Localization of plant N‐glycan processing enzymes along the secretory pathway

Abstract

N‐glycosylation is an abundant covalent protein modification in all eukaryotic cells. The biosynthesis and processing of protein N‐linked glycans results from a series of highly co‐ordinated step‐by‐step enzymatic conversions occurring mainly in the endoplasmic reticulum (ER) and Golgi apparatus. N‐glycan processing enzymes are thought to act on cargo glycoproteins in a highly ordered fashion in an assembly line. Thus, the subcellular localization of these enzymes together with their in vivo substrate specificity determines the carbohydrate structures of glycoproteins transported through the secretory pathway. While the substrate specificities of many plant N‐glycan processing enzymes are fairly well characterized, the molecular mechanisms underlying enzyme localization to the ER and Golgi have remained largely elusive so far. This review discusses current data on ER and Golgi localization of plant N‐glycan processing enzymes.     

The transmembrane domain of N –acetylglucosaminyltransferase I is the key determinant for its Golgi subcompartmentation

Golgi‐resident type–II membrane proteins are asymmetrically distributed across the Golgi stack. The intrinsic features of the protein that determine its subcompartment‐specific concentration are still largely unknown. Here, we used a series of chimeric proteins to investigate the contribution of the cytoplasmic, transmembrane and stem region of Nicotiana benthamiana N–acetylglucosaminyltransferase I (GnTI) for its cis/medial‐Golgi localization and for protein–protein interaction in the Golgi. The individual GnTI protein domains were replaced with those from the well‐known trans‐Golgi enzyme α2,6–sialyltransferase (ST) and transiently expressed in Nicotiana benthamiana. Using co‐localization analysis and N–glycan profiling, we show that the transmembrane domain of GnTI is the major determinant for its cis/medial‐Golgi localization. By contrast, the stem region of GnTI contributes predominately to homomeric and heteromeric protein complex formation. Importantly, in transgenic Arabidopsis thaliana, a chimeric GnTI variant with altered sub‐Golgi localization was not able to complement the GnTI‐dependent glycosylation defect. Our results suggest that sequence‐specific features in the transmembrane domain of GnTI account for its steady‐state distribution in the cis/medial‐Golgi in plants, which is a prerequisite for efficient N–glycan processing in vivo.     

Role of the Lectin VIP36 in Post‐ER Quality Control of Human α1‐Antitrypsin

The leguminous‐type (L‐type) lectin VIP36 localizes to the Golgi apparatus and cycles early in the secretory pathway. In vitro, VIP36 binds high‐mannose glycans with a pH optimum of 6.5, a value similar to the luminal pH of the Golgi apparatus. Although the sugar‐binding properties of VIP36 in vitro have been characterized in detail, the function of VIP36 in the intact cell remains unclear as no convincing glycoprotein cargo has been identified. Here, we used yellow fluorescent protein (YFP) fragment complementation to identify luminal interaction partners of VIP36. By screening a human liver cDNA library, we identified the glycoprotein α1‐antitrypsin (α1‐AT) as a cargo of VIP36. The VIP36/α1‐AT complex localized to Golgi and endoplasmic reticulum (ER). In the living cell, VIP36 bound exclusively to the high‐mannose form of α1‐AT. The binding was increased when complex glycosylation was prevented by kifunensine and abolished when the glycosylation sites of α1‐AT were inactivated by mutagenesis. Silencing VIP36 accelerated α1‐AT transport, arguing against a role of VIP36 in anterograde traffic. The complex formed by VIP36 and α1‐AT in the Golgi recycled back to the ER. The combined data are most consistent with a function of VIP36 in post‐ER quality control of α1‐AT.     

