The alpha subunit of the Saccharomyces cerevisiae oligosaccharyltransferase complex is essential for vegetative growth of yeast and is homologous to mammalian ribophorin I

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.

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.

Protein O-mannosyltransferases associate with the translocon to modify translocating polypeptide chains

O-Mannosylation and N-glycosylation are essential protein modifications that are initiated in the endoplasmic reticulum (ER). Protein translocation across the ER membrane and N-glycosylation are highly coordinated processes that take place at the translocon-oligosaccharyltransferase (OST) complex. In analogy, it was assumed that protein O-mannosyltransferases (PMTs) also act at the translocon, however, in recent years it turned out that prolonged ER residence allows O-mannosylation of un-/misfolded proteins or slow folding intermediates by Pmt1-Pmt2 complexes. Here, we reinvestigate protein O-mannosylation in the context of protein translocation. We demonstrate the association of Pmt1-Pmt2 with the OST, the trimeric Sec61, and the tetrameric Sec63 complex in vivo by co-immunoprecipitation. The coordinated interplay between PMTs and OST in vivo is further shown by a comprehensive mass spectrometry-based analysis of N-glycosylation site occupancy in pmtΔ mutants. In addition, we established a microsomal translation/translocation/O-mannosylation system. Using the serine/threonine-rich cell wall protein Ccw5 as a model, we show that PMTs efficiently mannosylate proteins during their translocation into microsomes. This in vitro system will help to unravel mechanistic differences between co- and post-translocational O-mannosylation.

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.

Over-expression of functional Saccharomyces cerevisiae GUP1, induces proliferation of intracellular membranes containing ER and Golgi resident proteins

High-level expression of the GUP1 gene in Saccharomyces cerevisiae resulted in the formation of proliferated structures, which hosted endoplasmic reticulum (ER), Golgi and itinerant proteins. The GUP1 over-expression enhanced ER biogenesis, as shown by the coordinated increased transcription rate of genes involved in both ER and Golgi metabolism and in phospholipids biosynthesis. The formation of Gup1-induced proliferation revealed that it depended on an intact unfolded protein response, because their assembly was reported to be lethal to yeast strains unable to initiate the UPR (Unfolded Protein Response) pathway. GUP1 over-expression affected global ER and Golgi structure and resulted in the biogenesis of novel membrane arrays with Golgi and ER hybrid composition. In fact, a number of ER and Golgi resident proteins together with itinerant proteins that normally cycle between ER and Golgi, were localized in the proliferated stacked membranes. The described assembling of novel membrane structures was affected by the functionality of the Gup1 O-acyltransferase domain, which regulates the Gup1 protein role as remodelase in the glycosylphosphatidylinositol (GPI) anchored proteins biosynthesis. To our knowledge, we presented the first evidence of sub cellular modifications in response over-expression of a GPI-anchor remodelase in S. cerevisiae.

Protein translocation across the ER membrane

Protein translocation into the endoplasmic reticulum (ER) is the first and decisive step in the biogenesis of most extracellular and many soluble organelle proteins in eukaryotic cells. It is mechanistically related to protein export from eubacteria and archaea and to the integration of newly synthesized membrane proteins into the ER membrane and the plasma membranes of eubacteria and archaea (with the exception of tail anchored membrane proteins). Typically, protein translocation into the ER involves cleavable amino terminal signal peptides in precursor proteins and sophisticated transport machinery components in the cytosol, the ER membrane, and the ER lumen. Depending on the hydrophobicity and/or overall amino acid content of the precursor protein, transport can occur co- or posttranslationally. The respective mechanism determines the requirements for certain cytosolic transport components. The two mechanisms merge at the level of the ER membrane, specifically, at the heterotrimeric Sec61 complex present in the membrane. The Sec61 complex provides a signal peptide recognition site and forms a polypeptide conducting channel. Apparently, the Sec61 complex is gated by various ligands, such as signal peptides of the transport substrates, ribosomes (in cotranslational transport), and the ER lumenal molecular chaperone, BiP. Binding of BiP to the incoming polypeptide contributes to efficiency and unidirectionality of transport. Recent insights into the structure of the Sec61 complex and the comparison of the transport mechanisms and machineries in the yeast Saccharomyces cerevisiae, the human parasite Trypanosoma brucei, and mammals have various important mechanistic as well as potential medical implications. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Oligosaccharyltransferase directly binds to ribosome at a location near the translocon-binding site

Oligosaccharyltransferase (OT) transfers high mannose-type glycans to the nascent polypeptides that are translated by the membrane-bound ribosome and translocated into the lumen of the endoplasmic reticulum through the Sec61 translocon complex. In this article, we show that purified ribosomes and OT can form a binary complex with a stoichiometry of approximately 1 to 1 in the presence of detergent. We present evidence that OT may bind to the large ribosomal subunit near the site where nascent polypeptides exit. We further show that OT and the Sec61 complex can simultaneously bind to ribosomes in vitro. Based on existing data and our findings, we propose that cotranslational translocation and N-glycosylation of nascent polypeptides are mediated by a ternary supramolecular complex consisting of OT, the Sec61 complex, and ribosomes.

Sec61p is required for ERAD-L: genetic dissection of the translocation and ERAD-L functions of Sec61P using novel derivatives of CPY

Misfolded proteins in the endoplasmic reticulum (ER) are exported to the cytosol for degradation by the proteasome in a process known as ER-associated degradation (ERAD). CPY* is a well characterized ERAD substrate whose degradation is dependent upon the Hrd1 complex. However, although the functions of some of the components of this complex are known, the nature of the protein dislocation channel remains obscure. Sec61p has been suggested as an obvious candidate because of its role as a protein-conducting channel through which polypeptides are initially translocated into the ER. However, it has not yet been possible to functionally dissect any role for Sec61p in dislocation from its essential function in translocation. By changing the translocation properties of a series of novel ERAD substrates, we are able to separate these two events and find that functional Sec61p is essential for the ERAD-L pathway.

Structure of the oligosaccharyl transferase complex at 12 A resolution

Oligosaccharyl transferase (OT) catalyzes the transfer of a lipid-linked oligosaccharide to the nascent polypeptide emerging from the translocon. Currently, there is no structural information on the membrane-embedded OT complex, which consists of eight different polypeptide chains. We report a 12 A resolution cryo-electron microscopy structure of OT from yeast. We mapped the locations of four essential OT subunits through a maltose-binding protein fusion strategy. OT was found to have a large domain in the lumenal side of endoplasmic reticulum where the catalysis occurs. The lumenal domain mainly comprises the catalytic Stt3p, the donor substrate-recognizing Wbp1p, and the acceptor substrate-recognizing Ost1p. A prominent groove was observed between these subunits, and we propose that the nascent polypeptide from the translocon threads through this groove while being scanned by the Ost1p subunit for the presence of the glycosylation sequon.

