Glycosyltransferases catalyze the synthesis of glycoconjugates, including glycolipids, glycoproteins, and polysaccharides, by transferring an activated mono- or oligosaccharide residue to an existing acceptor molecule for the initiation or elongation of the carbohydrate chain. Because the glycosylation reaction is highly specific with respect to both the anomeric configuration of the sugar residue and the site of the addition, it is expected that unique domain structures for substrate recognition and nucleotide-sugar binding are located within the enzyme molecule. Indeed, common amino acid sequences have been deduced for homologous binding sites. For example, sialyltransferases have sialylmotifs that may participate in the recognition of the donor substrate, CMP-sialic acid (Paulson and Colley, 1985; Datta and Paulson, 1995; Katsutoshi, 1996). The hexapeptide RDKKND in Gal[alpha]1-3 galactosyltransferase and RDKKNE in GlcNAc[beta]1-4 galactosyltransferase have been suggested as the binding site for UDP-Gal (Joziasse et al., 1985, 1989; Joziasse, 1992). These domains have been extensively reviewed in the literature and therefore are not a subject of this review. During the last few years, the amino acid sequences of a number of glycosyltransferases have been identified using sequence data provided by the complete genomic sequences obtained for such organisms as C.elegans, yeasts, several bacteria, and ongoing human and mouse genome projects. These advances have made it possible to deduce conserved domain structures in glycosyltransferases of diverse specificity.
Our interest in this area stems from our recent systematic analysis of the cDNAs encoding UDP-galactose: ceramide galactosyltransferases or CGT (EC 2.4.1.45), the enzyme responsible for the biosynthesis of galactosylceramide. This enzyme has been suggested to play a critical role in myelin formation (Costantino-Ceccarini and Suzuki, 1975; Costantino-Ceccarini and Poduslo, 1989), signal transduction (Dyer and Benjamins, 1989, 1991; Joshi and Mishra, 1992), viral (Baccetti et al., 1991, 1994; Bhat et al., 1991; Harouse et al., 1991; Yahi et al., 1992, 1995; Brogi et al., 1995; Toniolo et al., 1995) microbial adhesion (Garcia Monco et al., 1992; Khan et al., 1996; Kaneda et al., 1997), and oligodendrocyte development (Mirsky et al., 1980; Bansal and Pfeiffer, 1989). In our previous studies we have described the isolation of human, mouse, chicken, and bovine cDNAs for CGT and also the expression of human CGT cDNA (Kapitonov and Yu, 1997a,b). The CGT cDNA sequence appears to be evolutionarily conserved among higher vertebrates; the similarity of CGT from two different systematic classes of vertebrates exceeds 80% and from two different systematic orders of mammals is about 90%. CGT also has a high degree of sequence similarity with glucuronosyltransferases (UGT) (Schulte and Stoffel, 1993; Stahl et al., 1994). All species studied so far have only one copy of the CGT gene. The deduced amino acid sequence of human CGT reveals that it is a 61 kDa protein of 541 amino acid residues, 21 of which are charged. The protein has a calculated pI of 10.36. It maintains all the conserved features of other vertebrate CGTs: a C-terminal transmembrane hydrophobic domain, an endoplasmic reticulum retention signal, and three N-linked glycosylation sites at positions 83, 338, and 447 (none of these are conserved in glucuronosyltransferases). There are six potential sites for casein kinase 2 (CK2) phosphorylation (positions 85, 104, 234, 368, 408, and 410), two potential PKC phosphorylation sites (positions 100 and 426), and a potential N-terminal myristoylation site at position 16 that could be exposed upon cleavage of the signal peptide. Using the amino acid sequence information obtained from the four animal species, we deduced a consensus sequence of CGT, which we used to analyze the database for conserved domains of other glycosyltransferases. In this review, since we analyzed both known and putative glycosyltransferases in order to provide adequate description, all known glycosyltransferases are described by their proper protein names and organisms, while for putative glycosyltransferases from the database, an accession number immediately follows the particular organism. When several genes are located on the same cosmid, the accession number of the cosmid is followed by a dot and a gene number to indicate potential participation in the same biosynthetic pathway in lower eukaryotes. When one protein is chosen to represent a group of highly homologous enzymes, the name of the protein is followed by the number of the family members.
