Two proteins homologous to the N- and C-terminal domains of the bacterial glycosyltransferase Murg are required for the second step of dolichyl-linked oligosaccharide synthesis in Saccharomyces cerevisiae

Molecular characterization of Rft1, an ER membrane protein associated with congenital disorder of glycosylation RFT1-CDG

The oligosaccharide needed for protein N-glycosylation is assembled on a lipid carrier via a multistep pathway. Synthesis is initiated on the cytoplasmic face of the endoplasmic reticulum (ER) and completed on the luminal side after transbilayer translocation of a heptasaccharide lipid intermediate. More than 30 congenital disorders of glycosylation (CDGs) are associated with this pathway, including RFT1-CDG which results from defects in the membrane protein Rft1. Rft1 is essential for the viability of yeast and mammalian cells and was proposed as the transporter needed to flip the heptasaccharide lipid intermediate across the ER membrane. However, other studies indicated that Rft1 is not required for heptasaccharide lipid flipping in microsomes or unilamellar vesicles reconstituted with ER membrane proteins, nor is it required for the viability of at least one eukaryote. It is therefore not known what essential role Rft1 plays in N-glycosylation. Here, we present a molecular characterization of human Rft1, using yeast cells as a reporter system. We show that it is a multispanning membrane protein located in the ER, with its N and C termini facing the cytoplasm. It is not N-glycosylated. The majority of RFT1-CDG mutations map to highly conserved regions of the protein. We identify key residues that are important for Rft1's ability to support N-glycosylation and cell viability. Our results provide a necessary platform for future work on this enigmatic protein.

Molecular characterization of Rft1, an ER membrane protein associated with congenital disorder of glycosylation RFT1-CDG

The oligosaccharide needed for protein N-glycosylation is assembled on a lipid carrier via a multi-step pathway. Synthesis is initiated on the cytoplasmic face of the endoplasmic reticulum (ER) and completed on the luminal side after transbilayer translocation of a heptasaccharide lipid intermediate. More than 30 Congenital Disorders of Glycosylation (CDGs) are associated with this pathway, including RFT1-CDG which results from defects in the membrane protein Rft1. Rft1 is essential for the viability of yeast and mammalian cells and was proposed as the transporter needed to flip the heptasaccharide lipid intermediate across the ER membrane. However, other studies indicated that Rft1 is not required for heptasaccharide lipid flipping in microsomes or unilamellar vesicles reconstituted with ER membrane proteins, nor is it required for the viability of at least one eukaryote. It is therefore not known what essential role Rft1 plays in N-glycosylation. Here, we present a molecular characterization of human Rft1, using yeast cells as a reporter system. We show that it is a multi-spanning membrane protein located in the ER, with its N and C-termini facing the cytoplasm. It is not N-glycosylated. The majority of RFT1-CDG mutations map to highly conserved regions of the protein. We identify key residues that are important for Rft1's ability to support N-glycosylation and cell viability. Our results provide a necessary platform for future work on this enigmatic protein.

Mutations in Glycosyltransferases and Glycosidases: Implications for Associated Diseases

Glycosylation, a crucial and the most common post-translational modification, coordinates a multitude of biological functions through the attachment of glycans to proteins and lipids. This process, predominantly governed by glycosyltransferases (GTs) and glycoside hydrolases (GHs), decides not only biomolecular functionality but also protein stability and solubility. Mutations in these enzymes have been implicated in a spectrum of diseases, prompting critical research into the structural and functional consequences of such genetic variations. This study compiles an extensive dataset from ClinVar and UniProt, providing a nuanced analysis of 2603 variants within 343 GT and GH genes. We conduct thorough MTR score analyses for the proteins with the most documented variants using MTR3D-AF2 via AlphaFold2 (AlphaFold v2.2.4) predicted protein structure, with the analyses indicating that pathogenic mutations frequently correlate with Beta Bridge secondary structures. Further, the calculation of the solvent accessibility score and variant visualisation show that pathogenic mutations exhibit reduced solvent accessibility, suggesting the mutated residues are likely buried and their localisation is within protein cores. We also find that pathogenic variants are often found proximal to active and binding sites, which may interfere with substrate interactions. We also incorporate computational predictions to assess the impact of these mutations on protein function, utilising tools such as mCSM to predict the destabilisation effect of variants. By identifying these critical regions that are prone to disease-associated mutations, our study opens avenues for designing small molecules or biologics that can modulate enzyme function or compensate for the loss of stability due to these mutations.

An in vitro assay for enzymatic studies on human ALG13/14 heterodimeric UDP-N-acetylglucosamine transferase

The second step of eukaryotic lipid-linked oligosaccharide (LLO) biosynthesis is catalyzed by the conserved ALG13/ALG14 heterodimeric UDP-N-acetylglucosamine transferase (GnTase). In humans, mutations in ALG13 or ALG14 lead to severe neurological disorders with a multisystem phenotype, known as ALG13/14-CDG (congenital disorders of glycosylation). How these mutations relate to disease is unknown because to date, a reliable GnTase assay for studying the ALG13/14 complex is lacking. Here we describe the development of a liquid chromatography/mass spectrometry-based quantitative GnTase assay using chemically synthesized GlcNAc-pyrophosphate-dolichol as the acceptor and purified human ALG13/14 dimeric enzyme. This assay enabled us to demonstrate that in contrast to the literature, only the shorter human ALG13 isoform 2, but not the longer isoform 1 forms a functional complex with ALG14 that participates in LLO synthesis. The longer ALG13 isoform 1 does not form a complex with ALG14 and therefore lacks GnTase activity. Importantly, we further established a quantitative assay for GnTase activities of ALG13- and ALG14-CDG variant alleles, demonstrating that GnTase deficiency is the cause of ALG13/14-CDG phenotypes.