The ribophorin I from Penaeus monodon shrimp: cDNA cloning, expression and phylogenetic analysis

Ribophorin I, a 67 kDa subunit of the oligosaccharyl transferase complex, is involved in facilitating N-linked glycosylation of polypeptides. We have isolated a full length Penaeus monodon cDNA encoding an insect/mammalian ribophorin I homologue by screening a lymphoid cDNA library and by performing rapid amplification of cDNA ends polymerase chain reaction of lymphoid RNA. The cDNA clone of shrimp ribophorin I (PmRibI) consists of 2263 nucleotides encoding 601 amino acid residues. Primary structure analysis of PmRibI indicated that it is a type I transmembrane protein, comprising a cleavable signal sequence of 23 residues at the amino terminus, preceding 434 residues of the luminal domain, 17 residues of the transmembrane domain, and 150 residues of the cytoplasmic domain at the carboxy terminus. The protein has a calculated molecular mass of 67.98 kDa with a pI of 6.05. This putative PmRibI cDNA clone was also expressed as PmRibI-6His in Sf9 cells. The recombinant PmRibI has an apparent molecular weight of 70 kDa, similar to the MW calculated from the deduced cDNA sequence. The inferred protein sequence of PmRibI has 52% identity with that of Strongylocentrotus purpuratus, 49% identity with that of Danio rerio, and 47% identity with mammalian ribophorin I. Phylogenetic analysis showed that PmRibI is most closely related to the echinoderm ribophorin I. The expression of the ribophorin I gene is tissue specific, with its mRNA highly abundant in hemocytes, gill, lymphoid organ and hepatopancreas.

The actomyosin ring recruits early secretory compartments to the division site in fission yeast

The ultimate goal of cytokinesis is to establish a membrane barrier between daughter cells. The fission yeast Schizosaccharomyces pombe utilizes an actomyosin-based division ring that is thought to provide physical force for the plasma membrane invagination. Ring constriction occurs concomitantly with the assembly of a division septum that is eventually cleaved. Membrane trafficking events such as targeting of secretory vesicles to the division site require a functional actomyosin ring suggesting that it serves as a spatial landmark. However, the extent of polarization of the secretion apparatus to the division site is presently unknown. We performed a survey of dynamics of several fluorophore-tagged proteins that served as markers for various compartments of the secretory pathway. These included markers for the endoplasmic reticulum, the COPII sites, and the early and late Golgi. The secretion machinery exhibited a marked polarization to the division site. Specifically, we observed an enrichment of the transitional endoplasmic reticulum (tER) accompanied by Golgi cisternae biogenesis. These processes required actomyosin ring assembly and the function of the EFC-domain protein Cdc15p. Cdc15p overexpression was sufficient to induce tER polarization in interphase. Thus, fission yeast polarizes its entire secretory machinery to the cell division site by utilizing molecular cues provided by the actomyosin ring.

Ribophorin I acts as a substrate-specific facilitator of N-glycosylation

The mammalian oligosaccharyltransferase (OST) complex is composed of about eight subunits and mediates the N-glycosylation of nascent polypeptide chains entering the endoplasmic reticulum (ER). The conserved STT3 subunit of eukaryotic OST complexes has been identified as its catalytic centre, yet although many other subunits are equally well conserved their functions are unknown. We used RNA interference to investigate the function of ribophorin I, an ER-translocon-associated subunit of the OST complex previously shown to associate with newly synthesised membrane proteins. We show that ribophorin I dramatically enhances the N-glycosylation of selected membrane proteins and provide evidence that it is not essential for N-glycosylation per se. Parallel studies confirm that STT3 is essential for transferase activity of the complex, but reveal that the two mammalian isoforms are not functionally equivalent when modifying bona fide polypeptide substrates. We propose a new model for OST function where ribophorin I acts as a chaperone or escort to promote the N-glycosylation of selected substrates by the catalytic STT3 subunits.

Regulation of calnexin sub-cellular localization modulates endoplasmic reticulum stress-induced apoptosis in MCF-7 cells

The endoplasmic reticulum (ER) is the cellular compartment where proteins enter the secretory pathway, undergo post-translational modifications and acquire a correct conformation. If these functions are chronically altered, specific ER stress signals are triggered to promote cell death through the intrinsic apoptotic pathway. Here, we show that tunicamycin causes significant alteration of calnexin sub-cellular distribution in MCF-7 cells. Interestingly, this correlates with the absence of both tunicamycin-induced calnexin phosphorylation as well as tunicamycin-induced cell death. Under these conditions, calnexin-associated Bap31, an ER integral membrane protein, is subjected to a caspase-8 cleavage pattern within a specific sub-compartment of the ER. These results suggest that MCF-7 resistance to ER stress-induced apoptosis is partially mediated by the expression level of calnexin which in turn controls its sub-cellular localization, and its association with Bap31. These data may delineate a resistance mechanism to the ER stress-induced intrinsic apoptotic pathway.

An evolving view of the eukaryotic oligosaccharyltransferase

Asparagine-linked glycosylation (ALG) is one of the most common protein modification reactions in eukaryotic cells, as many proteins that are translocated across or integrated into the rough endoplasmic reticulum (RER) carry N-linked oligosaccharides. Although the primary focus of this review will be the structure and function of the eukaryotic oligosaccharyltransferase (OST), key findings provided by the analysis of the archaebacterial and eubacterial OST homologues will be reviewed, particularly those that provide insight into the recognition of donor and acceptor substrates. Selection of the fully assembled donor substrate will be considered in the context of the family of human diseases known as congenital disorders of glycosylation (CDG). The yeast and vertebrate OST are surprisingly complex hetero-oligomeric proteins consisting of seven or eight subunits (Ost1p, Ost2p, Ost3p/Ost6p, Ost4p, Ost5p, Stt3p, Wbp1p, and Swp1p in yeast; ribophorin I, DAD1, N33/IAP, OST4, STT3A/STT3B, Ost48, and ribophorin II in mammals). Recent findings from several laboratories have provided overwhelming evidence that the STT3 subunit is critical for catalytic activity. Here, we will consider the evolution and assembly of the eukaryotic OST in light of recent genomic evidence concerning the subunit composition of the enzyme in diverse eukaryotes.

The 3.4-kDa Ost4 protein is required for the assembly of two distinct oligosaccharyltransferase complexes in yeast

In the central reaction of N-linked glycosylation, the oligosaccharyltransferase (OTase) complex catalyzes the transfer of a lipid-linked core oligosaccharide onto asparagine residues of nascent polypeptide chains in the lumen of the endoplasmic reticulum (ER). The Saccharomyces cerevisiae OTase has been shown to consist of at least eight subunits. We analyzed this enzyme complex, applying the technique of blue native gel electrophoresis. Using available antibodies, six different subunits were detected in the wild-type (wt) complex, including Stt3p, Ost1p, Wbp1p, Swp1p, Ost3p, and Ost6p. We demonstrate that the small 3.4-kDa subunit Ost4p is required for the incorporation of either Ost3p or Ost6p into the complex, resulting in two, functionally distinct OTase complexes in vivo. Ost3p and Ost6p are not absolutely required for OTase activity, but modulate the affinity of the enzyme toward different protein substrates.

