STT3, a novel essential gene related to the PKC1/STT1 protein kinase pathway, is involved in protein glycosylation in yeast

The Arabidopsis tonoplast is almost devoid of glycoproteins with complex N-glycans, unlike the rat lysosomal membrane

The distribution of the N-glycoproteome in integral membrane proteins of the vacuolar membrane (tonoplast) or the plasma membrane of Arabidopsis thaliana and, for further comparison, of the Rattus norvegicus lysosomal and plasma membranes, was analyzed. In silico analysis showed that potential N-glycosylation sites are much less frequent in tonoplast proteins. Biochemical analysis of Arabidopsis subcellular fractions with the lectin concanavalin A, which recognizes mainly unmodified N-glycans, or with antiserum against Golgi-modified N-glycans confirmed the in silico results and showed that, unlike the plant plasma membrane, the tonoplast is almost or totally devoid of N-glycoproteins with Golgi-modified glycans. Lysosomes share with vacuoles the hydrolytic functions and the position along the secretory pathway; however, our results indicate that their membranes had a divergent evolution. We propose that protection against the luminal hydrolases that are abundant in inner hydrolytic compartments, which seems to have been achieved in many lysosomal membrane proteins by extensive N-glycosylation of the luminal domains, has instead been obtained in the vast majority of tonoplast proteins by limiting the length of such domains.

Protein Glycosylation in Aspergillus fumigatus Is Essential for Cell Wall Synthesis and Serves as a Promising Model of Multicellular Eukaryotic Development

Glycosylation is a conserved posttranslational modification that is found in all eukaryotes, which helps generate proteins with multiple functions. Our knowledge of glycosylation mainly comes from the investigation of the yeast Saccharomyces cerevisiae and mammalian cells. However, during the last decade, glycosylation in the human pathogenic mold Aspergillus fumigatus has drawn significant attention. It has been revealed that glycosylation in A. fumigatus is crucial for its growth, cell wall synthesis, and development and that the process is more complicated than that found in the budding yeast S. cerevisiae. The present paper implies that the investigation of glycosylation in A. fumigatus is not only vital for elucidating the mechanism of fungal cell wall synthesis, which will benefit the design of new antifungal therapies, but also helps to understand the role of protein glycosylation in the development of multicellular eukaryotes. This paper describes the advances in functional analysis of protein glycosylation in A. fumigatus.

Repression of N-glycosylation triggers the unfolded protein response (UPR) and overexpression of cell wall protein and chitin in Aspergillus fumigatus

Aspergillus fumigatus is the most common airborne fungal pathogen, causing fatal invasive aspergillosis in immunocompromised patients. The crude mortality is 60-90 % and remains around 29-42 % even with treatment. The main reason for patient death is the low efficiency of the drug therapies. As protein N-glycosylation is involved in cell wall biogenesis in A. fumigatus, a deeper understanding of its role in cell wall biogenesis will help to develop new drug targets. The Afstt3 gene encodes the essential catalytic subunit of oligosaccharyltransferase, an enzyme complex responsible for the transfer of the N-glycan to nascent polypeptides. To evaluate the role of N-glycosylation in cell wall biosynthesis, we constructed the conditional mutant strain CPR-stt3 by replacing the endogenous promoter of Afstt3 with the nitrogen-dependent niiA promoter. Repression of the Afstt3 gene in the CPR-stt3 strain led to a severe retardation of growth and a slight defect in cell wall integrity (CWI). One of the most interesting findings was that upregulation of the cell wall-related genes was not accompanied by an activation of the MpkA kinase, which has been shown to be a central element in the CWI signalling pathway in both Saccharomyces cerevisiae and A. fumigatus. Considering that the unfolded protein response (UPR) was found to be activated, which might upregulate the expression of cell wall protein and chitin, our data suggest that the UPR, instead of the MpkA-dependent CWI signalling pathway, is the major compensatory mechanism induced by repression but not abolition of N-glycosylation in A. fumigatus. Our finding is a key to understanding the complex compensatory mechanisms of cell wall biosynthesis and may provide a new strategy for drug development.

Characterization of N-linked protein glycosylation in Helicobacter pullorum

The first bacterial N-linked glycosylation system was discovered in Campylobacter jejuni, and the key enzyme involved in the coupling of glycan to asparagine residues within the acceptor sequon of the glycoprotein is the oligosaccharyltransferase PglB. Emerging genome sequence data have revealed that pglB orthologues are present in a subset of species from the Deltaproteobacteria and Epsilonproteobacteria, including three Helicobacter species: H. pullorum, H. canadensis, and H. winghamensis. In contrast to C. jejuni, in which a single pglB gene is located within a larger gene cluster encoding the enzymes required for the biosynthesis of the N-linked glycan, these Helicobacter species contain two unrelated pglB genes (pglB1 and pglB2), neither of which is located within a larger locus involved in protein glycosylation. In complementation experiments, the H. pullorum PglB1 protein, but not PglB2, was able to transfer C. jejuni N-linked glycan onto an acceptor protein in Escherichia coli. Analysis of the characterized C. jejuni N-glycosylation system with an in vitro oligosaccharyltransferase assay followed by matrix-assisted laser desorption ionization (MALDI) mass spectrometry demonstrated the utility of this approach, and when applied to H. pullorum, PglB1-dependent N glycosylation with a linear pentasaccharide was observed. This reaction required an acidic residue at the -2 position of the N-glycosylation sequon, as for C. jejuni. Attempted insertional knockout mutagenesis of the H. pullorum pglB2 gene was unsuccessful, suggesting that it is essential. These first data on N-linked glycosylation in a second bacterial species demonstrate the similarities to, and fundamental differences from, the well-studied C. jejuni system.

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.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

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.