The Impact of Glycoengineering on the Endoplasmic Reticulum Quality Control System in Yeasts

Yeasts are widely used and established production hosts for biopharmaceuticals. Despite of tremendous advances on creating human-type N-glycosylation, N-glycosylated biopharmaceuticals manufactured with yeasts are missing on the market. The N-linked glycans fulfill several purposes. They are essential for the properties of the final protein product for example modulating half-lives or interactions with cellular components. Still, while the protein is being formed in the endoplasmic reticulum, specific glycan intermediates play crucial roles in the folding of or disposal of proteins which failed to fold. Despite of this intricate interplay between glycan intermediates and the cellular machinery, many of the glycoengineering approaches are based on modifications of the N-glycan processing steps in the endoplasmic reticulum (ER). These N-glycans deviate from the canonical structures required for interactions with the lectins of the ER quality control system. In this review we provide a concise overview on the N-glycan biosynthesis, glycan-dependent protein folding and quality control systems and the wide array glycoengineering approaches. Furthermore, we discuss how the current glycoengineering approaches partially or fully by-pass glycan-dependent protein folding mechanisms or create structures that mimic the glycan epitope required for ER associated protein degradation.

Agl24 is an ancient archaeal homolog of the eukaryotic N-glycan chitobiose synthesis enzymes

Protein N-glycosylation is a post-translational modification found in organisms of all domains of life. The crenarchaeal N-glycosylation begins with the synthesis of a lipid-linked chitobiose core structure, identical to that in Eukaryotes, although the enzyme catalyzing this reaction remains unknown. Here, we report the identification of a thermostable archaeal β-1,4-N-acetylglucosaminyltransferase, named archaeal glycosylation enzyme 24 (Agl24), responsible for the synthesis of the N-glycan chitobiose core. Biochemical characterization confirmed its function as an inverting β-D-GlcNAc-(1→4)-α-D-GlcNAc-diphosphodolichol glycosyltransferase. Substitution of a conserved histidine residue, found also in the eukaryotic and bacterial homologs, demonstrated its functional importance for Agl24. Furthermore, bioinformatics and structural modeling revealed similarities of Agl24 to the eukaryotic Alg14/13 and a distant relation to the bacterial MurG, which are catalyzing the same or a similar reaction, respectively. Phylogenetic analysis of Alg14/13 homologs indicates that they are ancient in Eukaryotes, either as a lateral transfer or inherited through eukaryogenesis.

Structural Analysis of the Effect of Asn107Ser Mutation on Alg13 Activity and Alg13-Alg14 Complex Formation and Expanding the Phenotypic Variability of ALG13-CDG

Congenital Disorders of Glycosylation (CDG) are multisystemic metabolic disorders showing highly heterogeneous clinical presentation, molecular etiology, and laboratory results. Here, we present different transferrin isoform patterns (obtained by isoelectric focusing) from three female patients harboring the ALG13 c.320A>G mutation. Contrary to other known variants of type I CDGs, where transferrin isoelectric focusing revealed notably increased asialo- and disialotransferrin fractions, a normal glycosylation pattern was observed in the probands. To verify this data and give novel insight into this variant, we modeled the human Alg13 protein and analyzed the dynamics of the apo structure and the complex with the UDP-GlcNAc substrate. We also modeled the Alg13-Alg14 heterodimer and ran multiple simulations of the complex in the presence of the substrate. Finally, we proposed a plausible complex formation mechanism.

Structural and functional analysis of Alg1 beta-1,4 mannosyltransferase reveals the physiological importance of its membrane topology

In eukaryotes, the biosynthesis of a highly conserved dolichol-linked oligosaccharide (DLO) precursor Glc3Man9GlcNAc2-pyrophosphate-dolichol (PP-Dol) begins on the cytoplasmic face of the endoplasmic reticulum (ER) and ends within the lumen. Two functionally distinguished heteromeric glycosyltransferase (GTase) complexes are responsible for the cytosolic DLO assembly. Alg1, a β-1, 4 mannosyltransferase (MTase) physically interacts with Alg2 and Alg11 proteins to form the multienzyme complex which catalyzes the addition of all five mannose to generate the Man5GlcNAc2-PP-Dol intermediate. Despite the fact that Alg1 plays a central role in the formation of the multi-MTase has been confirmed, the topological information of Alg1 including the molecular mechanism of membrane association are still poorly understood. Using a combination of bioinformatics and biological approaches, we have undertaken a structural and functional study on Alg1 protein, in which the enzymatic activities of Alg1 and its variants were monitored by a complementation assay using the GALpr-ALG1 yeast strain, and further confirmed by a liquid chromatography-mass spectrometry-based in vitro quantitative assay. Computational and experimental evidence confirmed Alg1 shares structure similarity with Alg13/14 complex, which has been defined as a membrane-associated GT-B GTase. Particularly, we provide clear evidence that the N-terminal transmembrane domain including the following positively charged amino acids and an N-terminal amphiphilic-like α helix domain exposed on the protein surface strictly coordinate the Alg1 orientation on the ER membrane. This work provides detailed membrane topology of Alg1 and further reveals its biological importance at the spatial aspect in coordination of cytosolic DLO biosynthesis.

