Keratinocyte-associated protein 2 is a bona fide subunit of the mammalian oligosaccharyltransferase

Tandem HILIC-IMAC strategy for simultaneous N-glycoproteomics and phosphoproteomics in aging mouse brain

Abnormal glycosylation and phosphorylation are strongly associated with brain aging. N-glycosylation and phosphorylation are closely involved in pathological processes in a crosstalk dependent manner. However, simultaneous characterization of glycosylation and phosphorylation together in aging brain was uncommon. Herein we developed a novel tandem HILIC-IMAC strategy for simultaneous analysis of N-glycoproteomics and phosphoproteomics. This tandem method showed higher enrichment repeatability, more identifications of glycopeptides and phosphopeptides, and lower overlap. Application of the established method to mouse brain at two ages (8 weeks and 65 weeks) to explore changes in glycosylation and phosphorylation during aging. Up to 10,990 N-glycopeptides and 11,409 phosphopeptides were identified from mouse brain. Among these, differentially expressed phosphoproteins were involved in regulation of microtubule depolymerization, synapse, and transmission of nerve impulse. And glycoproteins differentially expressed with age were mostly related to cell adhesion processes and extracellular matrix. Furthermore, we found the opposite expression trends in glycosylation and phosphorylation during aging on Grin2b. Together, the HILIC-IMAC strategy has potential to discover aging biomarkers and analyze complex biosamples, paving the way for in-depth investigations for the changes of protein glycosylation and phosphorylation in aging brain.

Therapy-induced senescent cancer cells contribute to cancer progression by promoting ribophorin 1-dependent PD-L1 upregulation

Conventional chemotherapy- and radiotherapy-induced cancer senescence, which is characterized by poor proliferation, drug resistance, and senescence-associated secretory phenotype, has gained attention as contributing to cancer relapse and the development of an immunosuppressive tumor microenvironment. However, the association between cancer senescence and anti-tumor immunity is not fully understood. Here, we demonstrate that senescent cancer cells increase the level of PD-L1 by promoting its transcription and glycosylation. We identify ribophorin 1 as a key regulator of PD-L1 glycosylation during cancer senescence. Ribophorin 1 depletion reduces this elevated level of PD-L1 through the ER-lysosome-associated degradation pathway, thereby increasing the susceptibility of senescent cancer cells to T-cell-mediated killing. Consistently, ribophorin 1 depletion suppresses tumor growth by decreasing PD-L1 levels and boosting cytotoxic T lymphocyte activity in male mice. Moreover, ribophorin 1-targeted or anti-PD-1 therapy reduces the number of senescent cancer cells in irradiated tumors and suppresses cancer recurrence through the activation of cytotoxic T lymphocytes. These results provide crucial insights into how senescent cancer cells can escape T-cell immunity following cancer treatment and thereby contribute to cancer recurrence. Our findings also highlight the therapeutic promise of targeting senescent cancer cells for cancer treatment.

Induction of oxidative- and endoplasmic-reticulum-stress dependent apoptosis in pancreatic cancer cell lines by DDOST knockdown

The dolichyl-diphosphooligosaccharide-protein glycosyltransferase non-catalytic subunit (DDOST) is a key component of the oligosaccharyltransferase complex catalyzing N-linked glycosylation in the endoplasmic reticulum lumen. DDOST is associated with several cancers and congenital disorders of glycosylation. However, its role in pancreatic cancer remains elusive, despite its enriched pancreatic expression. Using quantitative mass spectrometry, we identify 30 differentially expressed proteins and phosphopeptides (DEPs) after DDOST knockdown in the pancreatic ductal adenocarcinoma (PDAC) cell line PA-TU-8988T. We evaluated DDOST / DEP protein-protein interaction networks using STRING database, correlation of mRNA levels in pancreatic cancer TCGA data, and biological processes annotated to DEPs in Gene Ontology database. The inferred DDOST regulated phenotypes were experimentally verified in two PDAC cell lines, PA-TU-8988T and BXPC-3. We found decreased proliferation and cell viability after DDOST knockdown, whereas ER-stress, ROS-formation and apoptosis were increased. In conclusion, our results support an oncogenic role of DDOST in PDAC by intercepting cell stress events and thereby reducing apoptosis. As such, DDOST might be a potential biomarker and therapeutic target for PDAC.

