Extreme C-terminal sites are posttranslocationally glycosylated by the STT3B isoform of the OST

An ATP13A1-assisted topogenesis pathway for folding multi-spanning membrane proteins

Many multi-spanning membrane proteins contain poorly hydrophobic transmembrane domains (pTMDs) protected from phospholipid in mature structure. Nascent pTMDs are difficult for translocon to recognize and insert. How pTMDs are discerned and packed into mature, muti-spanning configuration remains unclear. Here, we report that pTMD elicits a post-translational topogenesis pathway for its recognition and integration. Using six-spanning protein adenosine triphosphate-binding cassette transporter G2 (ABCG2) and cultured human cells as models, we show that ABCG2's pTMD2 can pass through translocon into the endoplasmic reticulum (ER) lumen, yielding an intermediate with inserted yet mis-oriented downstream TMDs. After translation, the intermediate recruits P5A-ATPase ATP13A1, which facilitates TMD re-orientation, allowing further folding and the integration of the remaining lumen-exposed pTMD2. Depleting ATP13A1 or disrupting pTMD-characteristic residues arrests intermediates with mis-oriented and exposed TMDs. Our results explain how a "difficult" pTMD is co-translationally skipped for insertion and post-translationally buried into the final correct structure at the late folding stage to avoid excessive lipid exposure.

N-glycosylation as a eukaryotic protective mechanism against protein aggregation

The tendency for proteins to form aggregates is an inherent part of every proteome and arises from the self-assembly of short protein segments called aggregation-prone regions (APRs). While posttranslational modifications (PTMs) have been implicated in modulating protein aggregation, their direct role in APRs remains poorly understood. In this study, we used a combination of proteome-wide computational analyses and biophysical techniques to investigate the potential involvement of PTMs in aggregation regulation. Our findings reveal that while most PTM types are disfavored near APRs, N-glycosylation is enriched and evolutionarily selected, especially in proteins prone to misfolding. Experimentally, we show that N-glycosylation inhibits the aggregation of peptides in vitro through steric hindrance. Moreover, mining existing proteomics data, we find that the loss of N-glycans at the flanks of APRs leads to specific protein aggregation in Neuro2a cells. Our findings indicate that, among its many molecular functions, N-glycosylation directly prevents protein aggregation in higher eukaryotes.

Structural and mechanistic studies of the N-glycosylation machinery: from lipid-linked oligosaccharide biosynthesis to glycan transfer

N-linked protein glycosylation is a post-translational modification that exists in all domains of life. It involves two consecutive steps: (i) biosynthesis of a lipid-linked oligosaccharide (LLO), and (ii) glycan transfer from the LLO to asparagine residues in secretory proteins, which is catalyzed by the integral membrane enzyme oligosaccharyltransferase (OST). In the last decade, structural and functional studies of the N-glycosylation machinery have increased our mechanistic understanding of the pathway. The structures of bacterial and eukaryotic glycosyltransferases involved in LLO elongation provided an insight into the mechanism of LLO biosynthesis, whereas structures of OST enzymes revealed the molecular basis of sequon recognition and catalysis. In this review, we will discuss approaches used and insight obtained from these studies with a special emphasis on the design and preparation of substrate analogs.

ER chaperones use a protein folding and quality control glyco-code

N-glycans act as quality control tags by recruiting lectin chaperones to assist protein maturation in the endoplasmic reticulum. The location and composition of N-glycans (glyco-code) are key to the chaperone-selection process. Serpins, a class of serine protease inhibitors, fold non-sequentially to achieve metastable active states. Here, the role of the glyco-code in assuring successful maturation and quality control of two human serpins, alpha-1 antitrypsin (AAT) and antithrombin III (ATIII), is described. We find that AAT, which has glycans near its N terminus, is assisted by early lectin chaperone binding. In contrast, ATIII, which has more C-terminal glycans, is initially helped by BiP and then later by lectin chaperones mediated by UGGT reglucosylation. UGGT action is increased for misfolding-prone disease variants, and these clients are preferentially glucosylated on their most C-terminal glycan. Our study illustrates how serpins utilize N-glycan presence, position, and composition to direct their proper folding, quality control, and trafficking.

STT3A-mediated viral N-glycosylation underlies the tumor selectivity of oncolytic virus M1

Oncolytic viruses are emerging as promising anticancer agents. Although the essential biological function of N-glycosylation on viruses are widely accepted, roles of N-glycan and glycan-processing enzyme in oncolytic viral therapy are remain elusive. Here, via cryo-EM analysis, we identified three distinct N-glycans on the envelope of oncolytic virus M1 (OVM) as being necessary for efficient receptor binding. E1-N141-glycan has immediate impact on the binding of MXRA8 receptor, E2-N200-glycan mediates the maturation of E2 from its precursor PE2 which is unable to bind with MXRA8, and E2-N262-glycan slightly promotes receptor binding. The necessity of OVM N-glycans in receptor binding make them indispensable for oncolysis in vitro and in vivo. Further investigations identified STT3A, a key catalytic subunit of oligosaccharyltransferase (OST), as the determinant of OVM N-glycosylation, and STT3A expression in tumor cells is positively correlated with OVM-induced oncolysis. Increased STT3A expression was observed in various solid tumors, pointing to a broad-spectrum anticancer potential of OVM. Collectively, our research supports the importance of STT3A-mediated N-glycosylation in receptor binding and oncolysis of OVM, thus providing a novel predictive biomarker for OVM.

Targeting N-linked Glycosylation for the Therapy of Aggressive Lymphomas

Diffuse large B-cell lymphoma (DLBCL) can be subdivided into the activated B-cell (ABC) and germinal center B cell-like (GCB) subtypes. Self-antigen engagement of B-cell receptors (BCR) in ABC tumors induces their clustering, thereby initiating chronic active signaling and activation of NF-κB and PI3 kinase. Constitutive BCR signaling is essential in some GCB tumors but primarily activates PI3 kinase. We devised genome-wide CRISPR-Cas9 screens to identify regulators of IRF4, a direct transcriptional target of NF-κB and an indicator of proximal BCR signaling in ABC DLBCL. Unexpectedly, inactivation of N-linked protein glycosylation by the oligosaccharyltransferase-B (OST-B) complex reduced IRF4 expression. OST-B inhibition of BCR glycosylation reduced BCR clustering and internalization while promoting its association with CD22, which attenuated PI3 kinase and NF-κB activation. By directly interfering with proximal BCR signaling, OST-B inactivation killed models of ABC and GCB DLBCL, supporting the development of selective OST-B inhibitors for the treatment of these aggressive cancers.

SIGNIFICANCE

DLBCL depends on constitutive BCR activation and signaling. There are currently no therapeutics that target the BCR directly and attenuate its pathologic signaling. Here, we unraveled a therapeutically exploitable, OST-B-dependent glycosylation pathway that drives BCR organization and proximal BCR signaling. This article is highlighted in the In This Issue feature, p. 1749.

