaluation and provide new immunotherapeutic targets for combating BC.

Overexpression of TaSTT3b-2B improves resistance to sharp eyespot and increases grain weight in wheat

STAUROSPORINE AND TEMPERATURE SENSITIVE3 (STT3) is a catalytic subunit of oligosaccharyltransferase, which is important for asparagine-linked glycosylation. Sharp eyespot, caused by the necrotrophic fungal pathogen Rhizoctonia cerealis, is a devastating disease of bread wheat. However, the molecular mechanisms underlying wheat defense against R. cerealis are still largely unclear. In this study, we identified TaSTT3a and TaSTT3b, two STT3 subunit genes from wheat and reported their functional roles in wheat defense against R. cerealis and increasing grain weight. The transcript abundance of TaSTT3b-2B was associated with the degree of wheat resistance to R. cerealis and induced by both R. cerealis and exogenous jasmonic acid (JA). Overexpression of TaSTT3b-2B significantly enhanced resistance to R. cerealis, grain weight, and JA content in transgenic wheat subjected to R. cerealis stress, while silencing of TaSTT3b-2B compromised resistance of wheat to R. cerealis. Transcriptomic analysis showed that TaSTT3b-2B affected the expression of a series of defense-related genes and JA biosynthesis-related genes, as well as genes coding starch synthase and sucrose synthase. Application of exogenous JA elevated expression levels of the abovementioned defense- and grain weight-related genes, and rescuing the resistance of TaSTT3b-2B-silenced wheat to R. cerealis, while pretreatment with sodium diethyldithiocarbamate, an inhibitor of JA synthesis, attenuated the TaSTT3b-2B-mediated resistance to R. cerealis, suggesting that TaSTT3b-2B played critical roles in regulating R. cerealis resistance and grain weight via JA biosynthesis. Altogether, this study reveals new functional roles of TaSTT3b-2B in regulating plant innate immunity and grain weight, and illustrates its potential application value for wheat molecular breeding.

Active site variants in STT3A cause a dominant type I congenital disorder of glycosylation with neuromusculoskeletal findings

Congenital disorders of glycosylation (CDGs) form a group of rare diseases characterized by hypoglycosylation. We here report the identification of 16 individuals from nine families who have either inherited or de novo heterozygous missense variants in STT3A, leading to an autosomal-dominant CDG. STT3A encodes the catalytic subunit of the STT3A-containing oligosaccharyltransferase (OST) complex, essential for protein N-glycosylation. Affected individuals presented with variable skeletal anomalies, short stature, macrocephaly, and dysmorphic features; half had intellectual disability. Additional features included increased muscle tone and muscle cramps. Modeling of the variants in the 3D structure of the OST complex indicated that all variants are located in the catalytic site of STT3A, suggesting a direct mechanistic link to the transfer of oligosaccharides onto nascent glycoproteins. Indeed, expression of STT3A at mRNA and steady-state protein level in fibroblasts was normal, while glycosylation was abnormal. In S. cerevisiae, expression of STT3 containing variants homologous to those in affected individuals induced defective glycosylation of carboxypeptidase Y in a wild-type yeast strain and expression of the same mutants in the STT3 hypomorphic stt3-7 yeast strain worsened the already observed glycosylation defect. These data support a dominant pathomechanism underlying the glycosylation defect. Recessive mutations in STT3A have previously been described to lead to a CDG. We present here a dominant form of STT3A-CDG that, because of the presence of abnormal transferrin glycoforms, is unusual among dominant type I CDGs.