Upregulation of ribosome complexes at the blood-brain barrier in Alzheimer's disease patients

The cerebrovascular-specific molecular mechanism in Alzheimer's disease (AD) was investigated by employing comprehensive and accurate quantitative proteomics. Highly purified brain capillaries were isolated from cerebral gray and white matter of four AD and three control donors, and examined by SWATH (sequential window acquisition of all theoretical fragment ion spectra) proteomics. Of the 29 ribosomal proteins that were quantified, 28 (RPLP0, RPL4, RPL6, RPL7A, RPL8, RPL10A, RPL11, RPL12, RPL14, RPL15, RPL18, RPL23, RPL27, RPL27A, RPL31, RPL35A, RPS2, RPS3, RPS3A, RPS4X, RPS7, RPS8, RPS14, RPS16, RPS20, RPS24, RPS25, and RPSA) were significantly upregulated in AD patients. This upregulation of ribosomal protein expression occurred only in brain capillaries and not in brain parenchyma. The protein expression of protein processing and N-glycosylation-related proteins in the endoplasmic reticulum (DDOST, STT3A, MOGS, GANAB, RPN1, RPN2, SEC61B, UGGT1, LMAN2, and SSR4) were also upregulated in AD brain capillaries and was correlated with the expression of ribosomal proteins. The findings reported herein indicate that the ribosome complex, the subsequent protein processing and N-glycosylation-related processes are significantly and specifically upregulated in the brain capillaries of AD patients.

Emerging roles of ER-resident selenoproteins in brain physiology and physiopathology

The brain has a very high oxygen consumption rate and is particularly sensitive to oxidative stress. It is also the last organ to suffer from a loss of selenium (Se) in case of deficiency. Se is a crucial trace element present in the form of selenocysteine, the 21st proteinogenic amino acid present in selenoproteins, an essential protein family in the brain that participates in redox signaling. Among the most abundant selenoproteins in the brain are glutathione peroxidase 4 (GPX4), which reduces lipid peroxides and prevents ferroptosis, and selenoproteins W, I, F, K, M, O and T. Remarkably, more than half of them are proteins present in the ER and recent studies have shown their involvement in the maintenance of ER homeostasis, glycoprotein folding and quality control, redox balance, ER stress response signaling pathways and Ca2+ homeostasis. However, their molecular functions remain mostly undetermined. The ER is a highly specialized organelle in neurons that maintains the physical continuity of axons over long distances through its continuous distribution from the cell body to the nerve terminals. Alteration of this continuity can lead to degeneration of distal axons and subsequent neuronal death. Elucidation of the function of ER-resident selenoproteins in neuronal pathophysiology may therefore become a new perspective for understanding the pathophysiology of neurological diseases. Here we summarize what is currently known about each of their molecular functions and their impact on the nervous system during development and stress.

Enhanced metabolism and negative regulation of ER stress support higher erythropoietin production in HEK293 cells

Recombinant protein production can cause severe stress on cellular metabolism, resulting in limited titer and product quality. To investigate cellular and metabolic characteristics associated with these limitations, we compare HEK293 clones producing either erythropoietin (EPO) (secretory) or GFP (non-secretory) protein at different rates. Transcriptomic and functional analyses indicate significantly higher metabolism and oxidative phosphorylation in EPO producers compared with parental and GFP cells. In addition, ribosomal genes exhibit specific expression patterns depending on the recombinant protein and the production rate. In a clone displaying a dramatically increased EPO secretion, we detect higher gene expression related to negative regulation of endoplasmic reticulum (ER) stress, including upregulation of ATF6B, which aids EPO production in a subset of clones by overexpression or small interfering RNA (siRNA) knockdown. Our results offer potential target pathways and genes for further development of the secretory power in mammalian cell factories.

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.