When comparing CGT and UGT to some microbial glycosyltransferases, we identified a region with a high degree of similarity, which we have named 'Nucleotide Recognition Domain type 1[beta]" or NRD1[beta], to account for the inversion of anomeric configuration and the resulting configuration of added sugar. This domain is located close to the C-terminal end before the C-terminal transmembrane domain, between residues 357-396 in CGT and between residues 369-407 in UGT. In both CGT and UGT this domain is encoded by one exon. A much smaller area of sequence similarity located closer to the N-terminus is identified as NRD1[beta] S (S for small). In order to understand the functional significance of the NRD1[beta], we generated a homologous amino acid sequence pattern and searched several DNA databases translated in all 6 frames using the programs FindPattern and TFastA (Wisconsin Package, GCG). We found 140 proteins containing this domain. Some representative sequences are shown in Figure
Figure 1. Multiple alignment of the NRD1[beta] family of proteins. All conservatively substituted residues that are present in more then 60% of the glycosyltransferases compared are shown as a NRD1[beta] Motif. Signal peptide (SIG), transmembrane domain (TM), potential endoplasmic reticulum retention signal (RETENTION), all truncated, as well as small NRD1[beta] S domain, and large NRD1[beta] L domain, are indicated in Domains. P* indicates the S/T residue in the middle of NRD1[beta] site that could be potentially phosphorylated by CK2. 1 and 2, residues that can potentially participate in the catalytic reaction (E at the position 2). The group number is followed by the type of glycosyltransferase (if known), then by the gene name (if given), then by organism and by accession number. For the multimember group the number of group members is indicated in brackets at the end. For C.elegans the accession number of a cosmid is followed by the gene number. S.c, Yeast Saccharomyces cerevisiae; C.e., Caenorhabditis elegans, nematoda; M.l., Mycobacterium leprae; M.t., Mycobacterium tuberculosis; St.c., Streptomyces capreolus; Ol, oleandomycin; S.a., Streptomyces antibioticus; Ml, macrolide; S.l., Streptomyces lividans; GlyT, glycosyltransferase; B.s., Bacillus subtilis; Z, zeaxanthin; E.u., Erwinia uredovora; A.o., Amycolatopsis orientalis; RhaT, rhamnosyltransferase; P.a., Pseudomonas aeruginosa; B, baumycin; St.c., Streptomyces C5; S.p., Streptomyces peucetius; DauT, daunosaminetransferse; GlcT, glucosyltransferase; S.t., Salmonella typhimurium; Synech, Synechocystis sp, cyanobacteria; Me.j., Methanococcus jannaschii, archaebacteria; G, glucose; N.t., Nicotiana tabacum; L.esc., Lycopersicon esculentum; F, flavonol; Z.m, Zea mais; Petun., Petunia hybrida; I, indole-3-acetate; A.t., Arabidopsis thaliana; C.eleg., Caenorhabditis elegans; UGT, glucuronosyltransferase; P.platessa, Pleuronectes platessa (plaice); CGT, ceramide galactosyltransferase; E, ecdysteroid; NPV, nuclear polyhedrosis virus. The transmembrane domain and signal peptide are truncated. Motif, amino acids residues occurring in more then 60% of UGT1, UGT2, CGT and NRD1[beta]. The conserved [alpha]-helical portion is underlined. Putative proton donor and nucleophile involved in reaction mechanism are marked as 1 and 2 correspondingly. Numbers on the left column indicate the position of the first amino acid residue of the domain; numbers on the right column indicate the total length of the protein. [Rgr], Total length of the protein. Psgene, possible pseudogene; altern, alternative splice sites have been chosen.
Figure 2. The evolutionary relationship between the NRD1[beta] family members. For the explanation of abbreviations used see the legend for Figure 1. All sequences are grouped in five classes. Members of Class I are underlined, members of Class II are shown in italics, Class III of microbial NRD1[beta] glycosyltransferases is shown dark gray, Class IV of plant NRD1[beta] glycosyltransferases is underlined, Class V of animal glycosyltransferases is shown in bold italic. C.elegans sequences are grouped according to similarity into 25 groups. Note that CGT, UGT1 and UGT2 appear as one group of Class V (shown in black). The corresponding division into classes is shown on the left margin by a dotted line.
A potential Ser/Thr residue phosphorylation site is located in the middle of the NRD1[beta] domain that follows the conserved S/TXXE/D pattern for casein kinase 2 (CK2) phosphorylation (where X can be any amino acid residue other than P). Phosphorylation of this site could potentially regulate enzyme activity.