The evolving genetic landscape of congenital disorders of glycosylation

Congenital Disorders of Glycosylation (CDG) are an expanding and complex group of rare genetic disorders caused by defects in the glycosylation of proteins and lipids. The genetic spectrum of CDG is extremely broad with mutations in over 140 genes leading to a wide variety of symptoms ranging from mild to severe and life-threatening. There has been an expansion in the genetic complexity of CDG in recent years. More specifically several examples of alternate phenotypes in recessive forms of CDG and new types of CDG following an autosomal dominant inheritance pattern have been identified. In addition, novel genetic mechanisms such as expansion repeats have been reported and several already known disorders have been classified as CDG as their pathophysiology was better elucidated. Furthermore, we consider the future and outlook of CDG genetics, with a focus on exploration of the non-coding genome using whole genome sequencing, RNA-seq and multi-omics technology.

N-Glycan Biosynthesis: Basic Principles and Factors Affecting Its Outcome

Carbohydrate chains are the most abundant and diverse of nature's biopolymers and represent one of the four fundamental macromolecular building blocks of life together with proteins, nucleic acids, and lipids. Indicative of their essential roles in cells and in multicellular organisms, genes encoding proteins associated with glycosylation account for approximately 2% of the human genome. It has been estimated that 50-80% of all human proteins carry carbohydrate chains-glycans-as part of their structure. Despite cells utilize only nine different monosaccharides for making their glycans, their order and conformational variation in glycan chains together with chain branching differences and frequent post-synthetic modifications can give rise to an enormous repertoire of different glycan structures of which few thousand is estimated to carry important structural or functional information for a cell. Thus, glycans are immensely versatile encoders of multicellular life. Yet, glycans do not represent a random collection of unpredictable structures but rather, a collection of predetermined but still dynamic entities that are present at defined quantities in each glycosylation site of a given protein in a cell, tissue, or organism.In this chapter, we will give an overview of what is currently known about N-glycan synthesis in higher eukaryotes, focusing not only on the processes themselves but also on factors that will affect or can affect the final outcome-the dynamicity and heterogeneity of the N-glycome. We hope that this review will help understand the molecular details underneath this diversity, and in addition, be helpful for those who plan to produce optimally glycosylated antibody-based therapeutics.

Uncoupling the hydrolysis of lipid-linked oligosaccharide from the oligosaccharyl transfer reaction by point mutations in yeast oligosaccharyltransferase

Oligosaccharyltransferase (OST) is responsible for the first step in the N-linked glycosylation, transferring an oligosaccharide chain onto asparagine residues to create glycoproteins. In the absence of an acceptor asparagine, OST hydrolyzes the oligosaccharide donor, releasing free N-glycans (FNGs) into the lumen of the endoplasmic reticulum (ER). Here, we established a purification method for mutated OSTs using a high-affinity epitope tag attached to the catalytic subunit Stt3, from yeast cells co-expressing the WT OST to support growth. The purified OST protein with mutations is useful for wide-ranging biochemical experiments. We assessed the effects of mutations in the Stt3 subunit on the two enzymatic activities in vitro, as well as their effects on the N-glycan attachment and FNG content levels in yeast cells. We found that mutations in the first DXD motif increased the FNG generation activity relative to the oligosaccharyl transfer activity, both in vitro and in vivo, whereas mutations in the DK motif had the opposite effect; the decoupling of the two activities may facilitate future deconvolution of the reaction mechanism. The isolation of the mutated OSTs also enabled us to identify different enzymatic properties in OST complexes containing either the Ost3 or Ost6 subunit and to find a 15-residue peptide as a better-quality substrate than shorter peptides. This toolbox of mutants, substrates, and methods will be useful for investigations of the molecular basis and physiological roles of the OST enzymes in yeast and other organisms.