An alternative pathway for membrane protein biogenesis at the endoplasmic reticulum

The heterotrimeric Sec61 complex is a major site for the biogenesis of transmembrane proteins (TMPs), accepting nascent TMP precursors that are targeted to the endoplasmic reticulum (ER) by the signal recognition particle (SRP). Unlike most single-spanning membrane proteins, the integration of type III TMPs is completely resistant to small molecule inhibitors of the Sec61 translocon. Using siRNA-mediated depletion of specific ER components, in combination with the potent Sec61 inhibitor ipomoeassin F (Ipom-F), we show that type III TMPs utilise a distinct pathway for membrane integration at the ER. Hence, following SRP-mediated delivery to the ER, type III TMPs can uniquely access the membrane insertase activity of the ER membrane complex (EMC) via a mechanism that is facilitated by the Sec61 translocon. This alternative EMC-mediated insertion pathway allows type III TMPs to bypass the Ipom-F-mediated blockade of membrane integration that is seen with obligate Sec61 clients.

Selenoprotein T: An Essential Oxidoreductase Serving as a Guardian of Endoplasmic Reticulum Homeostasis

Significance: Selenoproteins incorporate the essential nutrient selenium into their polypeptide chain. Seven members of this family reside in the endoplasmic reticulum (ER), the exact function of most of which is poorly understood. Especially, how ER-resident selenoproteins control the ER redox and ionic environment is largely unknown. Since alteration of ER function is observed in many diseases, the elucidation of the role of selenoproteins could enhance our understanding of the mechanisms involved in ER homeostasis. Recent Advances: Among selenoproteins, selenoprotein T (SELENOT) is remarkable as the most evolutionarily conserved and the only ER-resident selenoprotein whose gene knockout in mouse is lethal. Recent data indicate that SELENOT contributes to ER homeostasis: reduced expression of SELENOT in transgenic cell and animal models promotes accumulation of reactive oxygen and nitrogen species, depletion of calcium stores, activation of the unfolded protein response and impaired hormone secretion. Critical Issues: SELENOT is anchored to the ER membrane and associated with the oligosaccharyltransferase complex, suggesting that it regulates the early steps of N-glycosylation. Furthermore, it exerts a selenosulfide oxidoreductase activity carried by its thioredoxin-like domain. However, the physiological role of the redox activity of SELENOT is not fully understood. Likewise, the nature of its redox partners needs to be further characterized. Future Directions: Given the impact of ER stress in pathologies such as neurodegenerative, cardiovascular, metabolic and immune diseases, understanding the role of SELENOT and developing derived therapeutic tools such as selenopeptides to improve ER proteostasis and prevent ER stress could contribute to a better management of these diseases.

Intramembrane protease RHBDL4 cleaves oligosaccharyltransferase subunits to target them for ER-associated degradation

The endoplasmic reticulum (ER)-resident intramembrane rhomboid protease RHBDL4 generates metastable protein fragments and together with the ER-associated degradation (ERAD) machinery provides a clearance mechanism for aberrant and surplus proteins. However, the endogenous substrate spectrum and with that the role of RHBDL4 in physiological ERAD is mainly unknown. Here, we use a substrate trapping approach in combination with quantitative proteomics to identify physiological RHBDL4 substrates. This revealed oligosaccharyltransferase (OST) complex subunits such as the catalytic active subunit STT3A as substrates for the RHBDL4-dependent ERAD pathway. RHBDL4-catalysed cleavage inactivates OST subunits by triggering dislocation into the cytoplasm and subsequent proteasomal degradation. RHBDL4 thereby controls the abundance and activity of OST, suggesting a novel link between the ERAD machinery and glycosylation tuning.

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.

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.

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.