All NRD1[beta] glycosyltransferases can be divided into five classes based on the degree of sequence similarity. Class I is a group of relatively large proteins (950-1050 amino acid (aa) residues) that are homologous to glycosyltransferases from Saccharomyces cerevisiae and Caenorhabditis elegans (C.elegans). Class II constitutes proteins of intermediate size (about 700 amino acid residues) from C.elegans. All bacterial glycosyltransferases fall into Class III. Plant glycosyltransferases constitute Class IV. Class V consists of the remaining glycosyltransferases from C.elegans, glucuronosyltransferases (UGT), and ceramide galactosyltransferases (CGT). Proteins with high degrees of sequence similarity are found in these groups. NRD1[beta] glycosyltransferases are also found in archaebacteria Methanococcus jannaschii, indicating that these enzymes evolved prior to the separation of archaebacteria, eubacteria, and eukaryotes. Protozoa seldom have more than one NRD1[beta] glycosyltransferase, while metazoa have multiple groups of these enzymes.
The most studied metazoan organism, whose genome sequence was about 70% completed at the time of this analysis is C.elegans (Sulston et al., 1992; Watson et al., 1993; Wilson et al., 1994). C.elegans is a free-living nematode comprised of a total of 959 cells with a well-defined pattern of cell division. Based on total protein similarity, we have divided putative glycosyltransferases identified using sequence data from the C.elegans genome into 25 groups, 8 of which contain three proteins or more. Some of the ORF products have been previously identified as similar to UGT. Our analysis indicates that these proteins are not strictly glucuronosyltransferases, but rather glycosyltransferases that are potentially able to catalyze the transfer of either galactose, glucose, or glucuronic acid. In most of the C.elegans putative glycosyltransferases, the NRD1[beta] domain is encoded by one exon.
As a result of our database analysis, we have discovered a new family of glycosyltransferases which we name as the NRD1[beta] family. Class I NRD1[beta] glycosyltransferases include two proteins of 1198 and 949 amino acid residues from S.cerevisiae (Q06321) and C.elegans (Z71177.2), respectively. Class II NRD1[beta] proteins consist of six putative glycosyltransferases from C.elegans of 687-795 amino acid residues. Class III (microbial glycosyltransferases) NRD1[beta] proteins include zeaxanthin glucosyltransferases from erwinia (gracilicutes) and synechocystis (cyanobacteria) that catalyze the conversion of zeaxanthin (Figure
Figure 3. Chemical structure of the acceptor molecules of the NRD1[beta] glycosyltransferases.
Figure 4. The evolutionary relationship between the NRD1[beta] family members. (A) The evolutionary relationship between all known zeaxanthin glucosyltransferases. GlcT, Glucosyltransferase. (B) The evolutionary relationship between all known glycosyltransferases (GlyT) from Amycolatopsis orientalis; gtfB,C,D,E, glycosyltransferase genes B, C, D, and E. (C) Evolutionary relationship between mycobacterial glycosyltransferases. (D) Comparison of all known flavonol glucosyltransferases. GlcT, Glucosyltransferase; GlyT, glycosyltransferase; CGT7 Manihot, Manihot esculenta, gene 7. (E) Comparison of all known indole-3-acetate glucosyltransferases. GlcT, Glucosyltransferase; Nicotiana, Nicotiana tabacum; Arabidopsis, Arabidopsis thaliana. (F) Evolutionary relationship between all known glucose glucosyltransferases. GlcT, Glucosyltransferase; GlyT, glycosyltransferase; Solanum, Solanum tuberosum. (G) The evolutionary relationship of all known ecdysteroid glucosyltransferases (GlcT). GlcT, Glucosyltransferase; ACMNPV, Autographa californica nuclear polyhedrosis virus (NPV) (O'Reilly and Miller, 1989); BMNPV, Bombyx mori NPV; CFNPV, Choristoneura fumiferana NPV (Barrett et al., 1995); OP, Orgyia pseudotsugata (Pearson et al., 1993); MB, Mamestra brassicae; BS, Buzura suppressaria; LD, Lymantria dispar Multicapsid NPV (Riegel et al., 1994); SL, Spodoptera littoralis; LCGV, Lacanobia oleracea granulosis virus. All Class III microbial glycosyltransferases are shown in gray, all Class IV plant glycosyltransferases are shown in bold, and Class V glycosyltransferases are shown in light gray.