A 6-O-endosulfatase activity assay based on synthetic heparan sulfate oligomers

Sulf-2 is an extracellular heparan 6-O-endosulfatase involved in the postsynthetic editing of heparan sulfate (HS), which regulates many important biological processes. The activity of the Sulf-2 and its substrate specificity remain insufficiently characterized in spite of more than two decades of studies of this enzyme. This is due, in part, to the difficulties in the production and isolation of this highly modified protein and due to the lack of well-characterized synthetic substrates for the probing of its catalytic activity. We introduce synthetic HS oligosaccharides to fill this gap, and we use our recombinant Sulf-2 protein to show that a paranitrophenol (pNP)-labeled synthetic oligosaccharide allows a reliable quantification of its enzymatic activity. The substrate and products of the desulfation reaction are separated by ion exchange high-pressure liquid chromatography and quantified by UV absorbance. This simple assay allows the detection of the Sulf-2 activity at high sensitivity (nanograms of the enzyme) and specificity. The method also allowed us to measure the heparan 6-O-endosulfatase activity in biological samples as complex as the secretome of cancer cell lines. Our in vitro measurements show that the N-glycosylation of the Sulf-2 enzyme affects the activity of the enzyme and that phosphate ions substantially decrease the Sulf-2 enzymatic activity. This assay offers an efficient, sensitive, and specific measurement of the heparan 6-O-endosulfatase activity that could open avenues to in vivo activity measurements and improve our understanding of the enzymatic editing of the sulfation of heparan.

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.

Mechanism of Protein Translocation by the Sec61 Translocon Complex

The endoplasmic reticulum (ER) is a major site for protein synthesis, folding, and maturation in eukaryotic cells, responsible for production of secretory proteins and most integral membrane proteins. The universally conserved protein-conducting channel Sec61 complex mediates core steps in these processes by translocating hydrophilic polypeptide segments of client proteins across the ER membrane and integrating hydrophobic transmembrane segments into the membrane. The Sec61 complex associates with several other molecular machines and enzymes to enable substrate engagement with the channel and coordination of protein translocation with translation, protein folding, and/or post-translational modifications. Recent cryo-electron microscopy and functional studies of these translocon complexes have greatly advanced our mechanistic understanding of Sec61-dependent protein biogenesis at the ER. Here, we will review the current models for how the Sec61 channel performs its functions in coordination with partner complexes.

Assaying the Posttranslational Arginylation of Proteins in Cultured Cells

To evaluate the posttranslational arginylation of proteins in vivo, we describe a protocol for studying the ¹⁴C-Arg incorporation into proteins of cells in culture. The conditions determined for this particular modification contemplate both the biochemical requirements of the enzyme ATE1 and the adjustments that allowed the discrimination between posttranslational arginylation of proteins and de novo synthesis. These conditions are applicable for different cell lines or primary cultures, representing an optimal procedure for the identification and the validation of putative ATE1 substrates.

Glycan quality control in and out of the endoplasmic reticulum of mammalian cells

The endoplasmic reticulum (ER) is equipped with multiple quality control systems (QCS) that are necessary for shaping the glycoproteome of eukaryotic cells. These systems facilitate the productive folding of glycoproteins, eliminate defective products, and function as effectors to evoke cellular signaling in response to various cellular stresses. These ER functions largely depend on glycans, which contain sugar-based codes that, when needed, function to recruit carbohydrate-binding proteins that determine the fate of glycoproteins. To ensure their functionality, the biosynthesis of such glycans is therefore strictly monitored by a system that selectively degrades structurally defective glycans before adding them to proteins. This system, which is referred to as the glycan QCS, serves as a mechanism to reduce the risk of abnormal glycosylation under conditions where glycan biosynthesis is genetically or metabolically stalled. On the other hand, glycan QCS increases the risk of global hypoglycosylation by limiting glycan availability, which can lead to protein misfolding and the activation of unfolded protein response to maintaining cell viability or to initiate cell death programs. This review summarizes the current state of our knowledge of the mechanisms underlying glycan QCS in mammals and its physiological and pathological roles in embryogenesis, tumor progression, and congenital disorders associated with abnormal glycosylation.

ERO1 alpha deficiency impairs angiogenesis by increasing N-glycosylation of a proangiogenic VEGFA

N-glycosylation and disulfide bond formation are two essential steps in protein folding that occur in the endoplasmic reticulum (ER) and reciprocally influence each other. Here, to analyze crosstalk between N-glycosylation and oxidation, we investigated how the protein disulfide oxidase ERO1-alpha affects glycosylation of the angiogenic VEGF¹²¹, a key regulator of vascular homeostasis. ERO1 deficiency, while retarding disulfide bond formation in VEGF¹²¹, increased utilization of its single N-glycosylation sequon, which lies close to an intra-polypeptide disulfide bridge, and concomitantly slowed its secretion. Unbiased mass-spectrometric analysis revealed interactions between VEGF¹²¹ and N-glycosylation pathway proteins in ERO1-knockout (KO), but not wild-type cells. Notably, MAGT1, a thioredoxin-containing component of the post-translational oligosaccharyltransferase complex, was a major hit exclusive to ERO1-deficient cells. Thus, both a reduced rate of formation of disulfide bridges, and the increased trapping potential of MAGT1 may increase N-glycosylation of VEGF¹²¹. Extending our investigation to tissues, we observed altered lectin staining of ERO1 KO breast tumor xenografts, implicating ERO1 as a physiologic regulator of protein N-glycosylation. Our study, highlighting the effect of ERO1 loss on N-glycosylation of proteins, is particularly relevant not only to angiogenesis but also to other cancer patho-mechanisms in light of recent findings suggesting a close causal link between alterations in protein glycosylation and cancer development.

Proteome and Glycoproteome Analyses Reveal the Protein N-Linked Glycosylation Specificity of STT3A and STT3B

STT3A and STT3B are the main catalytic subunits of the oligosaccharyltransferase complex (OST-A and OST-B in mammalian cells), which primarily mediate cotranslational and post-translocational N-linked glycosylation, respectively. To determine the specificity of STT3A and STT3B, we performed proteomic and glycoproteomic analyses in the gene knock-out (KO) and wild-type HEK293 cells. In total, 3961 proteins, 4265 unique N-linked intact glycopeptides and 629 glycosites representing 349 glycoproteins were identified from all these cells. Deletion of the STT3A gene had a greater impact on the protein expression than deletion of STT3B, especially on glycoproteins. In addition, total mannosylated N-glycans were reduced and fucosylated N-glycans were increased in STT3A-KO cells, which were caused by the differential expression of glycan-related enzymes. Interestingly, hyperglycosylated proteins were identified in KO cells, and the hyperglycosylation of ENPL was caused by the endoplasmic reticulum (ER) stress due to the STT3A deletion. Furthermore, the increased expression of the ATF6 and PERK indicated that the unfolded protein response also happened in STT3A-KO cells. Overall, the specificity of STT3A and STT3B revealed that defects in the OST subunit not only broadly affect N-linked glycosylation of the protein but also affect protein expression.

Aberrant Cellular Glycosylation May Increase the Ability of Influenza Viruses to Escape Host Immune Responses through Modification of the Viral Glycome

Individuals with metabolic dysregulation of cellular glycosylation often experience severe influenza disease, with a poor immune response to the virus and low vaccine efficacy. Here, we investigate the consequences of aberrant cellular glycosylation for the glycome and the biology of influenza virus. We transiently induced aberrant N-linked glycosylation in cultured cells with an oligosaccharyltransferase inhibitor, NGI-1. Cells treated with NGI-1 produced morphologically unaltered viable influenza virus with sequence-neutral glycosylation changes (primarily reduced site occupancy) in the hemagglutinin and neuraminidase proteins. Hemagglutinin with reduced glycan occupancy required a higher concentration of surfactant protein D (an important innate immunity respiratory tract collectin) for inhibition compared to that with normal glycan occupancy. Immunization of mice with NGI-1-treated virus significantly reduced antihemagglutinin and antineuraminidase titers of total serum antibody and reduced hemagglutinin protective antibody responses. Our data suggest that aberrant cellular glycosylation may increase the risk of severe influenza as a result of the increased ability of glycome-modified influenza viruses to evade the immune response.