Golgi‐mediated Glycosylation Determines Residency of the T2 RNase Rny1p in Saccharomyces cerevisiae

The role of glycosylation in the function of the T2 family of RNases is not well understood. In this work, we examined how glycosylation affects the progression of the T2 RNase Rny1p through the secretory pathway in Saccharomyces cerevisiae. We found that Rny1p requires entering into the ER first to become active and uses the adaptor protein Erv29p for packaging into COPII vesicles and transport to the Golgi apparatus. While inside the ER, Rny1p undergoes initial N‐linked core glycosylation at four sites, N37, N70, N103 and N123. Rny1p transport to the Golgi results in the further attachment of high‐glycans. Whereas modifications with glycans are dispensable for the nucleolytic activity of Rny1p, Golgi‐mediated modifications are critical for its extracellular secretion. Failure of Golgi‐specific glycosylation appears to direct Rny1p to the vacuole as an alternative destination and/or site of terminal degradation. These data reveal a previously unknown function of Golgi glycosylation in a T2 RNase as a sorting and secretion signal .      

Arabidopsis thaliana FLA4 functions as a glycan‐stabilized soluble factor via its carboxy‐proximal Fasciclin 1 domain

Fasciclin‐like arabinogalactan proteins (FLAs) are involved in numerous important functions in plants but the relevance of their complex structure to physiological function and cellular fate is unresolved. Using a fully functional fluorescent version of Arabidopsis thaliana FLA4 we show that this protein is localized at the plasma membrane as well as in endosomes and soluble in the apoplast. FLA4 is likely to be GPI‐anchored, is highly N‐glycosylated and carries two O‐glycan epitopes previously associated with arabinogalactan proteins. The activity of FLA4 was resistant against deletion of the amino‐proximal fasciclin 1 domain and was unaffected by removal of the GPI‐modification signal, a highly conserved N‐glycan or the deletion of predicted O‐glycosylation sites. Nonetheless these structural changes dramatically decreased endoplasmic reticulum (ER)‐exit and plasma membrane localization of FLA4, with N‐glycosylation acting at the level of ER‐exit and O‐glycosylation influencing post‐secretory fate. We show that FLA4 acts predominantly by molecular interactions involving its carboxy‐proximal fasciclin 1 domain and that its amino‐proximal fasciclin 1 domain is required for stabilization of plasma membrane localization. FLA4 functions as a soluble glycoprotein via its carboxy‐proximal Fas1 domain and its normal cellular trafficking depends on N‐ and O‐glycosylation.     

Targeting and post‐translational processing of human α1‐antichymotrypsin in BY‐2 tobacco cultured cells†

The post‐translational processing of human α₁‐antichymotrypsin (AACT) in Bright Yellow‐2 (BY‐2) tobacco cells was assessed in relation to the cellular compartment targeted for accumulation. As determined by pulse‐chase labelling experiments and immunofluorescence microscopy, AACT sent to the vacuole or the endoplasmic reticulum (ER) was found mainly in the culture medium, similar to a secreted form targeted to the apoplast. Unexpectedly, AACT expressed in the cytosol was found in the nucleus under a stable, non‐glycosylated form, in contrast with secreted variants undergoing multiple post‐translational modifications during their transit through the secretory pathway. All secreted forms of AACT were N‐glycosylated, with the presence of complex glycans as observed naturally on human AACT. Proteolytic trimming was also observed for all secreted variants, both during their intracellular transit and after their secretion in the culture medium. Overall, the targeting of human AACT to different compartments of BY‐2 tobacco cells led to the production of two protein products: (i) a stable, non‐glycosylated protein accumulated in the nucleus; and (ii) a heterogeneous mixture of secreted variants resulting from post‐translational N‐glycosylation and proteolytic processing. Overall, these data suggest that AACT is sensitive to resident proteases in the ER, the Golgi and/or the apoplast, and that the production of intact AACT in the plant secretory pathway will require innovative approaches to protect its structural integrity in vivo. Studies are now needed to assess the activity of the different AACT variants, and to identify the molecular determinants for the nuclear localization of AACT expressed in the cytosol.     