Oleandomycin glycosyltransferase from Streptomyces antibioticus catalyzes the transfer of a glucosyl moiety from UDP-glucose to the 2[prime]-hydroxyl group of desosamine attached to the oleandomycin (Hernandez et al., 1993) (Figure
Class IV (plant glycosyltransferases) of the NRD1[beta] proteins is composed of several groups. Flavonol 3-O-glucosyltransferases from plants catalyze the transfer of the glucosyl residue from UDP-glucose to flavonol (Figure
A group of glucose-glucosyltransferases that catalyzes the transfer of a glucosyl residue from UDP-glucose to glucose is compared in Figure
In addition to CGT and UGT, there are other glycosyltransferases with known functions that belong to Class V of the NRD1[beta] family. Among them are the baculoviral (nucleopolyhedrovirus) ecdysteroid glucosyltransferases (EGTs) that catalyze the transfer of a glucosyl residue from UDP-glucose to ecdysteroids; the latter are insect molting hormones, e.g., ecdysone (Figure
Group 9 of C.elegans glycosyltransferases contains two proteins, one with 440 amino acid residues and the other with 435 amino acid residues. Both are encoded on the same cosmid: nine C.elegans (Z78200.8) and nine C.elegans (Z78200.7). UGT is a family of microsomal enzymes that transfer glucuronic acid (GlcA) to bilirubin, phenol, and certain toxic xenobiotics and endogenous steroid compounds. Currently there are two groups of highly homologous enzymes: UGT 1 and UGT 2. The UGT1 group consists of eight members: human bilirubin-specific UGT1A (P22309) and UGT1D (P22310), UGT1B (P36509), UGT1C (P35503), UGT1E (P35504), phenol-specific UGT1F (P19224) (Ritter et al., 1992a,b), rat p-nitrophenol-specific UGT1F (P08430) (Iyanagi et al., 1986), and bilirubin-specific UGT1A (P20720) (Sato et al., 1991). Most of these members are derived as a result of alternative splicing with a common C-terminal domain of 245 amino acid residues. The UGT2 group consists of 16 homologous members that include glucuronidate steroid hormones such as androgen, estradiol, and testosterone. Among the UGT2 group are rat 17-[beta]-hydroxysteroid-specific UGT2B6 (P19488) (Mackenzie, 1990), and UGT2B3 (P08542) (Mackenzie, 1987), 3-hydroxyandrogen-specific UGT2B2 (P08541), UGT2B1 (P09875) (Mackenzie, 1986), monoterpenoid alcohol-specific UGT2B12 (P36511) (Green et al., 1995), odorants-specific olfactorial UGT2A1 (P36510) (Lazard et al., 1990), mouse UGT2B5 (P17717) (Kimura and Owens, 1987), rabbit 4-hydroxybiphenyl-specific UGT2B13 (P36512) and UGT2B14 (P36513), UGT2X (P36514) (Tukey et al., 1993), human estriol-specific UGT2B8 (P2376) (Coffman et al., 1990) and UGT2B11 (P36538), UGT2B10 (P36537) (Jin et al., 1993), UGT2B15 (P54855), hyodeoxycholic acid-specific UGT2B4 (P06133) (Jackson et al., 1987), 3,4-catechol-estrogen-specific UGT2B7 (P16662) (Ritter et al., 1990). None of the Tetrapoda UGT has a potential phosphorylation site in the NRD1[beta] domain, except the UGT from fish species. The CGT group transfers galactose residue to ceramide. Currently rat (Schulte and Stoffel, 1993), mouse (Bosio et al., 1996b; Coetzee et al., 1996), human (Bosio et al., 1996a; Kapitonov and Yu, 1997a), chicken, and bovine (Kapitonov and Yu, 1997c) CGTs are cloned. All members of UGT and CGT families are compared in Figure
Figure 5. The evolutionary relationship between UGT superfamily (A), members of Groups 11, 13, 15, 16, 17 of C.elegans glycosyltransferases (B), and between members of the DAGGalT family (C).