The antibody-binding Fc gamma receptor IIIa / CD16a is N-glycosylated with high occupancy at all five sites

The antibody-binding Fc γ receptors (FcγRs) trigger life-saving immune responses and many therapeutic monoclonal antibodies require FcγR engagement for full effect. One proven strategy to improve the efficacy of antibody therapies is to increase receptor binding affinity, in particular binding to FcγRIIIa/CD16a. Currently, affinities are measured using recombinantly-expressed soluble extracellular FcγR domains and CD16a-mediated antibody-dependent immune responses are characterized using cultured cells. It is notable that CD16a is highly processed with multiple N-glycosylation sites, and preventing individual N-glycan modifications affects affinity. Furthermore, multiple groups have demonstrated that CD16a N-glycan composition is variable and composition impacts antibody binding affinity. The level of N-glycosylation at each site is not known though computational prediction indicates low to moderate potential at each site based on primary sequence (40-70%). Here we quantify occupancy of the extracellular domains using complementary mass spectrometry-based methods. All five sites of the tighter-binding CD16a V158 allotype showed 65-100% N-glycan occupancy in proteomics-based experiments. These observations were confirmed using intact protein mass spectrometry that demonstrated the predominant species corresponded to CD16a V158 with five N-glycans, with a smaller contribution from CD16a with four N-glycans. Occupancy was likewise high for the membrane-bound receptor at all detected N-glycosylation sites using CD16a purified from cultured human natural killer cells. Occupancy of the N162 site, critical for antibody binding, appeared independent of N169 occupancy based on analysis of the T171A mutant protein. The weaker-binding CD16a F158 allotype showed higher occupancy of >93% at each site.

The Molecular Biodiversity of Protein Targeting and Protein Transport Related to the Endoplasmic Reticulum

Looking at the variety of the thousands of different polypeptides that have been focused on in the research on the endoplasmic reticulum from the last five decades taught us one humble lesson: no one size fits all. Cells use an impressive array of components to enable the safe transport of protein cargo from the cytosolic ribosomes to the endoplasmic reticulum. Safety during the transit is warranted by the interplay of cytosolic chaperones, membrane receptors, and protein translocases that together form functional networks and serve as protein targeting and translocation routes. While two targeting routes to the endoplasmic reticulum, SRP (signal recognition particle) and GET (guided entry of tail-anchored proteins), prefer targeting determinants at the N- and C-terminus of the cargo polypeptide, respectively, the recently discovered SND (SRP-independent) route seems to preferentially cater for cargos with non-generic targeting signals that are less hydrophobic or more distant from the termini. With an emphasis on targeting routes and protein translocases, we will discuss those functional networks that drive efficient protein topogenesis and shed light on their redundant and dynamic nature in health and disease.

A cytosolic reductase pathway is required for efficient N-glycosylation of an STT3B-dependent acceptor site

N-linked glycosylation of proteins entering the secretory pathway is an essential modification required for protein stability and function. Previously, it has been shown that there is a temporal relationship between protein folding and glycosylation, which influences the occupancy of specific glycosylation sites. Here, we used an in vitro translation system that reproduces the initial stages of secretory protein translocation, folding and glycosylation under defined redox conditions. We found that the efficiency of glycosylation of hemopexin was dependent upon a robust NADPH-dependent cytosolic reductive pathway, which could be mimicked by the addition of a membrane-impermeable reducing agent. We identified a hypoglycosylated acceptor site that is adjacent to a cysteine involved in a short-range disulfide. We show that efficient glycosylation at this site is influenced by the cytosolic reductive pathway acting on both STT3A- and STT3B-dependent glycosylation. Our results provide further insight into the important role of the endoplasmic reticulum redox conditions in glycosylation site occupancy and demonstrate a link between redox conditions in the cytosol and glycosylation efficiency.

Substrate specificities and reaction kinetics of the yeast oligosaccharyltransferase isoforms

Oligosaccharyltransferase (OST) catalyzes the central step in N-linked protein glycosylation, the transfer of a preassembled oligosaccharide from its lipid carrier onto asparagine residues of secretory proteins. The prototypic hetero-octameric OST complex from the yeast Saccharomyces cerevisiae exists as two isoforms that contain either Ost3p or Ost6p, both noncatalytic subunits. These two OST complexes have different protein substrate specificities in vivo. However, their detailed biochemical mechanisms and the basis for their different specificities are not clear. The two OST complexes were purified from genetically engineered strains expressing only one isoform. The kinetic properties and substrate specificities were characterized using a quantitative in vitro glycosylation assay with short peptides and different synthetic lipid-linked oligosaccharide (LLO) substrates. We found that the peptide sequence close to the glycosylation sequon affected peptide affinity and turnover rate. The length of the lipid moiety affected LLO affinity, while the lipid double-bond stereochemistry had a greater influence on LLO turnover rates. The two OST complexes had similar affinities for both the peptide and LLO substrates but showed significantly different turnover rates. These data provide the basis for a functional analysis of the Ost3p and Ost6p subunits.

Vaccines and Therapeutics for COVID-19 - How Can Understanding SARS-CoV-2 Glycosylation Lead to Pharmaceutical Advances?

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected more than 160 million people worldwide. Researchers have targeted the SARS-CoV-2 structural proteins to better combat the pandemic. Of the four structural proteins, spike (S), membrane (M), envelope (E) and nucleocapsid (N), the S, M and E proteins are glycosylated whereas the N protein is phosphorylated. The glycosylation of the S protein has been reported previously by multiple research groups, and this knowledge has assisted the pharmaceutical industry in developing vaccines and treatment options. In the United States, there are currently three approved COVID-19 vaccines. All three of these vaccines use the S protein to teach host cells how to react when SARS-CoV-2 particles are present. Treatment options utilizing antivirals and immunosuppressants are being developed in addition to vaccines. Different treatment approaches are needed based on the severity of COVID-19 infection. The therapeutic options currently available are not derived through the direct knowledge on SARS-CoV-2 glycosylation. However, more research on the glycosylation of the structural proteins and how this effects SARS-CoV-2 and host cell binding could lead to new and more effective therapeutics. Herein we outline the current vaccine and therapeutic options against COVID-19 available to the public, as well as those still in development.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

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.

Deducing the N- and O-glycosylation profile of the spike protein of novel coronavirus SARS-CoV-2

The current emergence of the novel coronavirus pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) demands the development of new therapeutic strategies to prevent rapid progress of mortalities. The coronavirus spike (S) protein, which facilitates viral attachment, entry and membrane fusion is heavily glycosylated and plays a critical role in the elicitation of the host immune response. The spike protein is comprised of two protein subunits (S1 and S2), which together possess 22 potential N-glycosylation sites. Herein, we report the glycosylation mapping on spike protein subunits S1 and S2 expressed on human cells through high-resolution mass spectrometry. We have characterized the quantitative N-glycosylation profile on spike protein and interestingly, observed unexpected O-glycosylation modifications on the receptor-binding domain of spike protein subunit S1. Even though O-glycosylation has been predicted on the spike protein of SARS-CoV-2, this is the first report of experimental data for both the site of O-glycosylation and identity of the O-glycans attached on the subunit S1. Our data on the N- and O-glycosylation are strengthened by extensive manual interpretation of each glycopeptide spectra in addition to using bioinformatics tools to confirm the complexity of glycosylation in the spike protein. The elucidation of the glycan repertoire on the spike protein provides insights into the viral binding studies and more importantly, propels research toward the development of a suitable vaccine candidate.