A classical NLS and the SUN domain contribute to the targeting of SUN2 to the inner nuclear membrane

Integral membrane proteins of the inner nuclear membrane (INM) are inserted into the endoplasmic reticulum membrane during their biogenesis and are then targeted to their final destination. We have used human SUN2 to delineate features that are required for INM targeting and have identified multiple elements that collectively contribute to the efficient localization of SUN2 to the nuclear envelope (NE). One such targeting element is a classical nuclear localization signal (cNLS) present in the N‐terminal, nucleoplasmic domain of SUN2. A second motif proximal to the cNLS is a cluster of arginines that serves coatomer‐mediated retrieval of SUN2 from the Golgi. Unexpectedly, also the C‐terminal, lumenal SUN domain of SUN2 supports NE localization, showing that targeting elements are not limited to cytoplasmic or transmembrane domains of INM proteins. Together, SUN2 represents the first mammalian INM protein relying on a functional cNLS, a Golgi retrieval signal and a perinuclear domain to mediate targeting to the INM.     

N‐linked glycosylation of a subunit isoforms is critical for vertebrate vacuolar H+‐ATPase (V‐ATPase) biosynthesis

The a subunit of the V₀ membrane‐integrated sector of human V‐ATPase has four isoforms, a1‐a4, with diverse and crucial functions in health and disease. They are encoded by four conserved paralogous genes, and their vertebrate orthologs have positionally conserved N‐glycosylation sequons within the second extracellular loop, EL2, of the a subunit membrane domain. Previously, we have shown directly that the predicted sequon for the a4 isoform is indeed N‐glycosylated. Here we extend our investigation to the other isoforms by transiently transfecting HEK 293 cells to express cDNA constructs of epitope‐tagged human a1‐a3 subunits, with or without mutations that convert Asn to Gln at putative N‐glycosylation sites. Expression and N‐glycosylation were characterized by immunoblotting and mobility shifts after enzymatic deglycosylation, and intracellular localization was determined using immunofluorescence microscopy. All unglycosylated mutants, where predicted N‐glycosylation sites had been eliminated by sequon mutagenesis, showed increased relative mobility on immunoblots, identical to what was seen for wild‐type a subunits after enzymatic deglycosylation. Cycloheximide‐chase experiments showed that unglycosylated subunits were turned over at a higher rate than N‐glycosylated forms by degradation in the proteasomal pathway. Immunofluorescence colocalization analysis showed that unglycosylated a subunits were retained in the ER, and co‐immunoprecipitation studies showed that they were unable to associate with the V‐ATPase assembly chaperone, VMA21. Taken together with our previous a4 subunit studies, these observations show that N‐glycosylation is crucial in all four human V‐ATPase a subunit isoforms for protein stability and ultimately for functional incorporation into V‐ATPase complexes.     

Functional Roles of N‐Linked Glycosylation of Human Matrix Metalloproteinase 9

Matrix metalloproteinase‐9 (MMP‐9) is a secreted endoproteinase with a critical role in the regulation of the extracellular matrix and proteolytic activation of signaling molecules. Human (h)MMP‐9 has two well‐defined N‐glycosylation sites at residues N38 and N120; however, their role has remained mostly unexplored partly because expression of the N‐glycosylation‐deficient N38S has been difficult due to a recently discovered single nucleotide polymorphism‐dependent miRNA‐mediated inhibitory mechanism. hMMP‐9 cDNA encoding amino acid substitutions at residues 38 (modified‐S38, mS38) or 120 (N120S) were created in the background of a miRNA‐binding site disrupted template and expressed by transient transfection. hMMP‐9 harboring a single mS38 replacement secreted well, whereas N120S, or a double mS38/N120S hMMP‐9 demonstrated much reduced secretion. Imaging indicated endoplasmic reticulum (ER) retention of the non‐secreted variants and co‐immunoprecipitation confirmed an enhanced strong interaction between the non‐secreted hMMP‐9 and the ER‐resident protein calreticulin (CALR). Removal of N‐glycosylation at residue 38 revealed an amino acid‐dependent strong interaction with CALR likely preventing unloading of the misfolded protein from the ER chaperone down the normal secretory pathway. As with other glycoproteins, N‐glycosylation strongly regulates hMMP‐9 secretion. This is mediated, however, through a novel mechanism of cloaking an N‐glycosylation‐independent strong interaction with the ER‐resident CALR.      

