Photocross-linking of nascent chains to the STT3 subunit of the oligosaccharyltransferase complex

Subunits of the translocon interact with components of the oligosaccharyl transferase complex

Following initiation of translocation across the membrane of the endoplasmic reticulum via the translocon, polypeptide chains are N-glycosylated by the oligosaccharyl transferase (OT) enzyme complex. Translocation and N-glycosylation are concurrent events and would be expected to require juxtaposition of the translocon and the OT complex. To determine whether any of the subunits of the OT complex and translocon mediate interactions between the two complexes, we initiated a systematic study in budding yeast using the split-ubiquitin approach. Interestingly, the OT subunit Stt3p was found to interact only with Sec61p, whereas another OT subunit, Ost4p, was found to interact with all three components of the translocon, Sec61p, Sbh1p, and Sss1p. The OT subunit Wbp1p was found to interact very strongly with Sec61p and Sbh1p and weakly with Sss1p. Other OT subunits, Ost1p, Ost2p, and Swp1p were found to interact with Sec61p and either Sbh1p or Sss1p. Ost3p exhibited a weak interaction with Sec61p and Sbh1p, whereas Ost5p and Ost6p interacted very weakly with Sec61p and failed to interact with Sbh1p or Sss1p. We were able to confirm these split-ubiquitin findings by a chemical cross-linking technique. Based on our findings using these two techniques, we conclude that the association of these two complexes is stabilized via multiple protein-protein contacts. Based on extrapolation of the structural parameters of the crystal structure of the prokaryotic Sec complex to the eukaryotic complex, we propose a working model to understand the organization of the translocon-OT supercomplex.

Engineering N-linked protein glycosylation with diverse O antigen lipopolysaccharide structures in Escherichia coli

Campylobacter jejuni has a general N-linked protein glycosylation system that can be functionally transferred to Escherichia coli. In this study, we engineered E. coli cells in a way that two different pathways, protein N-glycosylation and lipopolysaccharide (LPS) biosynthesis, converge at the step in which PglB, the key enzyme of the C. jejuni N-glycosylation system, transfers O polysaccharide from a lipid carrier (undecaprenyl pyrophosphate) to an acceptor protein. PglB was the only protein of the bacterial N-glycosylation machinery both necessary and sufficient for the transfer. The relaxed specificity of the PglB oligosaccharyltransferase toward the glycan structure was exploited to create novel N-glycan structures containing two distinct E. coli or Pseudomonas aeruginosa O antigens. PglB-mediated transfer of polysaccharides might be valuable for in vivo production of O polysaccharides-protein conjugates for use as antibacterial vaccines.

The N-X-S/T consensus sequence is required but not sufficient for bacterial N-linked protein glycosylation

In the Gram-negative bacterium Campylobacter jejuni there is a pgl (protein glycosylation) locus-dependent general N-glycosylation system of proteins. One of the proteins encoded by pgl locus, PglB, a homolog of the eukaryotic oligosaccharyltransferase component Stt3p, is proposed to function as an oligosaccharyltransferase in this prokaryotic system. The sequence requirements of the acceptor polypeptide for N-glycosylation were analyzed by reverse genetics using the reconstituted glycosylation of the model protein AcrA in Escherichia coli. As in eukaryotes, the N-X-S/T sequon is an essential but not a sufficient determinant for N-linked protein glycosylation. This conclusion was supported by the analysis of a novel C. jejuni glycoprotein, HisJ. Export of the polypeptide to the periplasm was required for glycosylation. Our data support the hypothesis that eukaryotic and bacterial N-linked protein glycosylation are homologous processes.