PD-L1 Glycosylation and Its Impact on Binding to Clinical Antibodies

Immune checkpoint inhibitors, including PD-L1/PD-1, are key regulators of the immune response and promising targets in cancer immunotherapy. N-glycosylation of PD-L1 affects its interaction with PD-1, but little is known about the distribution of glycoforms at its four NXS/T sequons. We optimized LC-MS/MS methods using collision energy modulation for the site-specific resolution of specific glycan motifs. We demonstrate that PD-L1 on the surface of breast cancer cell line carries mostly complex glycans with a high proportion of polyLacNAc structures at the N219 sequon. Contrary to the full-length protein, the secreted form of PD-L1 expressed in breast MDA-MB-231 or HEK293 cells demonstrated minimum N219 occupancy and low contribution of the polyLacNAc structures. Molecular modeling of PD-L1/PD-1 interaction with N-glycans suggests that glycans at the N219 site of PD-L1 and N74 and N116 of PD-1 may be involved in glycan-glycan interactions, but the impact of this potential interaction on the protein function remains at this point unknown. The interaction of PD-L1 with clinical antibodies is also affected by glycosylation. In conclusion, PD-L1 expressed in the MDA-MB-231 breast cancer cell line carries polyLacNAc glycans mostly at the N219 sequon, which displays the highest variability in occupancy and is most likely to influence the interaction with PD-1.

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.

Defective glycosylation and multisystem abnormalities characterize the primary immunodeficiency XMEN disease

X-linked immunodeficiency with magnesium defect, EBV infection, and neoplasia (XMEN) disease are caused by deficiency of the magnesium transporter 1 (MAGT1) gene. We studied 23 patients with XMEN, 8 of whom were EBV naive. We observed lymphadenopathy (LAD), cytopenias, liver disease, cavum septum pellucidum (CSP), and increased CD4-CD8-B220-TCRαβ+ T cells (αβDNTs), in addition to the previously described features of an inverted CD4/CD8 ratio, CD4+ T lymphocytopenia, increased B cells, dysgammaglobulinemia, and decreased expression of the natural killer group 2, member D (NKG2D) receptor. EBV-associated B cell malignancies occurred frequently in EBV-infected patients. We studied patients with XMEN and patients with autoimmune lymphoproliferative syndrome (ALPS) by deep immunophenotyping (32 immune markers) using time-of-flight mass cytometry (CyTOF). Our analysis revealed that the abundance of 2 populations of naive B cells (CD20+CD27-CD22+IgM+HLA-DR+CXCR5+CXCR4++CD10+CD38+ and CD20+CD27-CD22+IgM+HLA-DR+CXCR5+CXCR4+CD10-CD38-) could differentially classify XMEN, ALPS, and healthy individuals. We also performed glycoproteomics analysis on T lymphocytes and show that XMEN disease is a congenital disorder of glycosylation that affects a restricted subset of glycoproteins. Transfection of MAGT1 mRNA enabled us to rescue proteins with defective glycosylation. Together, these data provide new clinical and pathophysiological foundations with important ramifications for the diagnosis and treatment of XMEN disease.

N-Glycans on EGF domain-specific O-GlcNAc transferase (EOGT) facilitate EOGT maturation and peripheral endoplasmic reticulum localization

Epidermal growth factor (EGF) domain-specific O-GlcNAc transferase (EOGT) is an endoplasmic reticulum (ER)-resident protein that modifies EGF repeats of Notch receptors and thereby regulates Delta-like ligand-mediated Notch signaling. Several EOGT mutations that may affect putative N-glycosylation consensus sites are recorded in the cancer database, but the presence and function of N-glycans in EOGT have not yet been characterized. Here, we identified N-glycosylation sites in mouse EOGT and elucidated their molecular functions. Three predicted N-glycosylation consensus sequences on EOGT are highly conserved among mammalian species. Within these sites, we found that Asn-263 and Asn-354, but not Asn-493, are modified with N-glycans. Lectin blotting, endoglycosidase H digestion, and MS analysis revealed that both residues are modified with oligomannose N-glycans. Loss of an individual N-glycan on EOGT did not affect its endoplasmic reticulum (ER) localization, enzyme activity, and ability to O-GlcNAcylate Notch1 in HEK293T cells. However, simultaneous substitution of both N-glycosylation sites affected both EOGT maturation and expression levels without an apparent change in enzymatic activity, suggesting that N-glycosylation at a single site is sufficient for EOGT maturation and expression. Accordingly, a decrease in O-GlcNAc stoichiometry was observed in Notch1 co-expressed with an N263Q/N354Q variant compared with WT EOGT. Moreover, the N263Q/N354Q variant exhibited altered subcellular distribution within the ER in HEK293T cells, indicating that N-glycosylation of EOGT is required for its ER localization at the cell periphery. These results suggest critical roles of N-glycans in sustaining O-GlcNAc transferase function both by maintaining EOGT levels and by ensuring its proper subcellular localization in the ER.

Efficient N-Glycosylation of the Heavy Chain Tailpiece Promotes the Formation of Plant-Produced Dimeric IgA

Production of monomeric IgA in mammalian cells and plant expression systems such as Nicotiana benthamiana is well-established and can be achieved by co-expression of the corresponding light and heavy chains. In contrast, the assembly of dimeric IgA requires the additional expression of the joining chain and remains challenging especially in plant-based systems. Here, we examined factors affecting the assembly and expression of HER2 binding dimeric IgA1 and IgA2m(2) variants transiently produced in N. benthamiana. While co-expression of the joining chain resulted in efficient formation of dimeric IgAs in HEK293F cells, a mixture of monomeric, dimeric and tetrameric variants was detected in plants. Mass-spectrometric analysis of site-specific glycosylation revealed that the N-glycan profile differed between monomeric and dimeric IgAs in the plant expression system. Co-expression of a single-subunit oligosaccharyltransferase from the protozoan Leishmania major in N. benthamiana increased the N-glycosylation occupancy at the C-terminal heavy chain tailpiece and changed the ratio of monomeric to dimeric IgAs. Our data demonstrate that N-glycosylation engineering is a suitable strategy to promote the formation of dimeric IgA variants in plants.

Structural Insight into the Mechanism of N-Linked Glycosylation by Oligosaccharyltransferase