Exome sequence identified a c.320A > G ALG13 variant in a female with infantile epileptic encephalopathy with normal glycosylation and random X inactivation: Review of the literature

Congenital Disorders of Glycosylation (CDG) are new and rapidly expanding neurometabolic disorders with multisystem involvements, broad phenotypic manifestations, and variable severity. The majority results from a defect of one of the steps involved with protein or lipid N-glycosylation pathway. Almost all are inherited in autosomal recessive patterns with a few exceptions such as the X-linked ALG13. Mutations of ALG13 are reported, so far in only 10 patients, all were ascertained through exome/genome sequencing. Specifically, the ALG13 c.320A > G (p.Asn107Ser) variant was reported only in females and in all were de novo mutations. These findings may suggest an X-linked dominant inheritance of this mutation with embryonic male lethality. These patients presented with severe infantile epileptic encephalopathy, global developmental delay, and multisystem abnormalities. Only two of these females had glycosylation studies done, and both showed normal pattern of glycosylated serum transferrin isoforms, and none had their X-chromosome inactivation patterns studied. Here, we report on another female patient who is heterozygous for the same ALG13 c.320A > G (p.Asn107Ser) variant. She presented with infantile spasms, epileptic encephalopathy, hypsarrhythmia, hypotonia, developmental delay, intellectual disability, abnormal coagulation profile, feeding problems, hypotonia, and dysmorphic features. The diagnosis of CGD was suspected clinically, but glycosylation studies were done twice and showed normal patterns on both occasions. Her X-inactivation study was also done and, surprisingly, showed a random pattern of X-inactivation, with no evidence of skewness.

The multiple evolutionary origins of the eukaryotic N-glycosylation pathway

BACKGROUND

The N-glycosylation is an essential protein modification taking place in the membranes of the endoplasmic reticulum (ER) in eukaryotes and the plasma membranes in archaea. It shares mechanistic similarities based on the use of polyisoprenol lipid carriers with other glycosylation pathways involved in the synthesis of bacterial cell wall components (e.g. peptidoglycan and teichoic acids). Here, a phylogenomic analysis was carried out to examine the validity of rival hypotheses suggesting alternative archaeal or bacterial origins to the eukaryotic N-glycosylation pathway.

RESULTS

The comparison of several polyisoprenol-based glycosylation pathways from the three domains of life shows that most of the implicated proteins belong to a limited number of superfamilies. The N-glycosylation pathway enzymes are ancestral to the eukaryotes, but their origins are mixed: Alg7, Dpm and maybe also one gene of the glycosyltransferase 1 (GT1) superfamily and Stt3 have proteoarchaeal (TACK superphylum) origins; alg2/alg11 may have resulted from the duplication of the original GT1 gene; the lumen glycosyltransferases were probably co-opted and multiplied through several gene duplications during eukaryogenesis; Alg13/Alg14 are more similar to their bacterial homologues; and Alg1, Alg5 and a putative flippase have unknown origins.

CONCLUSIONS

The origin of the eukaryotic N-glycosylation pathway is not unique and less straightforward than previously thought: some basic components likely have proteoarchaeal origins, but the pathway was extensively developed before the eukaryotic diversification through multiple gene duplications, protein co-options, neofunctionalizations and even possible horizontal gene transfers from bacteria. These results may have important implications for our understanding of the ER evolution and eukaryogenesis.

REVIEWERS

This article was reviewed by Pr. Patrick Forterre and Dr. Sergei Mekhedov (nominated by Editorial Board member Michael Galperin).

A case of fatal Type I congenital disorders of glycosylation (CDG I) associated with low dehydrodolichol diphosphate synthase (DHDDS) activity

BACKGROUND

Type I congenital disorders of glycosylation (CDG-I) are mostly complex multisystemic diseases associated with hypoglycosylated serum glycoproteins. A subgroup harbour mutations in genes necessary for the biosynthesis of the dolichol-linked oligosaccharide (DLO) precursor that is essential for protein N-glycosylation. Here, our objective was to identify the molecular origins of disease in such a CDG-Ix patient presenting with axial hypotonia, peripheral hypertonia, enlarged liver, micropenis, cryptorchidism and sensorineural deafness associated with hypo glycosylated serum glycoproteins.

RESULTS

Targeted sequencing of DNA revealed a splice site mutation in intron 5 and a non-sense mutation in exon 4 of the dehydrodolichol diphosphate synthase gene (DHDDS). Skin biopsy fibroblasts derived from the patient revealed ~20 % residual DHDDS mRNA, ~35 % residual DHDDS activity, reduced dolichol-phosphate, truncated DLO and N-glycans, and an increased ratio of [2-(3)H]mannose labeled glycoprotein to [2-(3)H]mannose labeled DLO. Predicted truncated DHDDS transcripts did not complement rer2-deficient yeast. SiRNA-mediated down-regulation of DHDDS in human hepatocellular carcinoma HepG2 cells largely mirrored the biochemical phenotype of cells from the patient. The patient also harboured the homozygous ALG6(F304S) variant, which does not cause CDG but has been reported to be more frequent in PMM2-CDG patients with severe/fatal disease than in those with moderate presentations. WES did not reveal other strong candidate causal genes.

CONCLUSIONS

We describe a patient presenting with severe multisystem disease associated with DHDDS deficiency. As retinitis pigmentosa is the only clinical sign in previously reported cases, this report broadens the spectrum of phenotypes associated with this condition.