Group 12 of the NRD1[beta] proteins consists of two proteins, Z75554.5 with 535 amino acid residues and Z75554.6 with 534 amino acid residues. Splice sites other than those predicted with the Genefinder Program are used in alignment on a coding negative strand for (Z75554.6):
Group 13 of the NRD1[beta] proteins consists of 4 proteins: U97009.3,10,1,11, containing 533 to 534 amino acid residues. All Group 13 members are compared in Figure
Group 16 of C.elegans NRD1[beta] proteins consists of four members with 520 to 542 amino acid residues. All members are compared in Figure
The NRD1[beta] family of glycosyltransferases encompasses enzymes from eukaryotes, prokaryotes and archaebacteria. Thus, the NRD1[beta] domain probably appeared prior to the separation of these three kingdoms. The acceptor substrates of these glycosyltransferases are either lipids, carbohydrates, proteins, or polycyclic compounds such as steroids, antibiotics, and pigments. These aglycones show little structural similarity (Figure
All C.elegans NRD1[beta] members in the database were detected with the help of the Genefinder Program. It should be noted that this program does make occasional mistakes in selecting the correct splice sites due probably to the lack of information about codon preferences and the third nucleotide bias in C.elegans. The program cannot discriminate between genes and pseudogenes. Occasionally, alternative splicing sites and alternative transcription initiation sites were used based on homology with close NRD1[beta] members. Apart from the complete genes, we found four examples of exon shuffling in the C.elegans genome, where the NRD1[beta]-encoding exon was relocated to other parts of the genome and had undergone several mutations. Gene duplications, deletions, frame shifts and exon shuffling have evolutionary significance in terms of the generation of new genes by recombining domains, point mutations, insertions, and deletions.
After we finished our initial analysis (Kapitonov and Yu, 1997a,b), a similar domain was described (Mackenzie et al., 1997), where the nomenclature for a UDP-glucuronosyltransferase gene superfamily was proposed. Our classification differs from theirs in the following aspects. (1) The NRD1[beta] family of glycosyltransferases is able to utilize not only a UDP-sugar as a donor molecule, but also a TDP-sugar. Therefore, rather than naming these enzymes as UDP glycosyltransferases, we identified them as NRD1[beta]-type glycosyltransferases in order to indicate the presence of a nucleotide recognition domain that can recognize either UDP or TDP. (2) We divide the NRD1[beta] proteins into the following five major classes based on protein sequence homology (Figure
All C.elegans NRD1[beta] family members have a C-terminal transmembrane domain and C-terminal motif similar to the endoplasmic reticulum retention signal. They can potentially transfer carbohydrate residues onto lipids, carbohydrates, oligopeptides, and polycyclic compounds such as steroid hormones. They also can take part in the biosynthesis of extracellular matrices, hormones, or certain extracellular signal factors. These factors may play an important role in the early stages of multicellular organism development.
CGT can transfer a galactose residue from UDP-galactose to diacylglycerol (DAG) and produce monogalactosyldiacylglycerol (Schaeren-Wiemers et al., 1995). The same function is performed in plants by DAG galactosyltransferase (DAGGalT) (EC 2.4.1.46, 522 amino acid residues), which produces the major structural lipid of chloroplast (Shimojima et al., 1997). DAGGalT shares homology with the murG gene from E.coli and Bacillus subtilis, whose product N-acetylglucosaminyltransferase (GlcNAcT) catalyzes the transfer of N-acetylglucosamine to N-acetylmuramyl-(pentapeptide) phosphoryl-undecaprenol in the last step of peptidoglycan synthesis (Mengin-Lecreulx et al., 1991). This homology implies that the chloroplast membrane biosynthesis mechanism is evolutionarily derived from that of the bacterial cell wall, supporting the endosymbiotic theory (Shimojima et al., 1997). Our computer analysis indicates the existence of two more putative glycosyltransferases from Bacillus subtilis (P54166) and Methanococcus jannaschii (Q58652). These sequences are compared in Figure
DAGGalT does not have a terminal transmembrane domain; the first 103 amino acid residues are cleaved upon translocation to the chloroplast. It appears that the murG gene product of E.coli and B.subtilis also lacks an identifiable terminal transmembrane domain although it is membrane associated.