Asparagine-linked glycosylation, also known as N-linked glycosylation is an essential and highly conserved post-translational protein modification that occurs in all three domains of life. This modification is essential for specific molecular recognition, protein folding, sorting in the endoplasmic reticulum, cell-cell communication, and stability. Defects in N-linked glycosylation results in a class of inherited diseases known as congenital disorders of glycosylation (CDG). N-linked glycosylation occurs in the endoplasmic reticulum (ER) lumen by a membrane associated enzyme complex called the oligosaccharyltransferase (OST). In the central step of this reaction, an oligosaccharide group is transferred from a lipid-linked dolichol pyrophosphate donor to the acceptor substrate, the side chain of a specific asparagine residue of a newly synthesized protein. The prokaryotic OST enzyme consists of a single polypeptide chain, also known as single subunit OST or ssOST. In contrast, the eukaryotic OST is a complex of multiple non-identical subunits. In this review, we will discuss the biochemical and structural characterization of the prokaryotic, yeast, and mammalian OST enzymes. This review explains the most recent high-resolution structures of OST determined thus far and the mechanistic implication of N-linked glycosylation throughout all domains of life. It has been shown that the ssOST enzyme, AglB protein of the archaeon Archaeoglobus fulgidus, and the PglB protein of the bacterium Campylobactor lari are structurally and functionally similar to the catalytic Stt3 subunit of the eukaryotic OST enzyme complex. Yeast OST enzyme complex contains a single Stt3 subunit, whereas the human OST complex is formed with either STT3A or STT3B, two paralogues of Stt3. Both human OST complexes, OST-A (with STT3A) and OST-B (containing STT3B), are involved in the N-linked glycosylation of proteins in the ER. The cryo-EM structures of both human OST-A and OST-B complexes were reported recently. An acceptor peptide and a donor substrate (dolichylphosphate) were observed to be bound to the OST-B complex whereas only dolichylphosphate was bound to the OST-A complex suggesting disparate affinities of two OST complexes for the acceptor substrates. However, we still lack an understanding of the independent role of each eukaryotic OST subunit in N-linked glycosylation or in the stabilization of the enzyme complex. Discerning the role of each subunit through structure and function studies will potentially reveal the mechanistic details of N-linked glycosylation in higher organisms. Thus, getting an insight into the requirement of multiple non-identical subunits in the N-linked glycosylation process in eukaryotes poses an important future goal.

Comparative and evolutionary analyses of the divergence of plant oligosaccharyltransferase STT3 isoforms

STT3 is a catalytic subunit of hetero-oligomeric oligosaccharyltransferase (OST), which is important for asparagine-linked glycosylation. In mammals and plants, OSTs with different STT3 isoforms exhibit distinct levels of enzymatic efficiency or different responses to stressors. Although two different STT3 isoforms have been identified in both plants and animals, it remains unclear whether these isoforms result from gene duplication in an ancestral eukaryote. Furthermore, the molecular mechanisms underlying the functional divergences between the two STT3 isoforms in plant have not been well elucidated. Here, we conducted phylogenetic analysis of the major evolutionary node species and suggested that gene duplications of STT3 may have occurred independently in animals and plants. Across land plants, the exon-intron structure differed between the two STT3 isoforms, but was highly conserved for each isoform. Most angiosperm STT3a genes had 23 exons with intron phase 0, while STT3b genes had 6 exons with intron phase 2. Characteristic motifs (motif 18 and 19) of STT3s were mapped to different structure domains in the plant STT3 proteins. These two motifs overlap with regions of high nonsynonymous-to-synonymous substitution rates, suggesting the regions may be related to functional difference between STT3a and STT3b. In addition, promoter elements and gene expression profiles were different between the two isoforms, indicating expression pattern divergence of the two genes. Collectively, the identified differences may result in the functional divergence of plant STT3s.

Cryo–electron microscopy structures of human oligosaccharyltransferase complexes OST-A and OST-B

Oligosaccharyltransferase (OST) catalyzes the transfer of a high-mannose glycan onto secretory proteins in the endoplasmic reticulum. Mammals express two distinct OST complexes that act in a cotranslational (OST-A) or posttranslocational (OST-B) manner. Here, we present high-resolution cryo–electron microscopy structures of human OST-A and OST-B. Although they have similar overall architectures, structural differences in the catalytic subunits STT3A and STT3B facilitate contacts to distinct OST subunits, DC2 in OST-A and MAGT1 in OST-B. In OST-A, interactions with TMEM258 and STT3A allow ribophorin-I to form a four-helix bundle that can bind to a translating ribosome, whereas the equivalent region is disordered in OST-B. We observed an acceptor peptide and dolichylphosphate bound to STT3B, but only dolichylphosphate in STT3A, suggesting distinct affinities of the two OST complexes for protein substrates.

Phase-Variable Glycosylation in Nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) is a leading cause of respiratory tract infections worldwide and continues to be a global health burden. Adhesion and colonization of host cells are crucial steps in bacterial pathogenesis, and in many strains of NTHi, the interaction with the host is mediated by the high molecular weight adhesins HMW1A and HMW2A. These adhesins are N-glycoproteins that are modified by cytoplasmic glycosyltransferases HMW1C and HMW2C. Phase variation in the number of short sequence repeats in the promoters of hmw1A and hmw2A directly affects their expression. Here, we report the presence of similar variable repeat elements in the promoters of hmw1C and hmw2C in diverse NTHi isolates. In an ex vivo assay, we systematically altered the substrate and glycosyltransferase expression and showed that both of these factors quantitatively affected the site-specific efficiency of glycosylation on HMW-A. This represents a novel mechanism through which phase variation can generate diversity in the quantitative extent of site-specific post-translational modifications on antigenic surface proteins. Glycosylation occupancy was incomplete at many sites, variable between sites, and generally lower close to the C-terminus of HMW-A. We investigated the causes of this variability. As HMW-C glycosylates HMW-A in the cytoplasm, we tested how secretion affected glycosylation on HMW-A and showed that retaining HMW-A in the cytoplasm indeed increased glycosylation occupancy across the full length of the protein. Site-directed mutagenesis showed that HMW-C had no inherent preference for glycosylating asparagines in NxS or NxT sequons. This work provides key insights into factors contributing to the heterogenous modifications of NTHi HMW-A adhesins, expands knowledge of NTHi population diversity and pathogenic capability, and is relevant to vaccine design for NTHi and related pathogens.

Oligosaccharyltransferase: A Gatekeeper of Health and Tumor Progression

Oligosaccharyltransferase (OST) is a multi-span membrane protein complex that catalyzes the addition of glycans to selected Asn residues within nascent polypeptides in the lumen of the endoplasmic reticulum. This process, termed N-glycosylation, is a fundamental post-translational protein modification that is involved in the quality control, trafficking of proteins, signal transduction, and cell-to-cell communication. Given these crucial roles, N-glycosylation is essential for homeostasis at the systemic and cellular levels, and a deficiency in genes that encode for OST subunits often results in the development of complex genetic disorders. A growing body of evidence has also demonstrated that the expression of OST subunits is cell context-dependent and is frequently altered in malignant cells, thus contributing to tumor cell survival and proliferation. Importantly, a recently developed inhibitor of OST has revealed this enzyme as a potential target for the treatment of incurable drug-resistant tumors. This review summarizes our current knowledge regarding the functions of OST in the light of health and tumor progression, and discusses perspectives on the clinical relevance of inhibiting OST as a tumor treatment.

Comprehensive Interactome Analysis Reveals that STT3B Is Required for N-Glycosylation of Lassa Virus Glycoprotein

Glycoproteins play vital roles in the arenavirus life cycle by facilitating virus entry and participating in the virus budding process. N-glycosylation of GPs is responsible for their proper functioning; however, little is known about the host factors on which the virus depends for this process. In this study, a comprehensive LASV GP interactome was characterized, and further study revealed that STT3B-dependent N-glycosylation was preferentially required by arenavirus GPs and critical for virus infectivity. The two specific thioredoxin subunits of STT3B-OST MAGT1 and TUSC3 were found to be essential for the N-glycosylation of viral GP. NGI-1, a small-molecule inhibitor of OST, also showed a robust inhibitory effect on arenavirus. Our study provides new insights into LASV GP-host interactions and extends the potential targets for the development of novel therapeutics against Lassa fever in the future.