Demonstration of an oligosaccharide-diphosphodolichol diphosphatase activity whose subcellular localization is different than those of dolichyl-phosphate-dependent enzymes of the dolichol cycle

Oligosaccharyl phosphates (OSPs) are hydrolyzed from oligosaccharide-diphosphodolichol (DLO) during protein N-glycosylation by an uncharacterized process. An OSP-generating activity has been reported in vitro, and here we asked if its biochemical characteristics are compatible with a role in endoplasmic reticulum (ER)-situated DLO regulation. We demonstrate a Co(2+)-dependent DLO diphosphatase (DLODP) activity that splits DLO into dolichyl phosphate and OSP. DLODP has a pH optimum of 5.5 and is inhibited by vanadate but not by NaF. Polyprenyl diphosphates inhibit [(3)H]OSP release from [(3)H]DLO, the length of their alkyl chains correlating positively with inhibition potency. The diphosphodiester GlcNAc2-PP-solanesol is hydrolyzed to yield GlcNAc2-P and inhibits [(3)H]OSP release from [(3)H]DLO more effectively than the diphosphomonoester solanesyl diphosphate. During subcellular fractionation of liver homogenates, DLODP codistributes with microsomal markers, and density gradient centrifugation revealed that the distribution of DLODP is closer to that of Golgi apparatus-situated UDP-galactose glycoprotein galactosyltransferase than those of dolichyl-P-dependent glycosyltransferases required for DLO biosynthesis in the ER. Therefore, a DLODP activity showing selectivity toward lipophilic diphosphodiesters such as DLO, and possessing properties distinct from other lipid phosphatases, is identified. Separate subcellular locations for DLODP action and DLO biosynthesis may be required to prevent uncontrolled DLO destruction.

X chromosome exome sequencing reveals a novel ALG13 mutation in a nonsyndromic intellectual disability family with multiple affected male siblings

X-linked intellectual disability (XLID) is a heterogeneous condition associated with mutations in >100 genes, accounting for over 10% of all cases of intellectual impairment. The majority of XLID cases show nonsyndromic forms (NSXLID), in which intellectual disability is the sole clinically consistent manifestation. Here we performed X chromosome exome (X-exome) sequencing to identify the causative mutation in an NSXLID family with four affected male siblings and five unaffected female siblings. The X-exome sequencing at 88× coverage in one affected male sibling revealed a novel missense mutation (p.Tyr1074Cys) in the asparagine-linked glycosylation 13 homolog (ALG13) gene. Segregation analysis by Sanger sequencing showed that the all affected siblings were hemizygous and the mother was heterozygous for the mutation. Recently, a de novo missense mutation in ALG13 has been reported in a patient with X-linked congenital disorders of glycosylation type I. Our study reports the first case of NSXLID caused by a mutation in ALG13 involved in protein N-glycosylation.

Novel exopolysaccharides produced by Lactococcus lactis subsp. lactis, and the diversity of epsE genes in the exopolysaccharide biosynthesis gene clusters

To characterize novel variations of exopolysaccharides (EPSs) produced by dairy strains of Lactococcus lactis subsp. lactis and subsp. cremoris, the EPSs of five dairy strains of L. lactis were purified. Sugar composition analysis showed two novel EPSs produced by strains of L. lactis subsp. lactis. One strain produced EPS lacking galactose, and the other produced EPS containing fucose. Among the eps gene clusters of these strains, the highly conserved epsD and its neighboring epsE were sequenced. Sequence and PCR analysis revealed that epsE genes were strain-specific. By Southern blot analysis using epsD, the eps gene cluster in each strain was found to locate to the chromosome or a very large plasmid. This is the first report on the identification of two novel EPSs in L. lactis subsp. lactis. The strains can be detected among other strains by using epsE genes specific to them.

Alg14 organizes the formation of a multiglycosyltransferase complex involved in initiation of lipid-linked oligosaccharide biosynthesis

Protein N-glycosylation begins with the assembly of a lipid-linked oligosaccharide (LLO) on the endoplasmic reticulum (ER) membrane. The first two steps of LLO biosynthesis are catalyzed by a functional multienzyme complex comprised of the Alg7 GlcNAc phosphotransferase and the heterodimeric Alg13/Alg14 UDP-GlcNAc transferase on the cytosolic face of the ER. In the Alg13/14 glycosyltransferase, Alg14 recruits cytosolic Alg13 to the ER membrane through interaction between their C-termini. Bioinformatic analysis revealed that eukaryotic Alg14 contains an evolved N-terminal region that is missing in bacterial orthologs. Here, we show that this N-terminal region of Saccharomyces cerevisiae Alg14 localize its green fluorescent protein fusion to the ER membrane. Deletion of this region causes defective growth at 38.5°C that can be partially complemented by overexpression of Alg7. Coimmunoprecipitation demonstrated that the N-terminal region of Alg14 is required for direct interaction with Alg7. Our data also show that Alg14 lacking the N-terminal region remains on the ER membrane through a nonperipheral association, suggesting the existence of another membrane-binding site. Mutational studies guided by the 3D structure of Alg14 identified a conserved α-helix involved in the second membrane association site that contributes to an integral interaction and protein stability. We propose a model in which the N- and C-termini of Alg14 coordinate recruitment of catalytic Alg7 and Alg13 to the ER membrane for initiating LLO biosynthesis.