Mutant SOD1 inhibits ER‐Golgi transport in amyotrophic lateral sclerosis

Cu/Zn‐superoxide dismutase is misfolded in familial and sporadic amyotrophic lateral sclerosis, but it is not clear how this triggers endoplasmic reticulum (ER) stress or other pathogenic processes. Here, we demonstrate that mutant SOD1 (mSOD1) is predominantly found in the cytoplasm in neuronal cells. Furthermore, we show that mSOD1 inhibits secretory protein transport from the ER to Golgi apparatus. ER‐Golgi transport is linked to ER stress, Golgi fragmentation and axonal transport and we also show that inhibition of ER‐Golgi trafficking preceded ER stress, Golgi fragmentation, protein aggregation and apoptosis in cells expressing mSOD1. Restoration of ER‐Golgi transport by over‐expression of coatomer coat protein II subunit Sar1 protected against inclusion formation and apoptosis, thus linking dysfunction in ER‐Golgi transport to cellular pathology. These findings thus link several cellular events in amyotrophic lateral sclerosis into a single mechanism occurring early in mSOD1 expressing cells.

     

LEW3, encoding a putative α‐1,2‐mannosyltransferase (ALG11) in N‐linked glycoprotein,plays vital roles in cell‐wall biosynthesis and the abiotic stress response in Arabidopsis thaliana

N‐linked glycosylation is an essential protein modification that helps protein folding, trafficking and translocation in eukaryotic systems. The initial process for N‐linked glycosylation shares a common pathway with assembly of a dolichol‐linked core oligosaccharide. Here we characterize a new Arabidopsis thaliana mutant lew3 (leaf wilting 3), which has a defect in an α‐1,2‐mannosyltransferase, a homolog of ALG11 in yeast, that transfers mannose to the dolichol‐linked core oligosaccharide in the last two steps on the cytosolic face of the ER in N‐glycan precursor synthesis. LEW3 is localized to the ER membrane and expressed throughout the plant. Mutation of LEW3 caused low‐level accumulation of Man₃GlcNAc₂ and Man₄GlcNAc₂ glycans, structures that are seldom detected in wild‐type plants. In addition, the lew3 mutant has low levels of normal high‐mannose‐type glycans, but increased levels of complex‐type glycans. The lew3 mutant showed abnormal developmental phenotypes, reduced fertility, impaired cellulose synthesis, abnormal primary cell walls, and xylem collapse due to disturbance of the secondary cell walls. lew3 mutants were more sensitive to osmotic stress and abscisic acid (ABA) treatment. Protein N‐glycosylation was reduced and the unfolded protein response was more activated by osmotic stress and ABA treatment in the lew3 mutant than in the wild‐type. These results demonstrate that protein N‐glycosylation plays crucial roles in plant development and the response to abiotic stresses.     

Disturbance of the secretory pathway inMicrasterias denticulata by tunicamycin and cyclopiazonic acid

Summary Both tunicamycin, an inhibitor of N-linked glycosylation of proteins, and cyclopiazonic acid, which inhibits the Ca²⁺-dependent ATPase in the ER, influence the secretory pathway at the ER level and lead to a cessation of cell growth inMicrasterias. Electron microscopical investigations reveal that the mode of action of the two inhibitors differs. While tunicamycin treatment results in a disintegration of the Golgi bodies into small vesicles, cyclopiazonic acid prevents products being supplied from the ER, resulting in the dilatation of ER cisternae and a reduction in the number of Golgi cisternae, combined with a loss of dictyosomal activity. The disturbed cell wall formation under tunicamycin indicates that N-linked glycosylation of proteins is required for normal cell growth inMicrasterias. Moreover, our studies reveal that changes in cytoplasmic free calcium concentration, as a consequence of ATPase inhibition in the ER by cyclopiazonic acid, may inhibit wall material secretion by interrupting the normal ER-dictyosome association.Abbreviations CPA cyclopiazonic acid - ER endoplasmic reticulum - TM tunicamycin     