Lassa virus (LASV) is the causative agent of a fatal hemorrhagic fever in humans. The glycoprotein (GP) of LASV mediates viral entry into host cells, and correct processing and modification of GP by host factors is a prerequisite for virus replication. Here, using an affinity purification-coupled mass spectrometry (AP-MS) strategy, 591 host proteins were identified as interactors of LASV GP. Gene ontology analysis was performed to functionally annotate these proteins, and the oligosaccharyltransferase (OST) complex was highly enriched. Functional studies conducted by using CRISPR-Cas9-mediated knockouts showed that STT3A and STT3B, the two catalytically active isoforms of the OST complex, are essential for the propagation of the recombinant arenavirus rLCMV/LASV glycoprotein precursor, mainly via affecting virus infectivity. Knockout of STT3B, but not STT3A, caused hypoglycosylation of LASV GP, indicating a preferential requirement of LASV for the STT3B-OST isoform. Furthermore, double knockout of magnesium transporter 1 (MAGT1) and tumor suppressor candidate 3 (TUSC3), two specific subunits of STT3B-OST, also caused hypoglycosylation of LASV GP and affected virus propagation. Site-directed mutagenesis analysis revealed that the oxidoreductase CXXC active-site motif of MAGT1 or TUSC3 is essential for the glycosylation of LASV GP. NGI-1, a small-molecule OST inhibitor, can effectively reduce virus infectivity without affecting cell viability. The STT3B-dependent N-glycosylation of GP is conserved among other arenaviruses, including both the Old World and New World groups. Our study provided a systematic view of LASV GP-host interactions and revealed the preferential requirement of STT3B for LASV GP N-glycosylation.

IMPORTANCE Glycoproteins play vital roles in the arenavirus life cycle by facilitating virus entry and participating in the virus budding process. N-glycosylation of GPs is responsible for their proper functioning; however, little is known about the host factors on which the virus depends for this process. In this study, a comprehensive LASV GP interactome was characterized, and further study revealed that STT3B-dependent N-glycosylation was preferentially required by arenavirus GPs and critical for virus infectivity. The two specific thioredoxin subunits of STT3B-OST MAGT1 and TUSC3 were found to be essential for the N-glycosylation of viral GP. NGI-1, a small-molecule inhibitor of OST, also showed a robust inhibitory effect on arenavirus. Our study provides new insights into LASV GP-host interactions and extends the potential targets for the development of novel therapeutics against Lassa fever in the future.

Asparagine-linked glycosylation is not directly coupled to protein translocation across the endoplasmic reticulum in Saccharomyces cerevisiae

Mammalian cells express two oligosaccharyltransferase complexes, STT3A and STT3B, that have distinct roles in N-linked glycosylation. The STT3A complex interacts directly with the protein translocation channel to mediate glycosylation of proteins using an N-terminal-to-C-terminal scanning mechanism. N-linked glycosylation of proteins in budding yeast has been assumed to be a cotranslational reaction. We have compared glycosylation of several glycoproteins in yeast and mammalian cells. Prosaposin, a cysteine-rich protein that contains STT3A-dependent glycosylation sites, is poorly glycosylated in yeast cells and STT3A-deficient human cells. In contrast, a protein with extreme C-terminal glycosylation sites was efficiently glycosylated in yeast by a posttranslocational mechanism. Posttranslocational glycosylation was also observed for carboxypeptidase Y-derived reporter proteins that contain closely spaced acceptor sites. A comparison of two recent protein structures indicates that the yeast OST is unable to interact with the yeast heptameric Sec complex via an evolutionarily conserved interface due to occupation of the OST binding site by the Sec63 protein. The efficiency of glycosylation in yeast is not enhanced for proteins that are translocated by the Sec61 or Ssh1 translocation channels instead of the Sec complex. We conclude that N-linked glycosylation and protein translocation are not directly coupled in yeast cells.

Quantitative glycoproteomics reveals new classes of STT3A- and STT3B-dependent N-glycosylation sites

Cherepanova et al. provide quantitative glycoproteomic analyses of human cells that lack either the STT3A or STT3B oligosaccharyltransferase (OST) complex, revealing new classes of STT3A- and STT3B-dependent glycosylation sites and indicating how cooperation between the OST complexes maximizes acceptor site occupancy in cellular glycoproteins.

Human cells express two oligosaccharyltransferase complexes (STT3A and STT3B) with partially overlapping functions. The STT3A complex interacts directly with the protein translocation channel to mediate cotranslational glycosylation, while the STT3B complex can catalyze posttranslocational glycosylation. We used a quantitative glycoproteomics procedure to compare glycosylation of roughly 1,000 acceptor sites in wild type and mutant cells. Analysis of site occupancy data disclosed several new classes of STT3A-dependent acceptor sites including those with suboptimal flanking sequences and sites located within cysteine-rich protein domains. Acceptor sites located in short loops of multi-spanning membrane proteins represent a new class of STT3B-dependent site. Remarkably, the lumenal ER chaperone GRP94 was hyperglycosylated in STT3A-deficient cells, bearing glycans on five silent sites in addition to the normal glycosylation site. GRP94 was also hyperglycosylated in wild-type cells treated with ER stress inducers including thapsigargin, dithiothreitol, and NGI-1.

Oligosaccharyltransferase structures provide novel insight into the mechanism of asparagine-linked glycosylation in prokaryotic and eukaryotic cells

Asparagine-linked (N-linked) glycosylation is one of the most common protein modification reactions in eukaryotic cells, occurring upon the majority of proteins that enter the secretory pathway. X-ray crystal structures of the single subunit OSTs from eubacterial and archaebacterial organisms revealed the location of donor and acceptor substrate binding sites and provided the basis for a catalytic mechanism. Cryoelectron microscopy structures of the octameric yeast OST provided substantial insight into the organization and assembly of the multisubunit oligosaccharyltransferases. Furthermore, the cryoelectron microscopy structure of a complex consisting of a mammalian OST complex, the protein translocation channel and a translating ribosome revealed new insight into the mechanism of cotranslational glycosylation.

Selective inhibition of N-linked glycosylation impairs receptor tyrosine kinase processing

Global inhibition of N-linked glycosylation broadly reduces glycan occupancy on glycoproteins, but identifying how this inhibition functionally impacts specific glycoproteins is challenging. This limits our understanding of pathogenesis in the congenital disorders of glycosylation (CDG). We used selective exo-enzymatic labeling of cells deficient in the two catalytic subunits of oligosaccharyltransferase - STT3A and STT3B - to monitor the presence and glycosylation status of cell surface glycoproteins. We show reduced abundance of two canonical tyrosine receptor kinases - the insulin receptor and insulin-like growth factor 1 receptor (IGF-1R) - at the cell surface in STT3A-null cells, due to decreased N-linked glycan site occupancy and proteolytic processing in combination with increased endoplasmic reticulum localization. Providing cDNA for Golgi-resident proprotein convertase subtilisin/kexin type 5a (PCSK5a) and furin cDNA to wild-type and mutant cells produced under-glycosylated forms of PCSK5a, but not furin, in cells lacking STT3A. Reduced glycosylation of PCSK5a in STT3A-null cells or cells treated with the oligosaccharyltransferase inhibitor NGI-1 corresponded with failure to rescue receptor processing, implying that alterations in the glycosylation of this convertase have functional consequences. Collectively, our findings show that STT3A-dependent inhibition of N-linked glycosylation on receptor tyrosine kinases and their convertases combines to impair receptor processing and surface localization. These results provide new insight into CDG pathogenesis and highlight how the surface abundance of some glycoproteins can be dually impacted by abnormal glycosylation.