Separation of the NRD1[beta] family members and the NRD1 domain of the DAGGalT family probably occurred prior to the separation of archaebacteria and bacteria; however, we still attribute NRD of the DAGGalT family to NRD1[beta] due to the significant similarity in the central portion of NRD1 domain. The central portion of the NRD1[beta] domain is very close to the conserved domain of a large group of glycosyltransferases. Since all known members of this group are retaining glycosyltransferases, we termed it NRD1[alpha] family to reflect the resulting anomeric configuration. This family is characterized by the presence of two conserved E residues, located at positions 1 and 2 (Figure
Figure 6. Members of the NRD1[alpha] family. The type of glycosyltransferase (if known) is followed by the gene name, organism and accession number. ManT, Mannosyltransferase; GalT, galactosyltransferase; GlcT, glucosyltransferase; GalAT, galacturonate transferase; GlyT, glycosyltransferase; GlcNAcT, N-acetylglucosaminyl transferase; CPS, capsular polysaccharide biosynthesis protein; EPS, exopolysaccharide biosynthesis protein; LPS, lipopolysaccharide biosynthesis protein; E.c., Escherichia coli; Vibr.ch., Vibrio cholera; Ye.e, Yersinia enterocolitica; Me.j., archaebacteria Methanococcus jannaschii; S. a., Staphylococcus aureus; St. t., Streptococcus thermophilus; K.p., Klebsiella pneumoniae; Pr. m., Proteus mirabilis; S. p., Streptococcus pneumoniae; C. h., Campylobacter hyoilei; S.e., Salmonella enterica; A.x., Acetobacter xylinum; An, Anabaena sp.; Synech, Synechocustis sp.; S.t, Salmonella typhimurium; Myc. tub., Mycobacterium tuberculosis; Arch. f., Archaeoglobus fulgidus; (17), group of 17 sucrose synthases from both monocotyledonae and dicotyledonae, shown are sucrose synthase from bean and barley; 5, group of five sucrose-phosphate synthases. Shown is sucrose phosphate synthase from rice, TRSD, E, H - LPS biosynthesis proteins; S.c., Saccharomyces cerevisiae; C.elegans, Caenorhabditis elegans; N-GlyT ALG2, asparagine-linked oligosaccharides biosynthesis protein ALG2; B.s., Bacillus subtilis. Motif, amino acid residues present in more then 60% of the family members. NRD1[beta] members MurG protein of Escherichia coli, diacylglycerol (DAG) galactosyltransferase from cucumber, oleandomycin glycosyltransferase, indole-3-acetate glucosyltransferase and CGT Motif are shown for comparison. 1 and 2, amino acid residues that we propose as a part of enzyme reaction mechanism. Residues predicted to be a part of [alpha]-helix in the middle of the proposed catalytic domain are underlined. The three-digit number on the left margin of the alignment indicates position of the first amino acid of the conserved domain; the number on the right margin indicates the total length of the protein.
Figure 7. The evolutionary relationship among members of the NRD1[alpha] family. For the explanation of abbreviations, see the Figure 6 caption. Eukaryotic enzymes are underlined; archaebacterial enzymes are shown in italics.
All of the bacterial glycosyltransferases participate in the formation of the exopolysaccharide (EPS), lipopolysaccharide (LPS), or capsular polysaccharide structures (CPS). Mannosyltransferase C (MtfC) from E.coli is the first ManT that transfers mannose residue to Glc-pyrophosphorylundecaprenol, forming an [alpha](1->3) bond in the O9 antigen biosynthesis pathway. ManT B (MtfB) is the second mannosyltransferase that transfers two mannose residues to form [alpha]-Man-(1->3)-[alpha]-Man-(1->3)-[alpha]-Man-(1->3)-Glc-PP-Undecaprenol intermediate. ManT A (MtfA) transfers three mannose residues to form [alpha]-Man-(1->2)-[alpha]-Man-(1->2)-[alpha]-Man-(1->2)-[alpha]-Man-(1->3)-
Therefore, NRD1[alpha] glycosyltransferases can be further divided into three groups: prokaryotic, eukaryotic plant, and animal glycosyltransferases (very much like NRD1[beta] glycosyltransferases). Apart from the domain shown in Figure
Human ceramide glucosyltransferase (CGlcT) catalyzes the transfer of the glucosyl residue from UDP-glucose onto ceramide (Ichikawa et al., 1996). In the NRD1[beta] family of enzymes, some members are able to use UDP-glucose as well as UDP-galactose as a donor substrate (e.g., ecdysteroid glycosyltransferase from Spodoptera frugiperda (O'Reilly et al., 1992). Therefore, we attempted to determine whether CGlcT belongs to the NRD1 family. Computer analysis indicated no significant similarity between CGT and CGlcT. Unlike CGT, CGlcT has an N-terminal transmembrane domain. Further analysis indicates that CGlcT belongs to another family of glycosyltransferases that share a common domain, which we have named NRD2. The NRD2 domain is located