Biochemical characterization, membrane association and identification of amino acids essential for the function of Alg11 from Saccharomyces cerevisiae, an alpha1,2-mannosyltransferase catalysing two sequential glycosylation steps in the formation of the lipid-linked core oligosaccharide

The biosynthesis of asparagine-linked glycans occurs in an evolutionarily conserved manner with the assembly of the unique lipid-linked oligosaccharide precursor Glc3Man9GlcNAc2-PP-Dol at the ER (endoplasmic reticulum). In the present study we characterize Alg11 from yeast as a mannosyltransferase catalysing the sequential transfer of two alpha1,2-linked mannose residues from GDP-mannose to Man3GlcNAc2-PP-Dol and subsequently to Man4GlcNAc2-PP-Dol forming the Man5GlcNAc2-PP-Dol intermediate at the cytosolic side of the ER before flipping to the luminal side. Alg11 is predicted to contain three hydrophobic transmembrane-spanning helices. Using Alg11 topology reporter fusion constructs, we show that only the N-terminal domain fulfils this criterion. Surprisingly, this domain can be deleted without disturbing glycosyltransferase function and membrane association, indicating also that the other two hydrophobic domains contribute to ER localization, but in a non-transmembrane manner. By site-directed mutagenesis we investigated amino acids important for transferase activity. We demonstrate that the first glutamate residue in the EX7E motif, conserved in a variety of glycosyltransferases, is more critical than the second, and loss of Alg11 function occurs only when both glutamate residues are exchanged, or when the mutation of the first glutamate residue is combined with replacement of another amino acid in the motif. This indicates that perturbations in EX7E are not restricted to the second glutamate residue. Moreover, Gly85 and Gly87, within a glycine-rich domain as part of a potential flexible loop, were found to be required for Alg11 function. Similarly, a conserved lysine residue, Lys319, was identified as being important for activity, which could be involved in the binding of the phosphate of the glycosyl donor.

Suggestive evidence for Darwinian Selection against asparagine-linked glycans of Plasmodium falciparum and Toxoplasma gondii

We are interested in asparagine-linked glycans (N-glycans) of Plasmodium falciparum and Toxoplasma gondii, because their N-glycan structures have been controversial and because we hypothesize that there might be selection against N-glycans in nucleus-encoded proteins that must pass through the endoplasmic reticulum (ER) prior to threading into the apicoplast. In support of our hypothesis, we observed the following. First, in protists with apicoplasts, there is extensive secondary loss of Alg enzymes that make lipid-linked precursors to N-glycans. Theileria makes no N-glycans, and Plasmodium makes a severely truncated N-glycan precursor composed of one or two GlcNAc residues. Second, secreted proteins of Toxoplasma, which uses its own 10-sugar precursor (Glc(3)Man(5)GlcNAc(2)) and the host 14-sugar precursor (Glc(3)Man(9)GlcNAc(2)) to make N-glycans, have very few sites for N glycosylation, and there is additional selection against N-glycan sites in its apicoplast-targeted proteins. Third, while the GlcNAc-binding Griffonia simplicifolia lectin II labels ER, rhoptries, and surface of plasmodia, there is no apicoplast labeling. Similarly, the antiretroviral lectin cyanovirin-N, which binds to N-glycans of Toxoplasma, labels ER and rhoptries, but there is no apicoplast labeling. We conclude that possible selection against N-glycans in protists with apicoplasts occurs by eliminating N-glycans (Theileria), reducing their length (Plasmodium), or reducing the number of N-glycan sites (Toxoplasma). In addition, occupation of N-glycan sites is markedly reduced in apicoplast proteins versus some secretory proteins in both Plasmodium and Toxoplasma.

Hetero-oligomeric interactions between early glycosyltransferases of the dolichol cycle

N-Linked glycosylation begins with the formation of a dolichol-linked oligosaccharide in the endoplasmic reticulum (ER). The first two steps of this pathway lead to the formation of GlcNAc(2)-PP-dolichol, whose synthesis is sequentially catalyzed by the Alg7p GlcNAc phosphotransferase and by the dimeric Alg13p/Alg14p UDP-GlcNAc transferase on the cytosolic face of the endoplasmic reticulum. Here, we show that the Alg7p, Alg13p, and Alg14p glycosyltransferases form a functional multienzyme complex. Coimmunoprecipitation and gel filtration assays demonstrate that the Alg7p/Alg13p/Alg14p complex is a hexamer with a native molecular weight of approximately 200 kDa and an Alg7p:Alg13:Alg14p stoichiometry of 1:1:1. These results highlight and extend the striking parallels that exist between these eukaryotic UDP-GlcNAc transferases and their bacterial MraY and MurG homologs that catalyze the first two steps of the lipid-linked peptidoglycan precursor. In addition to their preferred substrate and lipid acceptors, these enzymes are similar in their structure, chemistry, temporal, and spatial organization. These similarities point to an evolutionary link between the early steps of N-linked glycosylation and those of peptidoglycan synthesis.

Interaction between the C termini of Alg13 and Alg14 mediates formation of the active UDP-N-acetylglucosamine transferase complex

The second step of eukaryotic N-linked glycosylation in endoplasmic reticulum is catalyzed by an UDP-N-acetylglucosamine transferase that is comprised of two subunits, Alg13 and Alg14. The interaction between Alg13 and 14 is crucial for UDP-GlcNAc transferase activity, so formation of the Alg13/14 complex is likely to play a key role in the regulation of N-glycosylation. Using a combination of bioinformatics and molecular biological methods, we have undertaken a functional analysis of yeast Alg13 and Alg14 proteins to elucidate the mechanism of their interaction. Our mutational studies demonstrated that a short C-terminal alpha-helix of Alg13 is required for interaction with Alg14 and for enzyme activity. Electrostatic surface views of the modeled Alg13/14 complex suggest the presence of a hydrophobic cleft in Alg14 that provides a pocket for the Alg13 C-terminal alpha-helix. Co-immunoprecipitation assays confirmed the C-terminal three amino acids of Alg14 are required for maintaining the integrity of Alg13/Alg14 complex, and this depends on their hydrophobicity. Modeling studies place these three Alg14 residues at the entrance of the hydrophobic-binding pocket, suggesting their role in the stabilization of the interaction between the C termini of Alg13 and Alg14. Together, these results demonstrate that formation of this hetero-oligomeric complex is mediated by a short C-terminal alpha-helix of Alg13 in cooperation with the last three amino acids of Alg14. In addition, deletion of the N-terminal beta-strand of Alg13 caused the destruction of protein, indicating the structural importance of this region in protein stability.