Ribophorin I associates with a subset of membrane proteins after their integration at the sec61 translocon

The biosynthesis of membrane proteins at the endoplasmic reticulum (ER) involves the integration of the polypeptide at the Sec61 translocon together with a number of maturation events, such as N-glycosylation and signal sequence cleavage, that can occur both during and after synthesis. To better understand the events occurring after the release of the nascent chain from the ER translocon, we investigated the ER components adjacent to the transmembrane-spanning domain of a well characterized fragment of the amyloid precursor protein. Using individual cysteine residues as site-specific cross-linking targets, we found that several ER components can be cross-linked to the fully integrated polypeptide. We identified strong adducts with both the ribophorin I subunit of the oligosaccharyltransferase complex and the 25-kDa subunit of the signal peptidase complex. Focusing on the association with ribophorin I, we found that adduct formation occurred exclusively after the exit of the nascent chain from the Sec61 translocon and was unaffected by the N-glycosylation status of the associated precursor. Only a subset of newly made membrane proteins associated with ribophorin I in vitro, and we could recapitulate a specific association between the amyloid precursor protein fragment and ribophorin I in vivo. Taken together, our data suggest a model where ribophorin I may function to retain potential substrates in close proximity to the catalytic subunit of the oligosaccharyltransferase and thereby stochastically improve the efficiency of the N-glycosylation reaction in vivo. Alternatively ribophorin I may be multifunctional and facilitate additional processes, for example, ER quality control.

Studies on the N-glycosylation of the subunits of oligosaccharyl transferase in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, oligosaccharyl transferase (OT) consists of nine different subunits. Three of the essential gene products, Ost1p, Wbp1p, and Stt3p, are N-linked glycoproteins. To study the function of the N-glycosylation of these proteins, we prepared single or multiple N-glycosylation site mutations in each of them. We established that the four potential N-glycosylation sites in Ost1p and the two potential N-glycosylation sites in Wbp1p were occupied in the mature proteins. Interestingly, none of the N-glycosylation sites in these two proteins was conserved, and no defect in growth or OT activity was observed when the N-glycosylation sites were mutated to block N-glycosylation in either subunit. However, in the third glycosylated subunit, Stt3p, there are two adjacent potential N-glycosylation sites (N(535)NTWN(539)NT) that, in contrast to the other subunits, are highly conserved in eukaryotic organisms. Mass spectrometric analysis of a tryptic digest of Stt3p showed that the peptide containing the two adjacent N-glycosylation sites was N-glycosylated at one site. Furthermore, the glycan chain identified as Man(8)GlcNAc(2) is found linked only to Asn(539). Mutation experiments were carried out at these two sites. Four single amino acid mutations blocking either N-glycosylation site (N535Q, T537A, N539Q, and T541A) resulted in strains that were either lethal or extremely temperature sensitive. However, other mutations in the two N-glycosylation sites N(535)NTWN(539)NT (N536Q, T537S, N540Q, and T541S), did not exhibit growth defects. Based on these studies, we conclude that N-glycosylation of Stt3p at Asn(539) is essential for its function in the OT complex.

Roles of N-linked glycans in the endoplasmic reticulum

From a process involved in cell wall synthesis in archaea and some bacteria, N-linked glycosylation has evolved into the most common covalent protein modification in eukaryotic cells. The sugars are added to nascent proteins as a core oligosaccharide unit, which is then extensively modified by removal and addition of sugar residues in the endoplasmic reticulum (ER) and the Golgi complex. It has become evident that the modifications that take place in the ER reflect a spectrum of functions related to glycoprotein folding, quality control, sorting, degradation, and secretion. The glycans not only promote folding directly by stabilizing polypeptide structures but also indirectly by serving as recognition "tags" that allow glycoproteins to interact with a variety of lectins, glycosidases, and glycosyltranferases. Some of these (such as glucosidases I and II, calnexin, and calreticulin) have a central role in folding and retention, while others (such as alpha-mannosidases and EDEM) target unsalvageable glycoproteins for ER-associated degradation. Each residue in the core oligosaccharide and each step in the modification program have significance for the fate of newly synthesized glycoproteins.