Proteomic analysis of mammalian oligosaccharyltransferase reveals multiple subcomplexes that contain Sec61, TRAP, and two potential new subunits

Oligosaccharyltransferase (OST) catalyzes the cotranslational transfer of high-mannose sugars to nascent polypeptides during N-linked glycosylation in the rough endoplasmic reticulum lumen. Nine OST subunits have been identified in yeast. However, the composition and organization of mammalian OST remain unclear. Using two-dimensional Blue Native polyacrylamide gel electrophoresis/sodium dodecyl sulfate-polyacrylamide gel electrophoresis and mass spectrometry, we now demonstrate that mammalian OST can be isolated from solubilized, actively engaged ribosomes as multiple distinct protein complexes that range in size from approximately 500 to 700 kDa. These complexes exhibit different ribosome affinities and subunit compositions. The major complex, OSTC(I), had an apparent size of approximately 500 kDa and was readily released from ribosome translocon complexes after puromycin treatment under physiological salt conditions. Two additional complexes were released only after treatment with high salt: OSTC(II) ( approximately 600 kDa) and OSTC(III) ( approximately 700 kDa). Both remained stably associated with heterotrimeric Sec61alphabetagamma, while OSTC(III) also contained the tetrameric TRAP complex. All known mammalian OST subunits (STT3-A, ribophorin I, ribophorin II, OST48, and DAD1) were present in all complexes. In addition, two previously uncharacterized proteins were also copurified with OST. Mass spectrometry identified a 17 kDa protein as DC2 which is weakly homologous to the C-terminal half of yeast Ost3p and Ost6p. The second protein (14 kDa) was tentatively identified as keratinocyte-associated protein 2 (KCP2) and has no previously known function. Our results identify two potential new subunits of mammalian OST and demonstrate a remarkable heterogeneity in OST composition that may reflect a means for controlling nascent chain glycosylation.

Studies of yeast oligosaccharyl transferase subunits using the split-ubiquitin system: topological features and in vivo interactions

Oligosaccharyl transferase (OT) catalyzes the cotranslational N-glycosylation of nascent polypeptides in the endoplasmic reticulum in all eukaryotic systems. Due to the inherent difficulty in characterizing this membrane protein complex, the mode of enzymatic action has not been resolved. Here, we used a membrane protein two-hybrid approach, the split-ubiquitin system, to address two aspects of the enzyme complex in yeast: the topological features, as well as the in vivo interactions of all of the components. We investigated the N- and C-terminal orientation of these proteins and the presence or the absence of a cleavable signal sequence at their N termini. We found that Ost2p and Stt3p have only their N terminus located in the cytosol, whereas Ost3p and Swp1p have only their C terminus oriented in the cytosol. In the case of Ost5p and Ost6p, both their N and C termini are present in the cytosol. These findings also suggested that Ost2p, Stt3p, Ost5p, and Ost6p do not have a cleavable N-terminal signal sequence. The pairwise analysis of in vivo interactions among all of the OT subunits demonstrated that OT subunits display specific interactions with each other in a functional complex. By comparing this interaction pattern with that detected in vitro in a nonfunctional complex, we proposed that a distinct conformation rearrangement takes place when the enzyme complex changes from the nonfunctional state to the activated functional state. This finding is consistent with earlier work by others indicating that OT exhibits allosteric properties.

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.

New findings on interactions among the yeast oligosaccharyl transferase subunits using a chemical cross-linker

At present, there is very limited knowledge about the structural organization of the yeast oligosaccharyl transferase (OT) complex and the function of each of its nine subunits. Because of the failure of the yeast two-hybrid system to reveal interactions between luminal domains of these subunits, we utilized a membrane permeable, thiocleavable cross-linking reagent dithiobis-succinimidyl propionate to biochemically study the interactions of various OT subunits. Four essential gene products, Ost1p, Wbp1p, Swp1p, and Stt3p were shown to be cross-linked to each other in a pairwise fashion. In addition, Ost1p was found to be cross-linked to all other eight OT subunits individually. This led us to propose that Ost1p may reside in the core of the OT complex and could play an important role in its assembly. Ost4p and Ost5p were found to only interact with specific components of the OT complex and may function as an additional anchor for optimal stability of Stt3p and Ost1p in the membrane, respectively. Interestingly, Ost3p and Ost6p subunits exhibited a surprisingly identical pattern of cross-linking to other subunits, which is consistent with their proposed redundant function. Based on these findings, we analyzed the distribution of the lysine residues that are likely to be involved in cross-linking of OT subunits and propose that the OT subunits interact with each other through either their transmembrane domains and/or a region proximal to it, rather than through their luminal or cytoplasmic domains.

Determination of the membrane topology of Ost4p and its subunit interactions in the oligosaccharyltransferase complex in Saccharomyces cerevisiae

Ost4p is a minimembrane protein containing only 36 amino acids and is a subunit of oligosaccharyltransferase (OT) in Saccharomyces cerevisiae. It was found previously when amino acid residues 18-25 of Ost4p were mutated to ionizable amino acids and defects were observed in the interaction between Ost4p and either Stt3p or Ost3p, two other components of OT. The transmembrane segment of Ost4p is likely to extend from residues 10-25. This is consistent with the finding that alpha-helicity is estimated to be 36% by CD analysis of synthetic Ost4p in liposomes. This value is in reasonable agreement with the assumption that amino acids 10-25 (16 of 36 or 44%) are transmembrane. Therefore, the mutation-sensitive region (residues 18-25) is localized to only one half of the putative transmembrane domain of Ost4p. To learn where this region of Ost4p is situated in relation to the faces of endoplasmic reticulum (ER) membrane, we determined the membrane topology of Ost4p using an in vivo method and established that it is an Nlumen-Ccyto, type I membrane protein. These results indicate that the mutation-sensitive region of Ost4p is localized in the cytoplasmic leaflet of the ER membrane. In the current study, we also observed a loss of direct interaction between Ost3p and Stt3p in the presence of ost4 temperature-sensitive mutants, which indicates Ost4p, via interactions with amino acid residues in the cytosolic leaflet of the ER membrane, functions to bind these two proteins together in a subcomplex of OT.

Congenital disorders of glycosylation: review of their molecular bases, clinical presentations and specific therapies

Congenital disorders of glycosylation (CDG, formerly named carbohydrate-deficient glycoprotein syndromes) are a rapidly growing family of inherited disorders affecting the assembly or processing of glycans on glycoconjugates. The clinical spectrum of the different types of CDG discovered so far is variable, ranging from severe multisystemic disorders to disorders restricted to specific organs. This review deals with clinical, diagnostic, and biochemical aspects of all characterized CDGs, including a disorder affecting the N-glycosylation of erythrocytes, congenital dyserythropoietic anemia type II (CDA II/HEMPAS), and the first disorders affecting O-glycosylation. Since the clinical spectrum of symptoms in CDG is variable and may be unspecific, a generous selective screening for the presence of CDG is recommended.