Solution structure of Alg13: the sugar donor subunit of a yeast N-acetylglucosamine transferase

The solution structure of Alg13, the glycosyl donor-binding domain of an important bipartite glycosyltransferase in the yeast Saccharomyces cerevisiae, is presented. This glycosyltransferase is unusual in that it is active only in the presence of a binding partner, Alg14. Alg13 is found to adopt a unique topology among glycosyltransferases. Rather than the conventional Rossmann fold found in all GT-B enzymes, the N-terminal half of the protein is a Rossmann-like fold with a mixed parallel and antiparallel beta sheet. The Rossmann fold of the C-terminal half of Alg13 is conserved. However, although conventional GT-B enzymes usually possess three helices at the C terminus, only two helices are present in Alg13. Titration of Alg13 with both UDP-GlcNAc, the native glycosyl donor, and a paramagnetic mimic, UDP-TEMPO, shows that the interaction of Alg13 with the sugar donor is primarily through the residues in the C-terminal half of the protein.

Alg13p, the catalytic subunit of the endoplasmic reticulum UDP-GlcNAc glycosyltransferase, is a target for proteasomal degradation

The second step of dolichol-linked oligosaccharide synthesis in the N-linked glycosylation pathway at the endoplasmic reticulum (ER) membrane is catalyzed by an unusual hetero-oligomeric UDP-N-acetylglucosamine transferase that in most eukaryotes is comprised of at least two subunits, Alg13p and Alg14p. Alg13p is the cytosolic and catalytic subunit that is recruited to the ER by the membrane protein Alg14p. We show that in Saccharomyces cerevisiae, cytosolic Alg13p is very short-lived, whereas membrane-associated Alg13 is relatively stable. Cytosolic Alg13p is a target for proteasomal degradation, and the failure to degrade excess Alg13p leads to glycosylation defects. Alg13p degradation does not require ubiquitin but instead, requires a C-terminal domain whose deletion results in Alg13p stability. Conversely, appending this sequence onto normally long-lived beta-galactosidase causes it to undergo rapid degradation, demonstrating that this C-terminal domain represents a novel and autonomous degradation motif. These data lead to the model that proteasomal degradation of excess unassembled Alg13p is an important quality control mechanism that ensures proper protein complex assembly and correct N-linked glycosylation.

Glycosyltransferase mechanisms: impact of a 5-fluoro substituent in acceptor and donor substrates on catalysis

In glycosyltransferase-catalyzed reactions a new carbohydrate-carbohydrate bond is formed between a carbohydrate acceptor and the carbohydrate moiety of either a sugar nucleotide donor or lipid-linked saccharide donor. It is currently believed that most glycosyltransferase-catalyzed reactions occur via an electrophilic activation mechanism with the formation of an oxocarbenium ion-like transition state, a hypothesis that makes clear predictions regarding the charge development on the donor (strong positive charge) and acceptor (minimal negative charge) substrates. To better understand the mechanism of these enzyme-catalyzed reactions, we have introduced a strongly electron-withdrawing group (fluorine) at C-5 of both donor and acceptor substrates in order to explore its effect on catalysis. In particular, we have investigated the effects of the 5-fluoro analogues on the kinetics of two glycosyltransferase-catalyzed reactions mediated by UDP-GlcNAc:GlcNAc-P-P-Dol N-acetylglucosaminyltransferase (chitobiosyl-P-P-lipid synthase, CLS) and beta-N-acetylglucosaminyl-beta-1,4 galactosyltransferase (GalT). The 5-fluoro group has a marked effect on catalysis when inserted into the UDP-GlcNAc donor, with the UDP(5-F)-GlcNAc serving as a competitive inhibitor of CLS rather than a substrate. The (5-F)-GlcNAc beta-octyl glycoside acceptor, however, is an excellent substrate for GalT. Both of these results support a weakly associative transition state for glycosyltransferase-catalyzed reactions that proceed with inversion of configuration.

Membrane topology of the Alg14 endoplasmic reticulum UDP-GlcNAc transferase subunit

N-linked glycosylation begins in the endoplasmic reticulum with the synthesis of a highly conserved dolichol-linked oligosaccharide precursor. The UDP-GlcNAc glycosyltransferase catalyzing the second sugar addition of this precursor consists in most eukaryotes of at least two subunits, Alg14 and Alg13. Alg14 is a membrane protein that recruits the soluble Alg13 catalytic subunit from the cytosol to the face of the endoplasmic reticulum (ER) membrane where this reaction occurs. Here, we investigated the membrane topology of Saccharomyces cerevisiae Alg14 and its requirements for ER membrane association. Alg14 is predicted by most algorithms to contain one or more transmembrane spanning helices (transmembrane domains (TMDs)). We provide evidence that Alg14 contains a C-terminal cytosolic tail and an N terminus that resides within the ER lumen. However, we also demonstrate that Alg14 lacking this TMD is functional and remains peripherally associated with ER membranes, suggesting that additional domains can mediate ER association. These conclusions are based on the functional analysis of Alg13/Alg14 chimeras containing Alg13 fused at either end of Alg14 or truncated Alg14 variants lacking the predicted TMD; protease protection assays of Alg14 in intact ER membranes; and extraction of Alg14-containing ER membranes with high pH. These yeast Alg13-Alg14 chimeras recapitulate the phylogenetic diversity of Alg13-Alg14 domain arrangements that evolved in some protozoa. They encode single polypeptides containing an Alg13 domain fused to Alg14 domain in either orientation, including those lacking the Alg14 TMD. Thus, this Alg13-Alg14 UDP-GlcNAc transferase represents an unprecedented example of a bipartite glycosyltransferase that evolved by both fission and fusion.