Oligosaccharyltransferase isoforms that contain different catalytic STT3 subunits have distinct enzymatic properties

Oligosaccharyltransferase (OST) is an integral membrane protein that catalyzes N-linked glycosylation of nascent proteins in the lumen of the endoplasmic reticulum. Although the yeast OST is an octamer assembled from nonhomologous subunits (Ost1p, Ost2p, Ost3p/Ost6p, Ost4p, Ost5p, Wbp1p, Swp1p, and Stt3p), the composition of the vertebrate OST was less well defined. The roles of specific OST subunits remained enigmatic. Here we show that genomes of most multicellular eukaryotes encode two homologs of Stt3p and mammals express two homologs of Ost3p. The Stt3p and Ost3p homologs are assembled together with the previously described mammalian OST subunits (ribophorins I and II, OST48, and DAD1) into complexes that differ significantly in enzymatic activity. Tissue and cell type-specific differences in expression of the Stt3p homologs suggest that the enzymatic properties of oligosaccharyltransferase are regulated in eukaryotes to respond to alterations in glycoprotein flux through the secretory pathway and may contribute to tissue-specific glycan heterogeneity.

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.

Identification of differentially expressed genes following treatment of monkey kidney cells with the mycotoxin fumonisin B(1)

Fumonisin B(1) (FB(1)) is a mycotoxin produced by the phytopathogenic fungus Fusarium moniliforme, which structurally resembles sphingoid bases. FB(1) perturbs sphingolipid synthesis by inhibiting the activity of ceramide synthase. Depending on the host, ingestion of FB(1) causes equine leukoencephalomalacia or porcine pulmonary edema. It is also carcinogenic to rats and may play a role in certain human cancers. Previous studies showed that FB(1) repressed specific isoforms of protein kinase C and cyclin-dependent kinase 2 (CDK2) activity. Conversely, FB(1) induced expression of CDK inhibitors, p21(Waf1/Cip1), p27(Kip1), and p57(Kip2) in monkey kidney cells (CV-1). Consequently, FB(1) treatment of CV-1 cells leads to cell-cycle arrest and apoptosis. The baculovirus IAP gene (inhibitor of apoptosis), which blocks tumor necrosis factor (TNF)-induced apoptosis, protects several fibroblast cell types from apoptosis, suggesting the TNF pathway is important for FB(1)-induced apoptosis. To identify genes that are induced by FB(1), we used a PCR-based subtraction approach. Eight genes that showed high similarity (> 90%) to known mammalian genes were identified. These genes included: tumor necrosis factor type 1 receptor associated protein 2 (TRAP2), human leukemia virus receptor (GLVR1), human Scaffold attachment factor A (SAF-A) also called heterogeneous nuclear ribonucleoprotein U (hnRNP-U), human protein kinase C-binding protein (RACK7), human oligosaccharyl transferase STT3 subunit, mouse WW-domain binding protein 2 (WBP2), human fibronectin, and an unknown human clone. The ability of FB(1) to alter gene expression and signal transduction pathways may be necessary for its carcinogenic and toxic effects.

Evidence for interaction of yeast protein kinase C with several subunits of oligosaccharyl transferase

Oligosaccharyltransferase (OT) in Saccharomyces cerevisiae is an enzyme complex consisting of 8 transmembrane proteins located in the endoplasmic reticulum (ER). Studies on potential protein-protein interactions in OT using a two-hybrid library screen revealed that protein kinase C (Pkc1p) interacted with the lumenal domains of several OT subunits. Additional genetic experiments revealed that overexpression of two OT subunits rescued the growth defect caused by overexpression of a Pkc1 active site mutant, implying that there are specific genetic interactions between PKC1 and OT. These in vivo findings were complemented by in vitro studies that showed that several of the OT subunits bound to a fusion protein consisting of glutathione S-transferase linked via its C-terminus to Pkc1p. Assays of OT activity, in which glycosylation of a simple acceptor peptide was assayed in microsomes from wild-type and a pkc1 null revealed a 50% reduction in activity in the microsomes from the null strain. In contrast, strains containing null mutations of two other genes known to be downstream of Pkc1p in the PKC1-MAP kinase pathway had a level of OT activity comparable to that of wild-type cells. These in vivo and in vitro experiments suggest that in yeast cells Pkc1p may be involved in regulation of the N-glycosylation of proteins.

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.

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 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.

The lipopolysaccharide biosynthesis locus of Campylobacter jejuni 81116

Most Campylobacter jejuni strains express lipo-oligosaccharides. Some strains also express lipopolysaccharides (LPS), with O-antigen-like carbohydrate repeats. C. jejuni 81116 expresses an LPS containing both lipo-oligosaccharides and O-antigen-like repeats, but nothing is known about the structure or sugar composition of these LPS species. A cosmid library of the genome of C. jejuni 81116 was constructed and probed with Campylobacter hyoilei genes involved in LPS synthesis. Five cosmids hybridized with the probe and two of these expressed C. jejuni 81116 LPS in Escherichia coli. By subcloning, a 16 kb DNA region was identified which contains the genetic information required to express C. jejuni LPS. DNA sequence analysis revealed 11 ORFs homologous to genes involved in LPS synthesis of other bacteria. They consisted of three homologues of sugar biosynthesis genes, two homologues of transport genes and six homologues of sugar transferases.

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.

A review of phenotypes in Saccharomyces cerevisiae

A summary of previously defined phenotypes in the yeast Saccharomyces cerevisiae is presented. The purpose of this review is to provide a compendium of phenotypes that can be readily screened to identify pleiotropic phenotypes associated with primary or suppressor mutations. Many of these phenotypes provide a convenient alternative to the primary phenotype for following a gene, or as a marker for cloning a gene by genetic complementation. In many cases a particular phenotype or set of phenotypes can suggest a function for the product of the mutated gene.