Global view of human protein glycosylation pathways and functions

Glycosylation is the most abundant and diverse form of post-translational modification of proteins that is common to all eukaryotic cells. Enzymatic glycosylation of proteins involves a complex metabolic network and different types of glycosylation pathways that orchestrate enormous amplification of the proteome in producing diversity of proteoforms and its biological functions. The tremendous structural diversity of glycans attached to proteins poses analytical challenges that limit exploration of specific functions of glycosylation. Major advances in quantitative transcriptomics, proteomics and nuclease-based gene editing are now opening new global ways to explore protein glycosylation through analysing and targeting enzymes involved in glycosylation processes. In silico models predicting cellular glycosylation capacities and glycosylation outcomes are emerging, and refined maps of the glycosylation pathways facilitate genetic approaches to address functions of the vast glycoproteome. These approaches apply commonly available cell biology tools, and we predict that use of (single-cell) transcriptomics, genetic screens, genetic engineering of cellular glycosylation capacities and custom design of glycoprotein therapeutics are advancements that will ignite wider integration of glycosylation in general cell biology.

N-glycosylation is involved in stomatal development by modulating the release of active abscisic acid and auxin in Arabidopsis

Asparagine-linked glycosylation (N-glycosylation) is one of the most important protein modifications in eukaryotes, affecting the folding, transport, and function of a wide range of proteins. However, little is known about the roles of N-glycosylation in the development of stomata in plants. In the present study, we provide evidence that the Arabidopsis stt3a-2 mutant, defective in oligosaccharyltransferase catalytic subunit STT3, has a greater transpirational water loss and weaker drought avoidance, accompanied by aberrant stomatal distribution. Through physiological, biochemical, and genetic analyses, we found that the abnormal stomatal density of stt3a-2 was partially attributed to low endogenous abscisic acid (ABA) and auxin (IAA) content. Exogenous application of ABA or IAA could partially rescue the mutant's salt-sensitive and abnormal stomatal phenotype. Further analyses revealed that the decrease of IAA or ABA in stt3a-2 seedlings was associated with the underglycosylation of β-glucosidase (AtBG1), catalysing the conversion of conjugated ABA/IAA to active hormone. Our results provide strong evidence that N-glycosylation is involved in stomatal development and participates in abiotic stress tolerance by modulating the release of active plant hormones.

How glycosylation affects glycosylation: the role of N-glycans in glycosyltransferase activity

N-glycosylation is one of the most important posttranslational modifications of proteins. It plays important roles in the biogenesis and functions of proteins by influencing their folding, intracellular localization, stability and solubility. N-glycans are synthesized by glycosyltransferases, a complex group of ubiquitous enzymes that occur in most kingdoms of life. A growing body of evidence shows that N-glycans may influence processing and functions of glycosyltransferases, including their secretion, stability and substrate/acceptor affinity. Changes in these properties may have a profound impact on glycosyltransferase activity. Indeed, some glycosyltransferases have to be glycosylated themselves for full activity. N-glycans and glycosyltransferases play roles in the pathogenesis of many diseases (including cancers), so studies on glycosyltransferases may contribute to the development of new therapy methods and novel glycoengineered enzymes with improved properties. In this review, we focus on the role of N-glycosylation in the activity of glycosyltransferases and attempt to summarize all available data about this phenomenon.

The PAR2 signal peptide prevents premature receptor cleavage and activation

Unlike closely related GPCRs, protease-activated receptors (PAR1, PAR2, PAR3, and PAR4) have a predicted signal peptide at their N-terminus, which is encoded by a separate exon, suggesting that the signal peptides of PARs may serve an important and unique function, specific for PARs. In this report, we show that the PAR2 signal peptide, when fused to the N-terminus of IgG-Fc, effectively induced IgG-Fc secretion into culture medium, thus behaving like a classical signal peptide. The presence of PAR2 signal peptide has a strong effect on PAR2 cell surface expression, as deletion of the signal peptide (PAR2ΔSP) led to dramatic reduction of the cell surface expression and decreased responses to trypsin or the synthetic peptide ligand (SLIGKV). However, further deletion of the tethered ligand region (SLIGKV) at the N-terminus rescued the cell surface receptor expression and the response to the synthetic peptide ligand, suggesting that the signal peptide of PAR2 may be involved in preventing PAR2 from intracellular protease activation before reaching the cell surface. Supporting this hypothesis, an Arg36Ala mutation on PAR2ΔSP, which disabled the trypsin activation site, increased the receptor cell surface expression and the response to ligand stimulation. Similar effects were observed when PAR2ΔSP expressing cells were treated with protease inhibitors. Our findings indicated that there is a role of the PAR2 signal peptide in preventing the premature activation of PAR2 from intracellular protease cleavage before reaching the cells surface. The same mechanism may also apply to PAR1, PAR3, and PAR4.