Members of the evolutionarily conserved PMT family of protein O-mannosyltransferases form distinct protein complexes among themselves

Protein O-mannosyltransferases (PMTs) initiate the assembly of O-mannosyl glycans, an essential protein modification. Since PMTs are evolutionarily conserved in fungi but are absent in green plants, the PMT family is a putative target for new antifungal drugs, particularly in fighting the threat of phytopathogenic fungi. The PMT family is phylogenetically classified into PMT1, PMT2, and PMT4 subfamilies, which differ in protein substrate specificity. In the model organism Saccharomyces cerevisiae as well as in many other fungi the PMT family is highly redundant, and only the simultaneous deletion of PMT1/PMT2 and PMT4 subfamily members is lethal. In this study we analyzed the molecular organization of PMT family members in S. cerevisiae. We show that members of the PMT1 subfamily (Pmt1p and Pmt5p) interact in pairs with members of the PMT2 subfamily (Pmt2p and Pmt3p) and that Pmt1p-Pmt2p and Pmt5p-Pmt3p complexes represent the predominant forms. Under certain physiological conditions, however, Pmt1p interacts also with Pmt3p, and Pmt5p with Pmt2p, suggesting a compensatory cooperation that guarantees the maintenance of O-mannosylation. Unlike the PMT1/PMT2 subfamily members, the single member of the PMT4 subfamily (Pmt4p) acts as a homomeric complex. Using mutational analyses we demonstrate that the same conserved protein domains underlie both heteromeric and homomeric interactions, and we identify an invariant arginine residue of transmembrane domain two as essential for the formation and/or stability of PMT complexes in general. Our data suggest that protein-protein interactions between the PMT family members offer a point of attack to shut down overall protein O-mannosylation in fungi.

Studies on the function of oligosaccharyl transferase subunits: a glycosylatable photoprobe binds to the luminal domain of Ost1p

Oligosaccharyl transferase (OT) is a complex multisubunit enzyme that, in the case of Saccharomyces cerevisiae, contains nine different transmembrane proteins. One of our goals is to identify the OT subunit(s) responsible for recognizing the consensus sequence, -Asn-X-ThrSer-, and catalyzing the oligosaccharide transfer reaction. By using a substrate-based photoprobe, earlier we found that Ost1p was specifically linked to the radiolabeled photoprobe. We have now examined Ost1p in more detail. Deletion of the cytoplasmic tail of Ost1p caused no defects in growth and glycosylation. In addition, replacement of the transmembrane domain with other hydrophobic amino acids did not impair growth. In contrast, a construct containing only the luminal domain of Ost1p did not support cell growth. Given these observations, we concentrated on studying the luminal domain of Ost1p and localized the photoprobe attachment region within a sequence of nine amino acid residues. Because mutations in the photoprobe attachment region did not cause any severe growth or glycosylation defects, we conclude that this region is not involved in the recognition of the N-glycosylation site. By further mutagenesis of the conserved residues of Ost1p we conclude that the luminal domain mediates interactions with other subunits of OT and becomes labeled because of its proximity to the recognition andor catalytic subunit in the OT complex, Stt3p.

Studies on the function of oligosaccharyl transferase subunits. Stt3p is directly involved in the glycosylation process

In the yeast, Saccharomyces cerevisiae, oligosaccharyl transferase (OT) is composed of nine different transmembrane proteins. Using a glycosylatable peptide containing a photoprobe, we previously found that only one essential subunit, Ost1p, was specifically labeled by the photoprobe and recently have shown that it does not contain the recognition domain for the glycosylatable sequence Asn-Xaa-Thr/Ser. In this study we utilized additional glycosylatable peptides containing two photoreactive groups and found that these were linked to Stt3p and Ost3p. Stt3p is the most conserved subunit in the OT complex, and therefore 21 block mutants in the lumenal region were prepared. Of the 14 lethal mutant proteins only two, as well as one temperature-sensitive mutant protein, were incorporated into the OT complex. However, using microsomes prepared from these three strains, the labeling of Ost1p was markedly decreased upon photoactivation with the Asn-Bpa-Thr photoprobe. Based on the block mutants single amino acid mutations were prepared and analyzed. From all of these results, we conclude that the sequence from residues 516 to 520, WWDYG in Stt3p, plays a central role in glycosylatable peptide recognition and/or the catalytic glycosylation process.

Oligosaccharyl transferase: gatekeeper to the secretory pathway

Oligosaccharyl transferase is part of the macromolecular machinery that processes nascent proteins in the endoplasmic reticulum. The enzyme is highly conserved, catalyzes the initial step in the biosynthesis of N-linked glycoproteins and acts as a 'gatekeeper' for the secretory pathway. As more proteins associated with oligosaccharyl transferase are identified, the intricacies of the enzyme and the relationship with other proteins in the lumen of the endoplasmic reticulum are starting to be unraveled.

Interaction of the endoplasmic reticulum α1,2-mannosidase Mns1p with Rer1p using the split-ubiquitin system

The α1,2-mannosidase Mns1p involved in the N-glycosidic pathway in Saccharomyces cerevisiae is a type II membrane protein of the endoplasmic reticulum. The localization of Mns1p depends on retrieval from the Golgi through a mechanism that involves Rer1p. A chimera consisting of the transmembrane domain of Mns1p fused to the catalytic domain of the Golgi α1,2-mannosyltransferase Kre2p was localized in the endoplasmic reticulum of Δpep4 cells and in the vacuoles of rer1/Δpep4 by indirect immunofluorescence. The split-ubiquitin system was used to determine if there is an interaction between Mns1p and Rer1p in vivo. Co-expression of NubG-Mns1p and Rer1p-Cub-protein A-lexA-VP16 in L40 yeast cells resulted in cleavage of the reporter molecule, protein A-lexA-VP16, detected by western blot analysis and by expression of β-galactosidase activity. Sec12p, another endoplasmic reticulum protein that depends on Rer1p for its localization, also interacted with Rer1p using the split-ubiquitin assay, whereas the endoplasmic reticulum protein Ost1p showed no interaction. A weak interaction was observed between Alg5p and Rer1p. These results demonstrate that the transmembrane domain of Mns1p is sufficient for Rer1p-dependent endoplasmic reticulum localization and that Mns1p and Rer1p interact. Furthermore, the split-ubiquitin system demonstrates that the C-terminal of Rer1p is in the cytosol.

Oligosaccharyltransferase: a complex multisubunit enzyme of the endoplasmic reticulum

The attachment of N-linked oligosaccharide chains to proteins is an important cotranslational process. These chains can, in some cases, serve to stabilize the protein, while in other cases they function as recognition elements. A key enzyme in the N-glycosylation process is oligosaccharyltransferase (OT). In yeast this enzyme, which is found in the endoplasmic reticulum, consists of nine different transmembrane protein subunits. Our general aim is to learn more about the functions of the multiple subunits of yeast OT and their mode of interaction with each other. Using a combination of biochemical and genetic techniques the subunit Ost1p has been shown to recognize Asn-X-Ser/Thr glycosylation sites. The principle tool used in the identification process was a benzophenone-based glycosylation site peptide that was shown to be crosslinked to Ost1p. Our current objective is to identify the domain in the primary structure that is involved in recognition of the glycosylation site sequence. By use of bifunctional crosslinkers, the possible interaction of Ost1p with other subunits of OT will be studied. This work and other studies on the OT subunits are concisely summarized.