Follicle-Stimulating Hormone Glycobiology

FSH glycosylation varies in two functionally important aspects: microheterogeneity, resulting from oligosaccharide structure variation, and macroheterogeneity, arising from partial FSHβ subunit glycosylation. Although advances in mass spectrometry permit extensive characterization of FSH glycan populations, microheterogeneity remains difficult to illustrate, and comparisons between different studies are challenging because no standard format exists for rendering oligosaccharide structures. FSH microheterogeneity is illustrated using a consistent glycan diagram format to illustrate the large array of structures associated with one hormone. This is extended to commercially available recombinant FSH preparations, which exhibit greatly reduced microheterogeneity at three of four glycosylation sites. Macroheterogeneity is demonstrated by electrophoretic mobility shifts due to the absence of FSHβ glycans that can be assessed by Western blotting of immunopurified FSH. Initially, macroheterogeneity was hoped to matter more than microheterogeneity. However, it now appears that both forms of carbohydrate heterogeneity have to be taken into consideration. FSH glycosylation can reduce its apparent affinity for its cognate receptor by delaying initial interaction with the receptor and limiting access to all of the available binding sites. This is followed by impaired cellular signaling responses that may be related to reduced receptor occupancy or biased signaling. To resolve these alternatives, well-characterized FSH glycoform preparations are necessary.

Mutations in MAGT1 lead to a glycosylation disorder with a variable phenotype

Congenital disorders of glycosylation (CDG) are a group of rare metabolic diseases, due to impaired protein and lipid glycosylation. We identified two patients with defective serum transferrin glycosylation and mutations in the MAGT1 gene. These patients present with a phenotype that is mainly characterized by intellectual and developmental disability. MAGT1 has been described to be a subunit of the oligosaccharyltransferase (OST) complex and more specifically of the STT3B complex. However, it was also claimed that MAGT1 is a magnesium (Mg²⁺) transporter. So far, patients with mutations in MAGT1 were linked to a primary immunodeficiency, characterized by chronic EBV infections attributed to a Mg²⁺ homeostasis defect (XMEN). We compared the clinical and cellular phenotype of our two patients to that of an XMEN patient that we recently identified. All three patients have an N-glycosylation defect, as was shown by the study of different substrates, such as GLUT1 and SHBG, demonstrating that the posttranslational glycosylation carried out by the STT3B complex is dysfunctional in all three patients. Moreover, MAGT1 deficiency is associated with an enhanced expression of TUSC3, the homolog protein of MAGT1, pointing toward a compensatory mechanism. Hence, we delineate MAGT1-CDG as a disorder associated with two different clinical phenotypes caused by defects in glycosylation.

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.

An oligosaccharyltransferase from Leishmania major increases the N-glycan occupancy on recombinant glycoproteins produced in Nicotiana benthamiana

N-glycosylation is critical for recombinant glycoprotein production as it influences the heterogeneity of products and affects their biological function. In most eukaryotes, the oligosaccharyltransferase is the central-protein complex facilitating the N-glycosylation of proteins in the lumen of the endoplasmic reticulum (ER). Not all potential N-glycosylation sites are recognized in vivo and the site occupancy can vary in different expression systems, resulting in underglycosylation of recombinant glycoproteins. To overcome this limitation in plants, we expressed LmSTT3D, a single-subunit oligosaccharyltransferase from the protozoan Leishmania major transiently in Nicotiana benthamiana, a well-established production platform for recombinant proteins. A fluorescent protein-tagged LmSTT3D variant was predominately found in the ER and co-located with plant oligosaccharyltransferase subunits. Co-expression of LmSTT3D with immunoglobulins and other recombinant human glycoproteins resulted in a substantially increased N-glycosylation site occupancy on all N-glycosylation sites except those that were already more than 90% occupied. Our results show that the heterologous expression of LmSTT3D is a versatile tool to increase N-glycosylation efficiency in plants.

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

Oligosaccharyltransferases (OSTs) N-glycosylate proteins by transferring oligosaccharides from lipid-linked oligosaccharides (LLOs) to asparaginyl residues of Asn-Xaa-Ser/Thr acceptor sequons. Mammals have OST isoforms with STT3A or STT3B catalytic subunits for cotranslational or posttranslational N-glycosylation, respectively. OSTs also hydrolyze LLOs, forming free oligosaccharides (fOSs). It has been unclear whether hydrolysis is due to one or both OSTs, segregated from N-glycosylation, and/or regulated. Transfer and hydrolysis were assayed in permeabilized HEK293 kidney and Huh7.5.1 liver cells lacking STT3A or STT3B. Transfer by both STT3A-OST and STT3B-OST with synthetic acceptors was robust. LLO hydrolysis by STT3B-OST was readily detected and surprisingly modulated: Without acceptors, STT3B-OST hydrolyzed Glc₃Man₉GlcNAc₂-LLO but not Man₉GlcNAc₂-LLO, yet it hydrolyzed both LLOs with acceptors present. In contrast, LLO hydrolysis by STT3A-OST was negligible. STT3A-OST however may be regulatory, because it suppressed STT3B-OST-dependent fOSs. TREX1, a negative innate immunity factor that diminishes immunogenic fOSs derived from LLOs, acted through STT3B-OST as well. In summary, only STT3B-OST hydrolyzes LLOs, depending upon LLO quality and acceptor site occupancy. TREX1 and STT3A suppress STT3B-OST-dependent fOSs. Without strict kinetic limitations during posttranslational N-glycosylation, STT3B-OST can thus moonlight for LLO hydrolysis. In contrast, the STT3A-OST/translocon complex preserves LLOs for temporally fastidious cotranslational N-glycosylation.

Editing N-Glycan Site Occupancy with Small-Molecule Oligosaccharyltransferase Inhibitors

The oligosaccharyltransferase (OST) is a multisubunit enzyme complex that N-glycosylates proteins in the secretory pathway and is considered to be constitutive and unregulated. However, small-molecule OST inhibitors such as NGI-1 provide a pharmacological approach for regulating N-linked glycosylation. Herein we design cell models with knockout of each OST catalytic subunit (STT3A or STT3B) to screen the activity of NGI-1 and its analogs. We show that NGI-1 targets the function of both STT3A and STT3B and use structure-activity relationships to guide synthesis of catalytic subunit-specific inhibitors. Using this approach, pharmacophores that increase STT3B selectivity are characterized and an STT3B-specific inhibitor is identified. This inhibitor has discrete biological effects on endogenous STT3B target proteins such as COX2 but does not activate the cellular unfolded protein response. Together this work demonstrates that subsets of glycoproteins can be regulated through pharmacologic inhibition of N-linked glycosylation.

Molecular mechanisms of missense mutations that generate ectopic N-glycosylation sites in coagulation factor VIII

N-glycosylation is a common posttranslational modification of secreted and membrane proteins, catalyzed by the two enzymatic isoforms of the oligosaccharyltransferase, STT3A and STT3B. Missense mutations are the most common mutations in inherited diseases; however, missense mutations that generate extra, non-native N-glycosylation sites have not been well characterized. Coagulation factor VIII (FVIII) contains five consensus N-glycosylation sites outside its functionally dispensable B domain. We developed a computer program that identified hemophilia A mutations in FVIII that can potentially create ectopic glycosylation sites. We determined that 18 of these ectopic sites indeed become N-glycosylated. These sites span the domains of FVIII and are primarily associated with a severe disease phenotype. Using STT3A and STT3B knockout cells, we determined that ectopic glycosylation exhibited different degrees of dependence on STT3A and STT3B. By separating the effects of ectopic N-glycosylation from those due to underlying amino acid changes, we showed that ectopic glycans promote the secretion of some mutants, but impair the secretion of others. However, ectopic glycans that enhanced secretion could not functionally replace a native N-glycan in the same domain. Secretion-deficient mutants, but not mutants with elevated secretion levels, show increased association with the endoplasmic reticulum chaperones BiP (immunoglobulin heavy chain-binding protein) and calreticulin. Though secreted to different extents, all studied mutants exhibited lower relative activity than wild-type FVIII. Our results reveal differential impacts of ectopic N-glycosylation on FVIII folding, trafficking and activity, which highlight complex disease-causing mechanisms of FVIII missense mutations. Our findings are relevant to other secreted and membrane proteins with mutations that generate ectopic N-glycans.