A cell‐free platform for rapid synthesis and testing of active oligosaccharyltransferases

Protein glycosylation, or the attachment of sugar moieties (glycans) to proteins, is important for protein stability, activity, and immunogenicity. However, understanding the roles and regulations of site‐specific glycosylation events remains a significant challenge due to several technological limitations. These limitations include a lack of available tools for biochemical characterization of enzymes involved in glycosylation. A particular challenge is the synthesis of oligosaccharyltransferases (OSTs), which catalyze the attachment of glycans to specific amino acid residues in target proteins. The difficulty arises from the fact that canonical OSTs are large (>70 kDa) and possess multiple transmembrane helices, making them difficult to overexpress in living cells. Here, we address this challenge by establishing a bacterial cell‐free protein synthesis platform that enables rapid production of a variety of OSTs in their active conformations. Specifically, by using lipid nanodiscs as cellular membrane mimics, we obtained yields of up to 420 μg/ml for the single‐subunit OST enzyme, “Protein glycosylation B” (PglB) from Campylobacter jejuni, as well as for three additional PglB homologs from Campylobacter coli, Campylobacter lari, and Desulfovibrio gigas. Importantly, all of these enzymes catalyzed N‐glycosylation reactions in vitro with no purification or processing needed. Furthermore, we demonstrate the ability of cell‐free synthesized OSTs to glycosylate multiple target proteins with varying N‐glycosylation acceptor sequons. We anticipate that this broadly applicable production method will advance glycoengineering efforts by enabling preparative expression of membrane‐embedded OSTs from all kingdoms of life.

     

Oxysterol‐binding protein recruitment and activity at the endoplasmic reticulum‐Golgi interface are independent of Sac1

Oxysterol‐binding protein (OSBP) localizes to endoplasmic reticulum (ER)‐Golgi contact sites where it transports cholesterol and phosphatidylinositol 4‐phosphate (PI‐4P), and activates lipid transport and biosynthetic activities. The PI‐4P phosphatase Sac1 cycles between the ER and Golgi apparatus where it potentially regulates OSBP activity. Here we examined whether the ER‐Golgi distribution of endogenous or ectopically expressed Sac1 influences OSBP activity. OSBP and Sac1 co‐localized at apparent ER‐Golgi contact sites in response to 25‐hydroxycholesterol (25OH), cholesterol depletion and p38 MAPK inhibitors. A Sac1 mutant that is unable to exit the ER did not localize with OSBP, suggesting that sterol perturbations cause Sac1 transport to the Golgi apparatus. Ectopic expression of Sac1 in the ER or Golgi apparatus, or Sac1 silencing, did not affect OSBP localization to ER‐Golgi contact sites, OSBP‐dependent activation of sphingomyelin synthesis, or cholesterol esterification in the ER. p38 MAPK inhibition and retention of Sac1 in the Golgi apparatus also caused OSBP phosphorylation and OSBP‐dependent activation of sphingomyelin synthesis at ER‐Golgi contacts. These results demonstrate that Sac1 expression in either the ER or Golgi apparatus has a minimal impact on the PI‐4P that regulates OSBP activity or recruitment to contact sites.      