Expression of the 180-kD ribosome receptor induces membrane proliferation and increased secretory activity in yeast

Expression of the canine 180-kD ribosome receptor (p180) in yeast cells resulted in a marked proliferation of intracellular membranes. The type of membranes observed varied with the expression of specific portions of p180. Rough membranes predominated when the ribosome binding domain of p180 was present, whereas expression constructs lacking this region resulted in smooth membranes. Northern analysis indicated that expression of the NH(2)-terminal 767 amino acids (DeltaCT), which include the ribosome binding domain, upregulated the transcription and translation of genes involved in exocytosis. The membranes that were proliferated were functional as these cells overcame a temperature-sensitive translocation defect. Most significantly, cells that overexpressed DeltaCT and proliferated rough endoplasmic reticulum exhibited severalfold higher levels of secretion of an ectopically expressed secretory protein. We conclude that p180 expression triggers a cascade of events leading to an increase in secretory potential akin to the terminal differentiation of mammalian secretory cells and tissues.

The oligosaccharyltransferase complex from Saccharomyces cerevisiae. Isolation of the OST6 gene, its synthetic interaction with OST3, and analysis of the native complex

The key step of N-glycosylation of proteins, an essential and highly conserved protein modification, is catalyzed by the hetero-oligomeric protein complex oligosaccharyltransferase (OST). So far, eight genes have been identified in Saccharomyces cerevisiae that are involved in this process. Enzymatically active OST preparations from yeast were shown to be composed of four (Ost1p, Wbp1p, Ost3p, Swp1p) or six subunits (Ost2p and Ost5p in addition to the four listed). Genetic studies have disclosed Stt3p and Ost4p as additional proteins needed for N-glycosylation. In this study we report the identification and functional characterization of a new OST gene, designated OST6, that has homology to OST3 and in particular a strikingly similar membrane topology. Neither gene is essential for growth of yeast. Disruption of OST6 or OST3 causes only a minor defect in N-glycosylation, but an Deltaost3Deltaost6 double mutant displays a synthetic phenotype, leading to a severe underglycosylation of soluble and membrane-bound glycoproteins in vivo and to a reduced OST activity in vitro. Moreover, each of the two genes has also a specific function, since agents affecting cell wall biogenesis reveal different growth phenotypes in the respective null mutants. By blue native electrophoresis and immunodetection, a approximately 240-kDa complex was identified consisting of Ost1p, Stt3p, Wbp1p, Ost3p, Ost6p, Swp1p, Ost2p, and Ost5p, indicating that probably all so far identified OST proteins are constituents of the OST complex. It is also shown that disruption of OST3 and OST6 leads to a defect in the assembly of the complex. Hence, the function of these genes seems to be essential for recruiting a fully active complex necessary for efficient N-glycosylation.

Schizosaccharomyces pombe stt3+ is a functional homologue of Saccharomyces cerevisiae STT3 which regulates oligosaccharyltransferase activity

The Saccharomyces cerevisiae STT3 (ScSTT3) gene encodes a protein which is involved in protein glycosylation via the regulation of oligosaccharyltransferase activity. We have cloned and isolated the Schizosaccharomyces pombe STT3 homologous gene (Spstt3+). The Spstt3+ gene encodes a protein consisting of 749 amino acid residues which has significant homology with ScStt3p and the mouse Stt3p-homologue Itm1p. Disruption of the Spstt3+ gene shows that this gene is essential for growth. Like Itm1, Spstt3+ partially suppressed the temperature sensitivity of the stt3-1 mutation of S. cerevisiae, indicating that Spstt3+ is a functional and structural homologue of the ScSTT3 gene.

The Saccharomyces cerevisiae CWH8 gene is required for full levels of dolichol-linked oligosaccharides in the endoplasmic reticulum and for efficient N-glycosylation

The Saccharomyces cerevisiae mutant cwh8 was previously found to have an anomalous cell wall. Here we show that the cwh8 mutant has an N -glycosylation defect. We found that cwh8 cells were resistant to vanadate and sensitive to hygromycin B, and produced glycoforms of invertase and carboxypeptidase Y with a reduced number of N -chains. We have cloned the CWH8 gene. We found that it was nonessential and encoded a putative transmembrane protein of 239 amino acids. Comparison of the in vitro oligosaccharyl transferase activities of membrane preparations from wild type or cwh8 Delta cells revealed no differences in enzyme kinetic properties indicating that the oligosaccharyl transferase complex of mutant cells was not affected. cwh8 Delta cells also produced normal dolichols and dolichol-linked oligosaccharide intermediates including the full-length form Glc3Man9GlcNAc2. The level of dolichol-linked oligosaccharides in cwh8 Delta cells was, however, reduced to about 20% of the wild type. We propose that inefficient N -glycosylation of secretory proteins in cwh8 Delta cells is caused by an insufficient supply of dolichol-linked oligosaccharide substrate.

The Ost1p subunit of yeast oligosaccharyl transferase recognizes the peptide glycosylation site sequence, -Asn-X-Ser/Thr-

Other laboratories have established that oligosaccharyl transferase (OST) from Saccharomyces cerevisiae can be purified as a protein complex containing eight different subunits. To identify the OST subunit that recognizes the peptide sites that can be glycosylated, we developed photoaffinity probes containing a photoreactive benzophenone derivative, p-benzoylphenylalanine (Bpa), as part of an 125I-labeled peptide that could be expected to be glycosylated. We found that Asn-Bpa-Thr peptides served as substrates for OST and that photoactivation of these probes in the presence of microsomes abolished the OST activity. Photoactivation of 125I-labeled Asn-Bpa-Thr in the presence of microsomes resulted in specific covalent labeling of a protein doublet of molecular mass 62 and 64 kDa. By carrying out the photoactivation of the probe using microsomes containing epitope-tagged Ost1p, we demonstrated that the 125I-labeled protein was Ost1p. Radiolabeling of this protein was dependent on irradiation at 350 nm. No labeling was detected using a probe containing Ala instead of Thr as the third amino acid residue. We conclude that Ost1p is the subunit of the OST complex that recognizes the peptide sites in the nascent chains that are destined to be glycosylated.

The oligosaccharyltransferase complex from yeast

N-Glycosylation of eukaryotic secretory and membrane-bound proteins is an essential and highly conserved protein modification. The key step of this pathway is the en bloc transfer of the high mannose core oligosaccharide Glc3Man9GlcNAc2 from the lipid carrier dolichyl phosphate to selected Asn-X-Ser/Thr sequences of nascent polypeptide chains during their translocation across the endoplasmic reticulum membrane. The reaction is catalysed by the enzyme oligosaccharyltransferase (OST). Recent biochemical and molecular genetic studies in yeast have yielded novel insights into this enzyme with multiple tasks. Nine proteins have been shown to be OST components. These are assembled into a heterooligomeric membrane-bound complex and are required for optimal expression of OST activity in vivo in wild type cells. In accord with the evolutionary conservation of core N-glycosylation, there are significant homologies between the protein sequences of OST subunits from yeast and higher eukaryotes, and OST complexes from different sources show a similar organisation as well.