The atomic structure of a eukaryotic oligosaccharyltransferase complex

N-glycosylation is a ubiquitous modification of eukaryotic secretory and membrane-bound proteins; about 90% of glycoproteins are N-glycosylated. The reaction is catalyzed by an eight-protein oligosaccharyltransferase complex, OST, embedded in the ER membrane. Our understanding of eukaryotic protein N-glycosylation has been limited due to the lack of high-resolution structures. Here we report a 3.5-Å resolution cryo-EM structure of the Saccharomyces cerevisiae OST, revealing the structures of Ost1–5, Stt3, Wbp1, and Swp1. We found that seven phospholipids mediate many of the inter-subunit interactions, and an Stt3 N-glycan mediates interaction with Wbp1 and Swp1 in the lumen. Ost3 was found to mediate the OST-Sec61 translocon interface, funneling the acceptor peptide towards the OST catalytic site as the nascent peptide emerges from the translocon. The structure provides novel insights into co-translational protein N-glycosylation and may facilitate the development of small-molecule inhibitors targeting this process.

Structural Basis of Protein Asn-Glycosylation by Oligosaccharyltransferases

Glycosylation of asparagine residues is a ubiquitous protein modification. This N-glycosylation is essential in Eukaryotes, but principally nonessential in Prokaryotes (Archaea and Eubacteria), although it facilitates their survival and pathogenicity. In many reviews, Archaea have received far less attention than Eubacteria, but this review will cover the N-glycosylation in the three domains of life. The oligosaccharide chain is preassembled on a lipid-phospho carrier to form a donor substrate, lipid-linked oligosaccharide (LLO). The en bloc transfer of an oligosaccharide from LLO to selected Asn residues in the Asn-X-Ser/Thr (X≠Pro) sequons in a polypeptide chain is catalyzed by a membrane-bound enzyme, oligosaccharyltransferase (OST). Over the last 10 years, the three-dimensional structures of the catalytic subunits of the Stt3/AglB/PglB proteins, with an acceptor peptide and a donor LLO, have been determined by X-ray crystallography, and recently the complex structures with other subunits have been determined by cryo-electron microscopy . Structural comparisons within the same species and across the different domains of life yielded a unified view of the structures and functions of OSTs. A catalytic structure in the TM region accounts for the amide bond twisting, which increases the reactivity of the side-chain nitrogen atom of the acceptor Asn residue in the sequon. The Ser/Thr-binding pocket in the C-terminal domain explains the requirement for hydroxy amino acid residues in the sequon. As expected, the two functional structures are formed by the involvement of short amino acid motifs conserved across the three domains of life.

Construction of green fluorescence protein mutant to monitor STT3B-dependent N-glycosylation

Oligosaccharyltransferases (OSTs) mediate the en bloc transfer of N-glycan intermediates onto the asparagine residue in glycosylation sequons (N-X-S/T, X≠P). These enzymes are typically heteromeric complexes composed of several membrane-associated subunits, in which STT3 is highly conserved as a catalytic core. Metazoan organisms encode two STT3 genes (STT3A and STT3B) in their genome, resulting in the formation of at least two distinct OST isoforms consisting of shared subunits and complex specific subunits. The STT3A isoform of OST primarily glycosylates substrate polypeptides cotranslationally, whereas the STT3B isoform is involved in cotranslational and post-translocational glycosylation of sequons that are skipped by the STT3A isoform. Here, we describe mutant constructs of monomeric enhanced green fluorescent protein (mEGFP), which are susceptible to STT3B-dependent N-glycosylation. The endoplasmic reticulum-localized mEGFP (ER-mEGFP) mutants contained an N-glycosylation sequon at their C-terminus and exhibited increased fluorescence in response to N-glycosylation. Isoform-specific glycosylation of the constructs was confirmed by using STT3A- or STT3B-knockout cell lines. Among the mutant constructs that we tested, the ER-mEGFP mutant containing the N₁₈₅ -C₁₈₆ -T₁₈₇ sequon was the best substrate for the STT3B isoform in terms of glycosylation efficiency and fluorescence change. Our results suggest that the mutant ER-mEGFP is useful for monitoring STT3B-dependent post-translocational N-glycosylation in cells of interest, such as those from putative patients with a congenital disorder of glycosylation.

DC2 and KCP2 mediate the interaction between the oligosaccharyltransferase and the ER translocon

The STT3A isoform of the oligosaccharyltransferase is adjacent to the protein translocation channel to catalyze co-translational N-glycosylation of proteins in the endoplasmic reticulum. Shrimal et al. show that the DC2 and KCP2 subunits of the STT3A isoform of the oligosaccharyltransferase are responsible for mediating the interaction between the STT3A complex and the protein translocation channel to allow co-translational N-glycosylation of proteins.

In metazoan organisms, the STT3A isoform of the oligosaccharyltransferase is localized adjacent to the protein translocation channel to catalyze co-translational N-linked glycosylation of proteins in the endoplasmic reticulum. The mechanism responsible for the interaction between the STT3A complex and the translocation channel has not been addressed. Using genetically modified human cells that are deficient in DC2 or KCP2 proteins, we show that loss of DC2 causes a defect in co-translational N-glycosylation of proteins that mimics an STT3A^−/− phenotype. Biochemical analysis showed that DC2 and KCP2 are responsible for mediating the interaction between the protein translocation channel and the STT3A complex. Importantly, DC2- and KCP2-deficient STT3A complexes are stable and enzymatically active. Deletion mutagenesis revealed that a conserved motif in the C-terminal tail of DC2 is critical for assembly into the STT3A complex, whereas the lumenal loop and the N-terminal cytoplasmic segment are necessary for the functional interaction between the STT3A and Sec61 complexes.

N-glycosylation Triggers a Dual Selection Pressure in Eukaryotic Secretory Proteins

Nearly one third of the eukaryotic proteome traverses the secretory pathway and most of these proteins are N-glycosylated in the lumen of the endoplasmic reticulum. N-glycans fulfill multiple structural and biological functions, and are crucial for productive folding of many glycoproteins. N-glycosylation involves the attachment of an oligosaccharide to selected asparagine residues in the sequence N-X-S/T (X ≠ P), a motif known as an N-glycosylation’sequon’. Mutations that create novel sequons can cause disease due to the destabilizing effect of a bulky N-glycan. Thus, an analogous process must have occurred during evolution, whenever ancestrally cytosolic proteins were recruited to the secretory pathway. Here, we show that during evolution N-glycosylation triggered a dual selection pressure on secretory pathway proteins: while sequons were positively selected in solvent exposed regions, they were almost completely eliminated from buried sites. This process is one of the sharpest evolutionary signatures of secretory pathway proteins, and was therefore critical for the evolution of an efficient secretory pathway.