Trafficking of Acetyl‐C16‐Ceramide‐NBD with Long‐Term Stability and No Cytotoxicity into the Golgi Complex

The Golgi complex plays a prominent role in the modification and sorting of lipids and proteins, and is a highly dynamic organelle that is dispersed and rearranged before and after mitosis. Several reagents including 4‐nitrobenzo‐2‐oxa‐1,3‐diazole‐labeled C6‐ceramide (NBD‐C6‐ceramide, a ceramide having an NBD‐bound C6‐N‐acyl chain) and Golgi‐specific proteins that emit fluorescence are used as Golgi markers. In the present study, we synthesized a new ceramide analog, acetyl‐C16‐ceramide‐NBD (a ceramide having an acetylated C‐1 hydroxyl group, C16‐N‐acyl chain, and NBD‐bound C15‐sphingosine), and showed that it preferentially accumulated in the Golgi complex without cytotoxicity for over 24 h. Pathways for cellular uptake and interorganelle trafficking of acetyl‐C16‐ceramide‐NBD were investigated. Acetyl‐C16‐ceramide‐NBD was transported to the Golgi complex via ceramide transport proteins. In contrast to NBD‐C6‐ceramide, acetyl‐C16‐ceramide‐NBD was resistant to ceramide metabolic enzymes such as sphingomyelin synthase and glucosylceramide synthase. Because of its weaker cytotoxicity and resistance to ceramide metabolic enzymes, the localization of the Golgi complex could be observed in acetyl‐C16‐ceramide‐NBD‐labeled cells before and after mitosis.      

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.     

More than just sugars: Conserved oligomeric Golgi complex deficiency causes glycosylation‐independent cellular defects

The conserved oligomeric Golgi (COG) complex controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle trafficking within the Golgi. Human COG defects lead to severe multisystemic diseases known as COG‐congenital disorders of glycosylation (COG‐CDG). To gain better understanding of COG‐CDGs, we compared COG knockout cells with cells deficient to 2 key enzymes, Alpha‐1,3‐mannosyl‐glycoprotein 2‐beta‐N‐acetylglucosaminyltransferase and uridine diphosphate‐glucose 4‐epimerase (GALE), which contribute to proper N‐ and O‐glycosylation. While all knockout cells share similar defects in glycosylation, these defects only account for a small fraction of observed COG knockout phenotypes. Glycosylation deficiencies were not associated with the fragmented Golgi, abnormal endolysosomes, defective sorting and secretion or delayed retrograde trafficking, indicating that these phenotypes are probably not due to hypoglycosylation, but to other specific interactions or roles of the COG complex. Importantly, these COG deficiency specific phenotypes were also apparent in COG7‐CDG patient fibroblasts, proving the human disease relevance of our CRISPR knockout findings. The knowledge gained from this study has important implications, both for understanding the physiological role of COG complex in Golgi homeostasis in eukaryotic cells, and for better understanding human diseases associated with COG/Golgi impairment.      

A missense mutation in the vacuolar protein GOLD36 causes organizational defects in the ER and aberrant protein trafficking in the plant secretory pathway

A central question in cell biology is how the identity of organelles is established and maintained. Here, we report on GOLD36, an EMS mutant identified through a screen for partial displacement of the Golgi marker, ST‐GFP, to other organelles. GOLD36 showed partial distribution of ST‐GFP into a modified endoplasmic reticulum (ER) network, which formed bulges and large skein‐like structures entangling Golgi stacks. GOLD36 showed defects in ER protein export as evidenced by our observations that, besides the partial retention of Golgi markers in the ER, the trafficking of a soluble bulk‐flow marker to the cell surface was also compromised. Using a combination of classical mapping and next‐generation DNA sequencing approaches, we linked the mutant phenotype to a missense mutation of a proline residue in position 80 to a leucine residue in a small endomembrane protein encoded by the gold36 locus ( At1g54030 ). Subcellular localization analyses indicated that GOLD36 is a vacuolar protein and that its mutated form is retained in the ER. Interestingly also, a gold36 knock‐out mutant mirrored the GOLD36 subcellular phenotype. These data indicate that GOLD36 is a protein destined to post‐ER compartments and suggest that its export from the ER is a requirement to ensure steady‐state maintenance of the organelle’s organization and functional activity in relation to other secretory compartments. We speculate that GOLD36 may be a factor that is necessary for ER integrity because of its ability to limit deleterious effects of other secretory proteins on the ER.