A role for the DnaJ homologue Scj1p in protein folding in the yeast endoplasmic reticulum

Members of the eukaryotic heat shock protein 70 family (Hsp70s) are regulated by protein cofactors that contain domains homologous to bacterial DnaJ. Of the three DnaJ homologues in the yeast rough endoplasmic reticulum (RER; Scj1p, Sec63p, and Jem1p), Scj1p is most closely related to DnaJ, hence it is a probable cofactor for Kar2p, the major Hsp70 in the yeast RER. However, the physiological role of Scj1p has remained obscure due to the lack of an obvious defect in Kar2p-mediated pathways in scj1 null mutants. Here, we show that the Deltascj1 mutant is hypersensitive to tunicamycin or mutations that reduce N-linked glycosylation of proteins. Although maturation of glycosylated carboxypeptidase Y occurs with wild-type kinetics in Deltascj1 cells, the transport rate for an unglycosylated mutant carboxypeptidase Y (CPY) is markedly reduced. Loss of Scj1p induces the unfolded protein response pathway, and results in a cell wall defect when combined with an oligosaccharyltransferase mutation. The combined loss of both Scj1p and Jem1p exaggerates the sensitivity to hypoglycosylation stress, leads to further induction of the unfolded protein response pathway, and drastically delays maturation of an unglycosylated reporter protein in the RER. We propose that the major role for Scj1p is to cooperate with Kar2p to mediate maturation of proteins in the RER lumen.

DAD1 is required for the function and the structural integrity of the oligosaccharyltransferase complex

Asparagine-linked glycosylation is a highly conserved protein modification reaction that occurs in all eukaryotic organisms. The oligosaccharyltransferase (OST), which has its active site exposed on the luminal face of the endoplasmic reticulum (ER), catalyzes the transfer of preassembled high mannose oligosaccharides onto certain asparagine residues of nascent polypeptides. The mammalian OST complex was initially thought to be composed of three transmembrane proteins, ribophorin I (RI), ribophorin II (RII), and OST48. Most recently, a small integral membrane protein of 12 kDa called DAD1 has been identified as an additional member of the mammalian OST complex. A point mutation in the DAD1 gene is responsible for the temperature-sensitive phenotype of a baby hamster kidney-derived cell line (tsBN7) that undergoes apoptosis at the non-permissive temperature. Furthermore, the mutant protein DAD1 is not detectable in tsBN7 cells 6 h after shifting the cells to the non-permissive temperature. This temperature-sensitive cell line offered unique opportunities to study the effects caused by the loss of one OST subunit on the other three subunits and also on N-linked glycosylation. Western blot analysis of cell lysates showed that after 6 h at the non-permissive temperature, steady-state levels of the ribophorins were reduced by about 50%, and OST48 was barely detectable. On the other hand, steady-state levels of other components of the rough ER, such as the alpha-subunits of the TRAP (translocon-associated membrane protein) and the Sec61 complex, which are components of the translocation apparatus, are not affected by the instability of the OST subunits. Furthermore, N-glycosylation of the ribophorins was seriously affected 6 h after shifting the cells to the non-permissive temperature, and after 12 h they were synthesized only in the non-glycosylated form. As may be expected, this defect in the OST complex at the non-permissive temperature caused also the underglycosylation of a secretory glycoprotein. We concluded that degradation of DAD1 at the non-permissive temperature not only affects the stability of OST48 and the ribophorins but also results in the functional inactivation of the OST complex.

A genetic system based on split-ubiquitin for the analysis of interactions between membrane proteins in vivo

A detection system for interactions between membrane proteins in vivo is described. The system is based on split-ubiquitin [Johnsson, N. & Varshavsky, A. (1994) Proc. Natl. Acad. Sci. USA 91, 10340-10344]. Interaction between two membrane proteins is detected by proteolytic cleavage of a protein fusion. The cleavage releases a transcription factor, which activates reporter genes in the nucleus. As a result, interaction between membrane proteins can be analyzed by the means of a colorimetric assay. We use membrane proteins of the endoplasmic reticulum as a model system. Wbp1p and Ost1p are both subunits of the oligosaccharyl transferase membrane protein complex. The Alg5 protein also localizes to the membrane of the endoplasmic reticulum, but does not interact with the oligosaccharyltransferase. Specific interactions are detected between Wbp1p and Ost1p, but not between Wbp1p and Alg5p. The new system might be useful as a genetic and biochemical tool for the analysis of interactions between membrane proteins in vivo.

A contiguous 60 kb genomic stretch from barley reveals molecular evidence for gene islands in a monocot genome

The contiguous DNA sequence of a 60 kb genomic interval of barley chromosome 4HL has been assembled. The region harbours a single and novel gypsy -like retrotransposon, designated BAGY-1. Only three genes appear to reside in the genomic stretch. One predicts a plant homologue of ribophorin I, a subunit of the oligosaccharyltransferase-protein complex located in the rough endoplasmatic reticulum. The second is similar to the Drosophila g1 gene encoding a ring finger protein involved in developmental processes. The observed gene density is approximately 5-fold lower than in the best characterized dicot genome of Arabidopsis but 6- to 10-fold higher than expected from an equidistant gene distribution in the complex barley genome. Our data suggest that the 60 kb genomic interval represents part of a gene island, a seemingly distinctive feature of grass genomes.

Protein N-glycosylation: molecular genetics and functional significance

Protein N-glycosylation is a metabolic process that has been highly conserved in evolution. In all eukaryotes, N-glycosylation is obligatory for viability. It functions by modifying appropriate asparagine residues of proteins with oligosaccharide structures, thus influencing their properties and bioactivities. N-glycoprotein biosynthesis involves a multitude of enzymes, glycosyltransferases, and glycosidases, encoded by distinct genes. The majority of these enzymes are transmembrane proteins that function in the endoplasmic reticulum and Golgi apparatus in an ordered and well-orchestrated manner. The complexity of N-glycosylation is augmented by the fact that different asparagine residues within the same polypeptide may be modified with different oligosaccharide structures, and various proteins are distinguished from one another by the characteristics of their carbohydrate moieties. Furthermore, biological consequences of derivatization of proteins with N-glycans range from subtle to significant. In the past, all these features of N-glycosylation have posed a formidable challenge to an elucidation of the physiological role for this modification. Recent advances in molecular genetics, combined with the availability of diverse in vivo experimental systems ranging from yeast to transgenic mice, have expedited the identification, isolation, and characterization of N-glycosylation genes. As a result, rather unexpected information regarding relationships between N-glycosylation and other cellular functions--including secretion, cytoskeletal organization, proliferation, and apoptosis--has emerged. Concurrently, increased understanding of molecular details of N-glycosylation has facilitated the alignment between N-glycosylation deficiencies and human diseases, and has highlighted the possibility of using N-glycan expression on cells as potential determinants of disease and its progression. Recent studies suggest correlations between N-glycosylation capacities of cells and drug sensitivities, as well as susceptibility to infection. Therefore, knowledge of the regulatory features of N-glycosylation may prove useful in the design of novel therapeutics. While facing the demanding task of defining properties, functions, and regulation of the numerous, as yet uncharacterized, N-glycosylation genes, glycobiologists of the 21st century offer exciting possibilities for new approaches to disease diagnosis, prevention, and cure.