Spontaneous Glycan Reattachment Following N-Glycanase Treatment of Influenza and HIV Vaccine Antigens

In cells, asparagine/N-linked glycans are added to glycoproteins cotranslationally, in an attachment process that supports proper folding of the nascent polypeptide. We found that following pruning of N-glycan by the amidase PNGase F, the principal influenza vaccine antigen and major viral spike protein hemagglutinin (HA) spontaneously reattached N-glycan to its de-N-glycosylated positions when the amidase was removed from solution. This reaction, which we term N-glycanation, was confirmed by site-specific analysis of HA glycoforms by mass spectrometry prior to PNGase F exposure, during exposure to PNGase F, and after amidase removal. Iterative rounds of de-N-glycosylation followed by N-glycanation could be repeated at least three times and were observed for other viral glycoproteins/vaccine antigens, including the envelope glycoprotein (Env) from HIV. Covalent N-glycan reattachment was nonenzymatic as it occurred in the presence of metal ions that inhibit PNGase F activity. Rather, N-glycanation relied on a noncovalent assembly between protein and glycan, formed in the presence of the amidase, where linearization of the glycoprotein prevented this retention and subsequent N-glycanation. This reaction suggests that under certain experimental conditions, some glycoproteins can organize self-glycan addition, highlighting a remarkable self-assembly principle that may prove useful for re-engineering therapeutic glycoproteins such as influenza HA or HIV Env, where glycan sequence and structure can markedly affect bioactivity and vaccine efficacy.

Targeting STT3A-oligosaccharyltransferase with NGI-1 causes herpes simplex virus 1 dysfunction

Herpes simplex virus 1 (HSV-1) is a contagious neurotropic herpesvirus responsible for oral lesions and herpesviral encephalitis. The HSV-1 envelope contains N-glycosylated proteins involved in infection and that are candidate drug targets. NGI-1 is a small-molecule inhibitor of oligosaccharyltransferase (OST) complexes STT3A-OST and STT3B-OST, which catalyze cotranslational and post-translational N-glycosylation, respectively. Because host OSTs attach HSV-1 glycans, NGI-1 might have anti-HSV-1 activity. We evaluated HSV-1 function using NGI-1 and human embryonic kidney 293 knockout lines for OST isoform-specific catalytic and accessory subunits. N-glycosylation of 2 representative envelope proteins (gC and gD) was primarily dependent upon STT3A-OST, but to a large extent replaceable by STT3B-OST. Knockouts impairing STT3A- or STT3B-OST activity, by themselves, did not appreciably affect HSV-1 function (plaque-forming units, normalized to viral particles measured by unglycosylated capsid protein VP5 content). However, with cells lacking STT3B-OST activity (missing the catalytic subunit STT3B or the oxidoreductase subunits magnesium transporter 1/tumor suppressor candidate 3) and thus solely dependent upon STT3A-OST for N-glycosylation, NGI-1 treatment resulted in HSV-1 having cell type-dependent dysfunction (affecting infectivity with Vero cells much more than with the 293 lines). Ablation of post-translational N-glycosylation can therefore make HSV-1 infectivity, and possibly masking of immunogenic peptide epitopes by glycans, highly sensitive to pharmacological inhibition of cotranslational N-glycosylation.-Lu, H., Cherepanova, N. A., Gilmore, R., Contessa, J. N., Lehrman, M. A. Targeting STT3A-oligosaccharyltransferase with NGI-1 causes herpes simplex virus 1 dysfunction.