Analysis of substrate specificity of Trypanosoma brucei oligosaccharyltransferases (OSTs) by functional expression of domain-swapped chimeras in yeast

N-Linked protein glycosylation is an essential and highly conserved post-translational modification in eukaryotes. The transfer of a glycan from a lipid-linked oligosaccharide (LLO) donor to the asparagine residue of a nascent polypeptide chain is catalyzed by an oligosaccharyltransferase (OST) in the lumen of the endoplasmic reticulum (ER). Trypanosoma brucei encodes three paralogue single-protein OSTs called TbSTT3A, TbSTT3B, and TbSTT3C that can functionally complement the Saccharomyces cerevisiae OST, making it an ideal experimental system to study the fundamental properties of OST activity. We characterized the LLO and polypeptide specificity of all three TbOST isoforms and their chimeric forms in the heterologous expression host S. cerevisiae where we were able to apply yeast genetic tools and newly developed glycoproteomics methods. We demonstrated that TbSTT3A accepted LLO substrates ranging from Man₅GlcNAc₂ to Man₇GlcNAc₂ In contrast, TbSTT3B required more complex precursors ranging from Man₆GlcNAc₂ to Glc₃Man₉GlcNAc₂ structures, and TbSTT3C did not display any LLO preference. Sequence differences between the isoforms cluster in three distinct regions. We have swapped the individual regions between different OST proteins and identified region 2 to influence the specificity toward the LLO and region 1 to influence polypeptide substrate specificity. These results provide a basis to further investigate the molecular mechanisms and contribution of single amino acids in OST interaction with its substrates.

Selenoprotein T is a novel OST subunit that regulates UPR signaling and hormone secretion

Selenoprotein T (SelT) is a recently characterized thioredoxin-like protein whose expression is very high during development, but is confined to endocrine tissues in adulthood where its function is unknown. We report here that SelT is required for adaptation to the stressful conditions of high hormone level production in endocrine cells. Using immunofluorescence and TEM immunogold approaches, we find that SelT is expressed at the endoplasmic reticulum membrane in all hormone-producing pituitary cell types. SelT knockdown in corticotrope cells promotes unfolded protein response (UPR) and ER stress and lowers endoplasmic reticulum-associated protein degradation (ERAD) and hormone production. Using a screen in yeast for SelT-membrane protein interactions, we sort keratinocyte-associated protein 2 (KCP2), a subunit of the protein complex oligosaccharyltransferase (OST). In fact, SelT interacts not only with KCP2 but also with other subunits of the A-type OST complex which are depleted after SelT knockdown leading to POMC N-glycosylation defects. This study identifies SelT as a novel subunit of the A-type OST complex, indispensable for its integrity and for ER homeostasis, and exerting a pivotal adaptive function that allows endocrine cells to properly achieve the maturation and secretion of hormones.

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.

Mammalian cells lacking either the cotranslational or posttranslocational oligosaccharyltransferase complex display substrate-dependent defects in asparagine linked glycosylation

Asparagine linked glycosylation of proteins is an essential protein modification reaction in most eukaryotic organisms. Metazoan organisms express two oligosaccharyltransferase complexes that are composed of a catalytic subunit (STT3A or STT3B) assembled with a shared set of accessory subunits and one to two complex specific subunits. siRNA mediated knockdowns of STT3A and STT3B in HeLa cells have shown that the two OST complexes have partially non-overlapping roles in N-linked glycosylation. However, incomplete siRNA mediated depletion of STT3A or STT3B reduces the impact of OST complex loss, thereby complicating the interpretation of experimental results. Here, we have used the CRISPR/Cas9 gene editing technology to create viable HEK293 derived cells lines that are deficient for a single catalytic subunit (STT3A or STT3B) or two STT3B-specific accessory subunits (MagT1 and TUSC3). Analysis of protein glycosylation in the STT3A, STT3B and MagT1/TUSC3 null cell lines revealed that these cell lines are superior tools for investigating the in vivo role and substrate preferences of the STT3A and STT3B complexes.

Reduced expression of the oligosaccharyltransferase exacerbates protein hypoglycosylation in cells lacking the fully assembled oligosaccharide donor