The highly conserved Stt3 protein is a subunit of the yeast oligosaccharyltransferase and forms a subcomplex with Ost3p and Ost4p

The oligosaccharyltransferase has been purified from Saccharomyces cerevisiae as an hetero-oligomeric complex composed of four or six subunits. Here, the in vivo subunit composition and stoichiometry of the oligosaccharyltransferase were investigated by attaching an epitope coding sequence to a previously characterized subunit gene, OST3. Five (Ost1p, Wbp1p, Swp1p, Ost2p, and Ost5p) of the seven polypeptides that were coimmunoprecipitated with the epitope-tagged Ost3p were identical to those obtained by the conventional purification procedure. Two additional coprecipitating polypeptides with apparent molecular masses of 60 and 3.6 kDa were identified as the 78-kDa Stt3 protein and the 36-residue Ost4 protein, respectively. Stt3p and Ost4p were previously identified in screens for gene products involved in N-linked glycosylation. Quantification of the in vivo radiolabeled subunits and the radioiodinated purified enzyme shows that the yeast oligosaccharyltransferase is composed of equimolar amounts of eight subunits. Exposure of the immunoprecipitated oligosaccharyltransferase to mild protein denaturants yielded a subcomplex comprised of Stt3p, Ost3p, and Ost4p. These experiments, taken together with genetic and biochemical evidence for subunit interactions, suggest that the enzyme is composed of the following three subcomplexes: (a) Stt3p-Ost4p-Ost3p, (b) Swp1p-Wbp1p-Ost2p, and (c) Ost1p-Ost5p.

Interactions among subunits of the oligosaccharyltransferase complex

The mammalian oligosaccharyltransferase (OST) is an oligomeric complex composed of three membrane proteins of the endoplasmic reticulum: ribophorin I (RI), ribophorin II (RII), and OST48. In addition, sequence homology between the Ost2 subunit of the yeast OST complex and Dad1 (defender against apoptotic death) suggests that Dad1 may represent a fourth subunit of the mammalian OST complex. In attempts to elucidate the structural organization of this complex, we have studied the interactions among its subunits. Using the yeast two-hybrid system, we have shown that the luminal domains of RI and RII (RIL and RIIL, respectively) interacted with the luminal domain of OST48 (OST48L), but no direct interaction was observed between RIL and RIIL. These results were confirmed by biochemical assays. Deletion analyses using the yeast two-hybrid system showed that subdomain of RIL or RIIL adjacent to the respective transmembrane domains interacted with OST48L. Of the three equal length subdomains of OST48L, the one at the N terminus and the one next to the transmembrane domain interacted with RIL. None of these three subdomains of OST48L interacted with RIIL. The yeast two-hybrid assay also revealed affinity between the cytoplasmically located N-terminal region of Dad1 and the short cytoplasmic tail of OST48, thus placing Dad1 firmly into the OST complex. In addition, we found a homotypic interaction between the cytoplasmic domains of RI, which may play a role in the formation of the oligomeric array formed by components of the translocation machinery.

A specific screen for oligosaccharyltransferase mutations identifies the 9 kDa OST5 protein required for optimal activity in vivo and in vitro

The central reaction in the process of N-linked protein glycosylation in eukaryotic cells, the transfer of the oligosaccharide Glc(3)Man(9)GlcNAc(2) from the lipid dolicholpyrophosphate to selected asparagine residues, is catalyzed by the oligosaccharyltransferase (OTase). This enzyme consists of multiple subunits; however, purification of the complex has revealed different results with respect to its protein composition. To determine how many different loci are required for OTase activity in vivo, we performed a novel, specific screen for mutants with altered OTase activity. Based on the synthetic lethal phenotype of OTase mutants in combination with a deficiency of dolicholphosphoglucose biosynthesis which results in non-glucosylated lipid-linked oligosaccharide, we identified seven complementation groups with decreased OTase activity. Beside the known OTase loci, STT3, OST1, WBP1, OST3, SWP1 and OST2, a novel locus, OST5, was identified. OST5 is an intron-containing gene encoding a putative membrane protein of 9.5 kDa present in highly purified OTase preparations. OST5 protein is not essential for growth but its depletion results in a reduced OTase activity. Suppression of an ost1 mutation by overexpression of OST5 indicates that this small membrane protein directly interacts with other OTase components, most likely with Ost1p. A strong genetic interaction with a stt3 mutation implies a role in complex assembly.

Tumour-suppressor genes in prostatic oncogenesis: a positional approach

Genetic alterations, such as mutation, methylation and aneuploidy, are thought to underlie the multistep genesis and progression of many human cancers. However, the genetic events occurring in prostatic oncogenesis are still relatively poorly understood. This is especially so in early-stage tumours, in which mutations of known oncogenes or tumour-suppressor genes appear to be quite infrequent. Allelic losses of chromosome arms 7q, 8p, 10, 16q and 18q suggest the involvement of novel suppressor loci on these chromosomes; allelic losses of chromosome arm 8p are especially frequent and may be detected even in early-stage tumours. We have used a positional approach to seek novel genetic targets in prostate cancer, including allelic-loss mapping of chromosome 8p and physical mapping of chromosome band 8p22 around the MSR gene. A homozygous somatic deletion in one prostatic nodal metastasis was mapped in this region and spanned 730-970 kb. This region was then examined in detail for expressed sequences. One novel gene, called N33, was found to be silenced by a methylation mechanism in most colon cancer cell lines and some primary colorectal tumours. Characterization of additional chromosome 8p22 candidates is in progress.

A dual affinity tag on the 64-kDa Nlt1p subunit allows the rapid characterization of mutant yeast oligosaccharyl transferase complexes

Oligosaccharyl transferase catalyzes the glycosylation of selected asparagine residues of nascent polypeptide chains as they are translocated into the lumen of the endoplasmic reticulum. To date, this enzyme has been purified from a number of eukaryotic organisms. Purification of transferase activity has yielded polypeptide complexes of three to six subunits depending on the source organism. Here we present the purification of an affinity-tagged version of the enzyme complex from a membrane protein fraction of the yeast Saccharomyces cerevisiae. A yeast strain was created in which the essential 64-kDa glycoprotein Nlt1p subunit of the oligosaccharyl transferase was modified by the addition of a 22-residue carboxy-terminal affinity tag; the tag included both an 8-residue FLAG epitope and a 6-residue histidine motif. Facile purification of the oligosaccharyl transferase was achieved using affinity chromatography media specific for each segment of the tag. The enzyme was purified as a heteromeric complex of five subunits in agreement with previously reported characterizations of the yeast transferase. Yeast strains bearing affinity-tagged enzyme subunits allow the rapid characterization of native and mutant transferase complexes.