Acceptor substrate selectivity and kinetic mechanism of Bacillus subtilis TagA

Wall teichoic acids (WTAs) are anionic polymers that coat the cell walls of Gram-positive bacteria. Because they are essential for survival or virulence in many organisms, the enzymes involved in the biosynthesis of WTAs are attractive antibiotic targets. The first committed step in the WTA biosynthetic pathway in Bacillus subtilis is catalyzed by TagA, which transfers N-acetylmannosamine (ManNAc) to the C4 hydroxyl of a membrane-anchored N-acetylglucosaminyl diphospholipid (GlcNAc-pp-undecaprenyl, lipid I) to make ManNAc-beta-(1,4)-GlcNAc-pp-undecaprenyl (lipid II). We have previously shown that TagA utilizes an alternative substrate containing a saturated C(13)H(27) lipid chain. Here we use unnatural substrates and products to establish the lipid preferences of the enzyme and to characterize the kinetic mechanism. We report that TagA is a metal ion-independent glycosyltransferase that follows a steady-state ordered Bi-Bi mechanism in which UDP-ManNAc binds first and UDP is released last. TagA shares homology with a large family of bacterial glycosyltransferases, and the work described here should facilitate structural analysis of the enzyme in complex with its substrates.

Rooting the tree of life by transition analyses

BACKGROUND

Despite great advances in clarifying the family tree of life, it is still not agreed where its root is or what properties the most ancient cells possessed--the most difficult problems in phylogeny. Protein paralogue trees can theoretically place the root, but are contradictory because of tree-reconstruction artefacts or poor resolution; ribosome-related and DNA-handling enzymes suggested one between neomura (eukaryotes plus archaebacteria) and eubacteria, whereas metabolic enzymes often place it within eubacteria but in contradictory places. Palaeontology shows that eubacteria are much more ancient than eukaryotes, and, together with phylogenetic evidence that archaebacteria are sisters not ancestral to eukaryotes, implies that the root is not within the neomura. Transition analysis, involving comparative/developmental and selective arguments, can polarize major transitions and thereby systematically exclude the root from major clades possessing derived characters and thus locate it; previously the 20 shared neomuran characters were thus argued to be derived, but whether the root was within eubacteria or between them and archaebacteria remained controversial.

RESULTS

I analyze 13 major transitions within eubacteria, showing how they can all be congruently polarized. I infer the first fully resolved prokaryote tree, with a basal stem comprising the new infrakingdom Glidobacteria (Chlorobacteria, Hadobacteria, Cyanobacteria), which is entirely non-flagellate and probably ancestrally had gliding motility, and two derived branches (Gracilicutes and Unibacteria/Eurybacteria) that diverged immediately following the origin of flagella. Proteasome evolution shows that the universal root is outside a clade comprising neomura and Actinomycetales (proteates), and thus lies within other eubacteria, contrary to a widespread assumption that it is between eubacteria and neomura. Cell wall and flagellar evolution independently locate the root outside Posibacteria (Actinobacteria and Endobacteria), and thus among negibacteria with two membranes. Posibacteria are derived from Eurybacteria and ancestral to neomura. RNA polymerase and other insertions strongly favour the monophyly of Gracilicutes (Proteobacteria, Planctobacteria, Sphingobacteria, Spirochaetes). Evolution of the negibacterial outer membrane places the root within Eobacteria (Hadobacteria and Chlorobacteria, both primitively without lipopolysaccharide): as all phyla possessing the outer membrane beta-barrel protein Omp85 are highly probably derived, the root lies between them and Chlorobacteria, the only negibacteria without Omp85, or possibly within Chlorobacteria.

CONCLUSION

Chlorobacteria are probably the oldest and Archaebacteria the youngest bacteria, with Posibacteria of intermediate age, requiring radical reassessment of dominant views of bacterial evolution. The last ancestor of all life was a eubacterium with acyl-ester membrane lipids, large genome, murein peptidoglycan walls, and fully developed eubacterial molecular biology and cell division. It was a non-flagellate negibacterium with two membranes, probably a photosynthetic green non-sulphur bacterium with relatively primitive secretory machinery, not a heterotrophic posibacterium with one membrane.

Bioinformatics for comprehensive finding and analysis of glycosyltransferases

Bioinformatics is a very powerful tool in the field of glycoproteomics as well as genomics and proteomics. As a part of the Glycogene Project (GG project), we have developed a novel bioinformatics system for the comprehensive identification and in silico cloning of human glycogenes. Using our system, a total of 105 candidate human glycogenes were identified and then engineered for heterologous expression. Of these candidates, 38 recombinant proteins were successfully identified for their enzyme activity and substrate specificity. We also classified 47 out of 60 carbohydrate-active enzyme glycosyltransferase families into 4 superfamilies using the profile Hidden Markov Model method. On the basis of our classification and the relationship between glycosylation pathways and superfamilies, we propose the evolution of glycosyltransferases.