A defect in the assembly of the oligosaccharide donor (Dol-PP-GlcNAc(2)Man(9)Glc(3)) for N-linked glycosylation causes hypoglycosylation of proteins by the oligosaccharyltransferase (OST). Mammalian cells express two OST complexes that have different catalytic subunits (STT3A or STT3B). We monitored glycosylation of proteins in asparagine-linked glycosylation 6 (ALG6) deficient cell lines that assemble Dol-PP-GlcNAc(2)Man(9) as the largest oligosaccharide donor. Based upon pulse labeling experiments, 30-40% of STT3A-dependent glycosylation sites and 20% of STT3B-dependent sites are skipped in ALG6-congenital disorders of glycosylation fibroblasts supporting previous evidence that the STT3B complex has a relaxed preference for the fully assembled oligosaccharide donor. Glycosylation of STT3B-dependent sites was more severely reduced in the ALG6 deficient MI8-5 cell line. Protein immunoblot analysis and RT-PCR revealed that MI8-5 cells express 2-fold lower levels of STT3B than the parental Chinese hamster ovary cells. The combination of reduced expression of STT3B and the lack of the optimal Dol-PP-GlcNAc(2)Man(9)Glc(3) donor synergize to cause very severe hypoglycosylation of proteins in MI8-5 cells. Thus, differences in OST subunit expression can modify the severity of hypoglycosylation displayed by cells with a primary defect in the dolichol oligosaccharide assembly pathway.

Cotranslational and posttranslocational N-glycosylation of proteins in the endoplasmic reticulum

Asparagine linked glycosylation of proteins is an essential protein modification reaction in most eukaryotic organisms. N-linked oligosaccharides are important for protein folding and stability, biosynthetic quality control, intracellular traffic and the physiological function of many N-glycosylated proteins. In metazoan organisms, the oligosaccharyltransferase is composed of a catalytic subunit (STT3A or STT3B) and a set of accessory subunits. Duplication of the catalytic subunit gene allowed cells to evolve OST complexes that act sequentially to maximize the glycosylation efficiency of the large number of proteins that are glycosylated in metazoan organisms. We will summarize recent progress in understanding the mechanism of (a) cotranslational glycosylation by the translocation channel associated STT3A complex, (b) the role of the STT3B complex in mediating cotranslational or posttranslocational glycosylation of acceptor sites that have been skipped by the STT3A complex, and (c) the role of the oxidoreductase MagT1 in STT3B-dependent glycosylation of cysteine-proximal acceptor sites.

Oxidoreductase activity is necessary for N-glycosylation of cysteine-proximal acceptor sites in glycoproteins

Stabilization of protein tertiary structure by disulfides can interfere with glycosylation of acceptor sites (NXT/S) in nascent polypeptides. Here, we show that MagT1, an ER-localized thioredoxin homologue, is a subunit of the STT3B isoform of the oligosaccharyltransferase (OST). The lumenally oriented active site CVVC motif in MagT1 is required for glycosylation of STT3B-dependent acceptor sites including those that are closely bracketed by disulfides or contain cysteine as the internal residue (NCT/S). The MagT1- and STT3B-dependent glycosylation of cysteine-proximal acceptor sites can be reduced by eliminating cysteine residues. The predominant form of MagT1 in vivo is oxidized, which is consistent with transient formation of mixed disulfides between MagT1 and a glycoprotein substrate to facilitate access of STT3B to unmodified acceptor sites. Cotranslational N-glycosylation by the STT3A isoform of the OST, which lacks MagT1, allows efficient modification of acceptor sites in cysteine-rich protein domains before disulfide bond formation. Thus, mammalian cells use two mechanisms to achieve N-glycosylation of cysteine proximal acceptor sites.

Comparative Analysis of Protein Glycosylation Pathways in Humans and the Fungal Pathogen Candida albicans

Protein glycosylation pathways are present in all kingdoms of life and are metabolic pathways found in all the life kingdoms. Despite sharing commonalities in their synthesis, glycans attached to glycoproteins have species-specific structures generated by the presence of different sets of enzymes and acceptor substrates in each organism. In this review, we present a comparative analysis of the main glycosylation pathways shared by humans and the fungal pathogen Candida albicans: N-linked glycosylation, O-linked mannosylation and glycosylphosphatidylinositol-anchorage. The knowledge of similarities and divergences between these metabolic pathways could help find new pharmacological targets for C. albicans infection.