The G beta gamma complex of the yeast pheromone response pathway. Subcellular fractionation and protein-protein interactions

Genetic evidence suggests that the yeast STE4 and STE18 genes encode G beta and G gamma subunits, respectively, that the G betagamma complex plays a positive role in the pheromone response pathway, and that its activity is subject to negative regulation by the G alpha subunit (product of the GPA1 gene) and to positive regulation by cell-surface pheromone receptors. However, as yet there is no direct biochemical evidence for a G betagamma protein complex associated with the plasma membrane. We found that the products of the STE4 and STE18 genes are stably associated with plasma membrane as well as with internal membranes and that 30% of the protein pool is not tightly associated with either membrane fraction. A slower-migrating, presumably phosphorylated, form of Ste4p is enriched in the non-membrane fraction. The Ste4p and Ste18p proteins that had been extracted from plasma membranes with detergent were found to co-sediment as an 8 S particle under low salt conditions and as a 6 S particle in the presence of 0.25 M NaCl; the Ste18p in these fractions was precipitated with anti-Ste4p antiserum. Under the conditions of our assay, Gpa1p was not associated with either particle. The levels of Ste4p and Ste18p accumulation in mutant cells provided additional evidence for a G betagamma complex. Ste18p failed to accumulate in ste4 mutant cells, and Ste4p showed reduced levels of accumulation and an increased rate of turnover in ste18 mutant cells. The gpa1 mutant blocked stable association of Ste4p with the plasma membrane, and the ste18 mutant blocked stable association of Ste4p with both plasma membranes and internal membranes. The membrane distribution of Ste4p was unaffected by the ste2 mutation or by down-regulation of the cell-surface receptors. These results indicate that at least 40% of Ste4p and Ste18p are part of a G betagamma complex at the plasma membrane and that stable association of this complex with the plasma membrane requires the presence of G alpha.

The OST4 gene of Saccharomyces cerevisiae encodes an unusually small protein required for normal levels of oligosaccharyltransferase activity

Sodium vanadate is an effective drug for the enrichment of yeast mutants defective in glycosylation reactions that are carried out in the Golgi complex. We have isolated vanadate-resistant, hygromycin B-sensitive mutants that act at very early steps of N-linked glycosylation, occurring in the endoplasmic reticulum. Here we describe the phenotypic characterization of ost4, a vanadate-resistant mutant that is defective in oligosaccharyltransferase (OTase) activity both in vivo and in vitro. The OST4 open reading frame is unusual in that it predicts a protein of only 36 amino acids. We demonstrate that the OST4 gene product is, in fact, an unusually small protein of approximately 3.6 kDa, predicted to lie almost entirely in the hydrophobic environment of the membrane. Strains carrying a disruption of the OST4 gene are viable but grow poorly at 25 degrees C. The null mutant is inviable at 37 degrees C, demonstrating that the OST4 gene product is essential for growth at high temperatures. Deletion of the OST4 gene greatly diminishes OTase activity but does not abolish it. These results suggest that the OST4 gene encodes a subunit or accessory component of OTase that is essential at high temperature.

The essential OST2 gene encodes the 16-kD subunit of the yeast oligosaccharyltransferase, a highly conserved protein expressed in diverse eukaryotic organisms

Oligosaccharyltransferase catalyzes the transfer of a preassembled high mannose oligosaccharide from a dolichol-oligosaccharide donor to consensus glycosylation acceptor sites in newly synthesized proteins in the lumen of the rough endoplasmic reticulum. The Saccharomyces cerevisiae oligosaccharyltransferase is an oligomeric complex composed of six non-identical subunits (alpha-zeta). The alpha, beta, gamma, and delta subunits of the oligosaccharyltransferase are encoded by the OST1, WBP1, OST3, and SWP1 genes, respectively. Here we describe the functional characterization of the OST2 gene that encodes the epsilon-subunit of the oligosaccharyltransferase. Genomic disruption of the OST2 locus was lethal in haploid yeast showing that expression of the Ost2 protein is essential for viability. Overexpression of the Ost2 protein suppresses the temperature-sensitive phenotype of the wbp1-2 allele and increases in vivo and in vitro oligosaccharyltransferase activity in a wbp1-2 strain. An analysis of a series of conditional ost2 mutants demonstrated that defects in the Ost2 protein cause pleiotropic underglycosylation of soluble and membrane-bound glycoproteins. Microsomal membranes isolated from ost2 mutant yeast show marked reductions in the in vitro transfer of high mannose oligosaccharide from exogenous lipid-linked oligosaccharide to a glycosylation site acceptor tripeptide. Surprisingly, the Ost2 protein was found to be 40% identical to the DAD1 protein (defender against apoptotic cell death), a highly conserved protein initially identified in vertebrate organisms. The protein sequence of ost2 mutant alleles revealed mutations at highly conserved residues in the Ost2p/DAD1 protein sequence.

Functional characterization of Ost3p. Loss of the 34-kD subunit of the Saccharomyces cerevisiae oligosaccharyltransferase results in biased underglycosylation of acceptor substrates

Within the lumen of the rough endoplasmic reticulum, oligosaccharyltransferase catalyzes the en bloc transfer of a high mannose oligosaccharide moiety from the lipid-linked oligosaccharide donor to asparagine acceptor sites in nascent polypeptides. The Saccharomyces cerevisiae oligosaccharyltransferase was purified as a heteroligomeric complex consisting of six subunits (alpha-zeta) having apparent molecular masses of 64 kD (Ost1p), 45 kD (Wbp1p), 34 kD, 30 kD (Swp1p), 16 kD, and 9 kD. Here we report a structural and functional characterization of Ost3p which corresponds to the 34-kD gamma-subunit of the oligosaccharyltransferase. Unlike Ost1p, Wbp1p, and Swp1p, expression of Ost3p is not essential for viability of yeast. Instead, ost3 null mutant yeast grow at wild-type rates on solid or in liquid media irrespective of culture temperature. Nonetheless, detergent extracts prepared from ost3 null mutant membranes are twofold less active than extracts prepared from wild-type membranes in an in vitro oligosaccharyltransferase assay. Furthermore, loss of Ost3p is accompanied by significant underglycosylation of soluble and membrane-bound glycoproteins in vivo. Compared to the previously characterized ost1-1 mutant in the oligosaccharyltransferase, and the alg5 mutant in the oligosaccharide assembly pathway, ost3 null mutant yeast appear to be selectively impaired in the glycosylation of several membrane glycoproteins. The latter observation suggests that Ost3p may enhance oligosaccharide transfer in vivo to a subset of acceptor substrates.