Genetic, biochemical, and morphological evidence for the involvement of N-glycosylation in biosynthesis of the cell wall beta1,6-glucan of Saccharomyces cerevisiae

Recent evidence indicates that Stt3p plays a central role in the recognition and/or catalytic step in N-glycosylation (asparagine-linked glycosylation) in the lumen of the endoplasmic reticulum. It is known that stt3 mutants exhibit certain phenotypic features that are suggestive of a cell wall defect. To understand the basis of these phenotypes, we devised a genetic screen to isolate strains bearing mutations that lead to synthetic lethality in combination with the stt3-1 mutation. Using this screen, we were surprised to identify two KRE genes (KRE5 and KRE9) that are involved in the biosynthesis of the cell wall beta1,6-glucan. This finding led us to propose that the N-glycosylation process is essential in the biosynthesis of cell wall beta1,6-glucan. This proposal was supported by the observation that several stt3 mutants exhibited a 60-70% reduction in the content of cell wall beta1,6-glucan as compared with WT cells. Transmission electron microscopy revealed that the stt3 mutant strains exhibit a diffused cell wall with loss of the outer mannoprotein layer as compared with the WT cells. Thus, we provide genetic, morphological, and biochemical evidence for the critical involvement of N-glycosylation in some step in assembly of the cell wall beta1,6-glucan in Saccharomyces cerevisiae.

Insight into Functional Aspects of Stt3p, a Subunit of the Oligosaccharyl Transferase

Over a decade ago, the gene STT3 was identified in a staurosporine and temperature sensitivity screen of yeast. Subsequently the product of this gene was shown to be a subunit of the endoplasmic reticulum-localized oligosaccharyl transferase (OT) complex. Although stt3 mutants are known to be staurosporine-sensitive, we found that mutants of other OT subunits (except ost4 Delta) are staurosporine-resistant, which indicates that this phenotype of stt3 mutants is not simply a consequence of their defect in glycosylation, as previously speculated. Staurosporine sensitivity was found to be an allele-specific phenotype restricted to cells harboring mutations in highly conserved residues in the N-terminal domain of the STT3 protein. Cells bearing mutations in one of the cytosolic-oriented loops (amino acids 158-168) in the N terminus of Stt3p were found to be specifically susceptible to staurosporine. Staurosporine is a specific inhibitor of Pkc1p, and a genetic link had previously been suggested between PKC1 and STT3. It is known that overexpression of PKC1 suppresses the staurosporine sensitivity of the stt3 mutants in an allele-specific manner, which is typical of mutants of Pkc1p cascade. It has been shown that the pkc1 null mutant exhibits lowered OT activity. Our results combined with these previous observations indicate that the N-terminal domain of Stt3p may interact with members of the Pkc1p cascade and consequently mutations in this domain result in staurosporine sensitivity. We further speculate that the Pkc1p regulates OT activity through the N-terminal domain of Stt3p, the C-terminal domain of which possesses the recognition and/or catalytic site of the OT complex.

The STT3a subunit isoform of the Arabidopsis oligosaccharyltransferase controls adaptive responses to salt/osmotic stress

Arabidopsis stt3a-1 and stt3a-2 mutations cause NaCl/osmotic sensitivity that is characterized by reduced cell division in the root meristem. Sequence comparison of the STT3a gene identified a yeast ortholog, STT3, which encodes an essential subunit of the oligosaccharyltransferase complex that is involved in protein N-glycosylation. NaCl induces the unfolded protein response in the endoplasmic reticulum (ER) and cell cycle arrest in root tip cells of stt3a seedlings, as determined by expression profiling of ER stress-responsive chaperone (BiP-GUS) and cell division (CycB1;1-GUS) genes, respectively. Together, these results indicate that plant salt stress adaptation involves ER stress signal regulation of cell cycle progression. Interestingly, a mutation (stt3b-1) in another Arabidopsis STT3 isogene (STT3b) does not cause NaCl sensitivity. However, the stt3a-1 stt3b-1 double mutation is gametophytic lethal. Apparently, STT3a and STT3b have overlapping and essential functions in plant growth and developmental processes, but the pivotal and specific protein glycosylation that is a necessary for recovery from the unfolded protein response and for cell cycle progression during salt/osmotic stress recovery is associated uniquely with the function of the STT3a isoform.