N-linked protein glycosylation in the endoplasmic reticulum

The attachment of glycans to asparagine residues of proteins is an abundant and highly conserved essential modification in eukaryotes. The N-glycosylation process includes two principal phases: the assembly of a lipid-linked oligosaccharide (LLO) and the transfer of the oligosaccharide to selected asparagine residues of polypeptide chains. Biosynthesis of the LLO takes place at both sides of the endoplasmic reticulum (ER) membrane and it involves a series of specific glycosyltransferases that catalyze the assembly of the branched oligosaccharide in a highly defined way. Oligosaccharyltransferase (OST) selects the Asn-X-Ser/Thr consensus sequence on polypeptide chains and generates the N-glycosidic linkage between the side-chain amide of asparagine and the oligosaccharide. This ER-localized pathway results in a systemic modification of the proteome, the basis for the Golgi-catalyzed modification of the N-linked glycans, generating the large diversity of N-glycoproteome in eukaryotic cells. This article focuses on the processes in the ER. Based on the highly conserved nature of this pathway we concentrate on the mechanisms in the eukaryotic model organism Saccharomyces cerevisiae.

OST4 is a subunit of the mammalian oligosaccharyltransferase required for efficient N-glycosylation

The eukaryotic oligosaccharyltransferase (OST) is a membrane-embedded protein complex that catalyses the N-glycosylation of nascent polypeptides in the lumen of the endoplasmic reticulum (ER), a highly conserved biosynthetic process that enriches protein structure and function. All OSTs contain a homologue of the catalytic STT3 subunit, although in many cases this is assembled with several additional components that influence function. In S. cerevisiae, one such component is Ost4p, an extremely small membrane protein that appears to stabilise interactions between subunits of assembled OST complexes. OST4 has been identified as a putative human homologue, but to date neither its relationship to the OST complex, nor its role in protein N-glycosylation, have been directly addressed. Here, we establish that OST4 is assembled into native OST complexes containing either the catalytic STT3A or STT3B isoforms. Co-immunoprecipitation studies suggest that OST4 associates with both STT3 isoforms and with ribophorin I, an accessory subunit of mammalian OSTs. These presumptive interactions are perturbed by a single amino acid change in the transmembrane region of OST4. Using siRNA knockdowns and native gel analysis, we show that OST4 plays an important role in maintaining the stability of native OST complexes. Hence, upon OST4 depletion well-defined OST complexes are partially destabilised and a novel ribophorin I-containing subcomplex can be detected. Strikingly, cells depleted of either OST4 or STT3A show a remarkably similar defect in the N-glycosylation of endogenous prosaposin. We conclude that OST4 most likely promotes co-translational N-glycosylation by stabilising STT3A-containing OST isoforms.

N-linked protein glycosylation in the ER

N-linked protein glycosylation in the endoplasmic reticulum (ER) is a conserved two phase process in eukaryotic cells. It involves the assembly of an oligosaccharide on a lipid carrier, dolichylpyrophosphate and the transfer of the oligosaccharide to selected asparagine residues of polypeptides that have entered the lumen of the ER. The assembly of the oligosaccharide (LLO) takes place at the ER membrane and requires the activity of several specific glycosyltransferases. The biosynthesis of the LLO initiates at the cytoplasmic side of the ER membrane and terminates in the lumen where oligosaccharyltransferase (OST) selects N-X-S/T sequons of polypeptide and generates the N-glycosidic linkage between the side chain amide of asparagine and the oligosaccharide. The N-glycosylation pathway in the ER modifies a multitude of proteins at one or more asparagine residues with a unique carbohydrate structure that is used as a signalling molecule in their folding pathway. In a later stage of glycoprotein processing, the same systemic modification is used in the Golgi compartment, but in this process, remodelling of the N-linked glycans in a protein-, cell-type and species specific manner generates the high structural diversity of N-linked glycans observed in eukaryotic organisms. This article summarizes the current knowledge of the N-glycosylation pathway in the ER that results in the covalent attachment of an oligosaccharide to asparagine residues of polypeptide chains and focuses on the model organism Saccharomyces cerevisiae. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Mixed disulfide formation in vitro between a glycoprotein substrate and yeast oligosaccharyltransferase subunits Ost3p and Ost6p