either immediately adjacent to the N-terminus transmembrane domain, or at the N-terminus. This recognition domain is bipartite: a large NRD2L part of about 50 amino acid residues, located near the N-terminus (amino acid residues 51-93 in the human CGlcT), and a small NRD2S part of 11-17 amino acids (amino acids 136-153 in the human CGlcT). In different NRD2 family members, NRD2L and NRD2S are separated by 27-41 amino acid residues. Human CGlcT has three close homologs, one from Synechocystis and two from C.elegans. Based on the conserved sequence, we have generated a motif and searched the database in order to find additional members of the NRD2 family. We found very similar domains in the NodC protein, cellulose synthase, rhamnosyltransferase, hyaluronate synthase, and some other proteins (Figure
Figure 8. The conserved domain of the NRD2 family members. DPManT, dolichol-phosphate mannosyltransferase; S.c., Saccharomyces cerevisiae; DPGlcT, dolichol-phosphate glucosyltransferase; NODC, nodulation protein C; Rh.m., Rhizobium meliloti; FBFA, fruit body formation protein A; St.a., Stigmatella aurantiaca; HASA, hyaluronate synthase; S.py., Streptococcus pyogenes; EXOM, O, U, W, and A, exoolygosaccharide biosynthesis proteins M, O, U, and W (glucosyltransferases) and A; R. m., Rhizobium meliloti; GlcNAcT, N-acetyl glucosaminyltransferase; S.e., Staphylococcus epidermidis, ICAA protein; EPSI and EPSG, exopolycaccharide biosynthesis proteins I and G; S.t., Streptococcus thermophilus; GalT, galactosyltransferase; CPSJ and CPS I, capsid polysaccharide biosynthesis proteins J and I; L.l., Lactococcus lactis; GGAB and GGAA TeicAc, teichoic acid biosynthesis protein B and A; B.s., Bacillus subtilis; S.p., Streptococcus pneumoniae; GlyT KFIC, glycosyltransferase KFIC; E. c., Escherichia coli; GlyNAcT LGTA, N-acetyl glucosaminyl/galactosaminyltransferase LGTA protein; N.m., Neisseria meningitidis; M.j., Methanococcus jannaschii; VIRB V.a., Vibrio anguillarum, VIRB protein; RFBV, B, Q, glycosyltransferases; Sa.t., Salmonella typhimurium; Y.e., Yersinia enterocolitica; RhaT RFBN and RFBQ, rhamnosyltransferase RFBN and Q BM_1_proteins; Sa.t., Salmonella typhimurium; S.d., Shigella dysentariae; EXPE2, exopolysaccharide biosynthesis protein E2; S.m., Sinorhizobium meliloti; ACSA, cellulose synthase; A.x., Acetobacter xylinum; CerGlcT, ceramide glucosyltransferase. NRD2 Motif, amino acid residues that are present in the most of the NRD2 family members. Number in front of the name indicates the belonging to a group of several members. The number of group members is shown in brackets. 28-42aa, Nonconserved region of 28-42 amino acid residues not shown in the figure.
Figure 9. The evolutionary relationship among the NRD2 family members. Eukaryotic proteins are underlined and archaebacterial proteins are shown in italics. Number in front of the name indicates a group of several enzymes that share higher homology with the members of a group, than with other members of NRD2 family. For an explanation of the abbreviations see the Figure 8 caption.
Based on sequence similarities, the NRD2 family of glycosyltransferases can be divided into the following groups. Group 1 consists of GDP-mannose: dolichol-phosphate O-[beta]-d-mannosyltransferase (EC 2.4.1.83). This is an essential membrane-bound enzyme responsible for the synthesis of dolichol-phosphate-mannose (DPM), a key glycosyl donor for the synthesis of the N-linked oligosaccharide chains of glycoproteins (Orlean et al., 1988; Orlean, 1990; Schutzbach et al., 1993). Other enzymes of the NRD2 family that belong to this group are DPM from yeast, DPM1 from Trypanosoma brucei (Q26732) and Ustilago maydis (P54856), hypothetical proteins from Synechocystis sp (P74505) and Mycobacterium tuberculosis (P71781), and two proteins from E.coli (P77293 and P77757). Dolichol-phosphate glucosyltransferase from Saccharomyces cerevisiae transfers glucose from UDP-glucose to dolichol phosphate (Heesen et al., 1994). An absence of this enzyme leads to underglycosylation of the secreted proteins.
Group 2 of the NRD2 glycosyltransferase family consists of NODC proteins. These proteins are produced by rhizobial microorganisms that fix nitrogen inside specialized nodule structures in the root. NODC proteins possess N-acetylglucosaminyl transferase activities (chitin synthases) that are involved in the formation of lipo-chito-oligosaccharide Nod factors that initiate root nodule morphogenesis in legumes. NODC proteins are responsible for the synthesis of trimers to pentamers of [beta]-1,4-linked GlcNAc residues called Nod factors (Figure
Figure 10. Chemical structure of the acceptors of the NRD2 glycosyltransferases.