Asparagine-linked protein glycosylation: from eukaryotic to prokaryotic systems

Asparagine-linked protein glycosylation is a prevalent protein modification reaction in eukaryotic systems. This process involves the co-translational transfer of a pre-assembled tetradecasaccharide from a dolichyl-pyrophosphate donor to the asparagine side chain of nascent proteins at the endoplasmic reticulum (ER) membrane. Recently, the first such system of N-linked glycosylation was discovered in the Gram-negative bacterium, Campylobacter jejuni. Glycosylation in this organism involves the transfer of a heptasaccharide from an undecaprenyl-pyrophosphate donor to the asparagine side chain of proteins at the bacterial periplasmic membrane. Here we provide a detailed comparison of the machinery involved in the N-linked glycosylation systems of eukaryotic organisms, exemplified by the yeast Saccharomyces cerevisiae, with that of the bacterial system in C. jejuni. The two systems display significant similarities and the relative simplicity of the bacterial glycosylation process could provide a model system that can be used to decipher the complex eukaryotic glycosylation machinery.

In vitro reconstitution of two essential steps in wall teichoic acid biosynthesis

Wall teichoic acids (WTAs) are anionic polymers that decorate the cell walls of many gram-positive bacteria. These structures are essential for survival or virulence in many organisms, which makes the enzymes involved in their biosynthesis attractive targets for the development of new antibacterial agents. We present a strategy to obtain WTA biosynthetic intermediates that involves a combination of chemical and enzymatic transformations. Using these intermediates, we have reconstituted the first two committed steps in the biosynthetic pathway. This work enables the exploration of WTA-synthesizing enzymes as antibiotic targets.

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.

Alg14 Recruits Alg13 to the Cytoplasmic Face of the Endoplasmic Reticulum to Form a Novel Bipartite UDP-N-acetylglucosamine Transferase Required for the Second Step of N-Linked Glycosylation

N-linked glycosylation requires the synthesis of an evolutionarily conserved lipid-linked oligosaccharide (LLO) precursor that is essential for glycoprotein folding and stability. Despite intense research, several of the enzymes required for LLO synthesis have not yet been identified. Here we show that two poorly characterized yeast proteins known to be required for the synthesis of the LLO precursor, GlcNAc2-PP-dolichol, interact to form an unusual hetero-oligomeric UDP-GlcNAc transferase. Alg13 contains a predicted catalytic domain, but lacks any membrane-spanning domains. Alg14 spans the membrane but lacks any sequences predicted to play a direct role in sugar catalysis. We show that Alg14 functions as a membrane anchor that recruits Alg13 to the cytosolic face of the ER, where catalysis of GlcNAc2-PP-dol occurs. Alg13 and Alg14 physically interact and under normal conditions, are associated with the ER membrane. Overexpression of Alg13 leads to its cytosolic partitioning, as does reduction of Alg14 levels. Concomitant Alg14 overproduction suppresses this cytosolic partitioning of Alg13, demonstrating that Alg14 is both necessary and sufficient for the ER localization of Alg13. Further evidence for the functional relevance of this interaction comes from our demonstration that the human ALG13 and ALG14 orthologues fail to pair with their yeast partners, but when co-expressed in yeast can functionally complement the loss of either ALG13 or ALG14. These results demonstrate that this novel UDP-GlcNAc transferase is a unique eukaryotic ER glycosyltransferase that is comprised of at least two functional polypeptides, one that functions in catalysis and the other as a membrane anchor.

Biosynthesis of lipid-linked oligosaccharides in Saccharomyces cerevisiae: Alg13p and Alg14p form a complex required for the formation of GlcNAc(2)-PP-dolichol

N-Glycosylation in the endoplasmic reticulum is an essential protein modification and highly conserved in evolution from yeast to man. Here we identify and characterize two essential yeast proteins having homology to bacterial glycosyltransferases, designated Alg13p and Alg14p, as being required for the formation of GlcNAc(2)-PP-dolichol (Dol), the second step in the biosynthesis of the unique lipid-linked core oligosaccharide. Down-regulation of each gene led to a defect in protein N-glycosylation and an accumulation of GlcNAc(1)-PP-Dol in vivo as revealed by metabolic labeling with [(3)H]glucosamine. Microsomal membranes from cells repressed for ALG13 or ALG14, as well as detergent-solubilized extracts thereof, were unable to catalyze the transfer of N-acetylglucosamine from UDP-GlcNAc to [(14)C]GlcNAc(1)-PP-Dol, but did not impair the formation of GlcNAc(1)-PP-Dol or GlcNAc-GPI. Immunoprecipitating Alg13p from solubilized extracts resulted in the formation of GlcNAc(2)-PP-Dol but required Alg14p for activity, because an Alg13p immunoprecipitate obtained from cells in which ALG14 was down-regulated lacked this activity. In Western blot analysis it was demonstrated that Alg13p, for which no well defined transmembrane segment has been predicted, localizes both to the membrane and cytosol; the latter form, however, is enzymatically inactive. In contrast, Alg14p is exclusively membrane-bound. Repression of the ALG14 gene causes a depletion of Alg13p from the membrane. By affinity chromatography on IgG-Sepharose using Alg14-ZZ as bait, we demonstrate that Alg13-myc co-fractionates with Alg14-ZZ. The data suggest that Alg13p associates with Alg14p to a complex forming the active transferase catalyzing the biosynthesis of GlcNAc(2)-PP-Dol.