Oligosaccharyltransferase (OTase) glycosylates selected asparagine residues in secreted and membrane proteins in eukaryotes, and asparagine (N)-glycosylation affects the folding, stability and function of diverse glycoproteins. The range of acceptor protein substrates that are efficiently glycosylated depends on the action of several accessory subunits of OTase, including in yeast the homologous proteins Ost3p and Ost6p. A model of Ost3p and Ost6p function has been proposed in which their thioredoxin-like active site cysteines form transient mixed disulfide bonds with cysteines in substrate proteins to enhance the glycosylation of nearby asparagine residues. We tested aspects of this model with a series of in vitro assays. We developed a whole protein mixed disulfide interaction assay that showed that Ost6p could form mixed disulfide bonds with selected cysteines in pre-reduced yeast Gas1p, a model glycoprotein substrate of Ost3p and Ost6p. A complementary peptide affinity chromatography assay for mixed disulfide bond formation showed that Ost3p could also form mixed disulfide bonds with cysteines in selected reduced tryptic peptides from Gas1p. Together, these assays showed that the thioredoxin-like active sites of Ost3p and Ost6p could form transient mixed disulfide bonds with cysteines in a model substrate glycoprotein, consistent with the function of Ost3p and Ost6p in modulating N-glycosylation substrate selection by OTase in vivo.

Tandem affinity purification in transgenic mouse embryonic stem cells identifies DDOST as a novel PPP1CC2 interacting protein

Members of the PP1 family of protein phosphatases achieve functional diversity through numerous and varied protein-protein interactions. In mammals, there are four PP1 isoforms, the ubiquitously expressed PPP1CA, PPP1CB, and PPP1CC1, and the testis specific splice isoform PPP1CC2. When the mouse Ppp1cc gene is deleted, the only phenotypic consequence is a failure of spermatogenesis in homozygous males. To elucidate the function of the Ppp1cc gene, we sought to identify novel protein-protein interactions. To this end, we have created SBP-3XFLAG-PPP1CC1 and SBP-3XFLAG-PPP1CC2 knock-in mouse embryonic stem cell lines using a gene-trap-based system. Tandem affinity purification using our knock-in cell lines identified 11 significant protein-protein interactions, including nine known PP1 interacting proteins and two additional proteins (ATP5C1 and DDOST). Reciprocal in vitro sedimentation assays confirmed the interaction between PPP1CC2 and DDOST that may have physiological implications in spermatogenesis. Immunolocalization studies revealed that DDOST localized to the nuclear envelope in dissociated spermatogenic cells and persists throughout spermatogenesis. The knock-in system described in this paper can be applied in creating tandem affinity-tagged knock-in embryonic stem cell lines with any gene for which a compatible gene-trap line is available.

The oligosaccharyltransferase subunits OST48, DAD1 and KCP2 function as ubiquitous and selective modulators of mammalian N-glycosylation

Protein N-glycosylation is an essential modification that occurs in all eukaryotes and is catalysed by the oligosaccharyltransferase (OST) in the endoplasmic reticulum. Comparative studies have clearly shown that eukaryotic STT3 proteins alone can fulfil the enzymatic requirements for N-glycosylation, yet in many cases STT3 homologues form stable complexes with a variety of non-catalytic OST subunits. Whereas some of these additional components might play a structural role, others appear to increase or modulate N-glycosylation efficiency for certain precursors. Here, we have analysed the roles of three non-catalytic mammalian OST components by studying the consequences of subunit-specific knockdowns on the stability and enzymatic activity of the OST complex. Our results demonstrate that OST48 and DAD1 are required for the assembly of both STT3A- and STT3B-containing OST complexes. The structural perturbations of these complexes we observe in OST48- and DAD1-depleted cells underlie their pronounced hypoglycosylation phenotypes. Thus, OST48 and DAD1 are global modulators of OST stability and hence N-glycosylation. We show that KCP2 also influences protein N-glycosylation, yet in this case, the effect of its depletion is substrate specific, and is characterised by the accumulation of a novel STT3A-containing OST subcomplex. Our results suggest that KCP2 acts to selectively enhance the OST-dependent processing of specific protein precursors, most likely co-translational substrates of STT3A-containing complexes, highlighting the potential for increased complexity of OST subunit composition in higher eukaryotes.