Another example of cell to cell interaction in prokaryotes can be found in the development of the myxobacterium Stigmatella aurantiaca, which undergoes a multicellular cycle of development resulting in the formation of fruiting bodies. The protein FBFA (Q53680), which participates in the fruiting body formation, is a glycosyltransferase that shares a higher degree of homology with the NODC protein than it does with any other protein of the NRD2 family. Inactivation of FBFA leads to the formation of nonstructured clumps rather than the structured fruiting body (Silakowski et al., 1996). Together with two other putative enzymes from Bacillus subtilis (P96587) and Synechocystis sp (P74165), FBFA forms a third group of NRD2 glycosyltransferases.
Several groups of the NRD2 enzymes participate in cell wall formation. Hyaluronan synthase, or HASA, from group A streptococci (e.g., Streptococcus pyogenes), produces the antiphagocytic EPS hyaluronate capsule by alternating the addition of UDP-N-acetyl-d-glucosamine and UDP-d-glucuronic acid. The enzyme apparently has both glycosyltransferase activities (Dougherty and van de Rijn, 1992; DeAngelis et al., 1993). Streptococcal hyaluronate is chemically indistinguishable from that found in animal connective tissues (Figure
EXO is a group of glycosyltransferases responsible for EPS type I succinoglycan biosynthesis in Rhizobium meliloti. EPS plays an important role during interaction with its host alfalfa. EXO glycosyltransferases participate in the biosynthesis of the repeating unit of succinoglycan (Figure
The ICAA protein from Staphylococcus epidermidis participates in the synthesis of the polysaccharide intercellular adhesin (PIA), which is located on the cell surface and is responsible for the formation of the biofilm and the large cell clusters. PIA is a linear [beta]-1,6-linked glycosaminoglycan composed of at least 130 2-deoxy-2-amino-d-glucopyranosyl residues, most of which are N-acetylated. ICAA has been shown to be a membrane-bound N-acetylglucosaminyltransferase that forms [beta]-1,6-linked N-acetyl-d-glucosaminyl polymers (Heilmann et al., 1996).
Group 4 of the NRD2 glycosyltransferase family is the group of 13 enzymes responsible for cell surface polysaccharide (CSP) biosynthesis. CSP can be attached to the cell membrane as the O-antigen of lipopolysaccharides (LPS) to form a capsule around the cell as capsular polysaccharides (CPS). It can also be completely excreted as exopolysaccharides (EPS). The biological functions of the polysaccharides are diverse, including roles in pathogenesis and symbiosis, protection of the cell from desiccation or other environmental stresses, and/or facilitating adherence of bacteria to cell surfaces.
The EPSI glycosyltransferase from Streptococcus thermophilus Sfi6 (Q56046) participates in the synthesis of excreted EPS having the repeating unit ->3)-[beta]-d-Galp-(1->3)-[[alpha]-d-Galp-(1->6)]-[beta]-d-Glcp-(1->3)-[alpha]-d-GalpNAc-(1-> which is responsible for the extremely slimy texture of some yogurt products (Stingele et al., 1996). The EPSG glycosyltransferase from Lactococcus lactis (O06035) participates in the synthesis of similar EPS (van Kranenburg et al., 1997).
CPSJ and I proteins of Streptococcus pneumoniae (O07340 and O07339) are glycosyltransferases that transfer the 4th sugar (galactose) and 3rd sugar (N-acetylglucosamine), respectively, to the glycolipid intermediate (L):
Minor teichoic acid synthesis proteins GGAA and GGAB from Bacillus subtilis (P46917 and P46918) are glycosyltransferases that are involved in the synthesis of poly(3-O-[beta]-d-glucopyranosyl N-acetylgalactosamine-1-phosphate), a secondary anionic polymer of gram-positive bacteria (Mauel et al., 1994; Lazarevic et al., 1995). In E.coli KFIC protein (Q47330) is responsible for the synthesis of the capsular polysaccharide, K5 antigen, a polymer of 4)-[beta]-GlcA-( 1,4)-[alpha]GlcNAc-( 1. KFIC can use both UDP-glucuronic acid (UDP-GlcA) and UDP-N-acetyl-glucosamine (UDP-GlcNAc) as substrates (Petit et al., 1995).
Other proteins that belong to Group 4 of the NRD2 glycosyltransferases are Haemophilus influenzae (Q57022 and Q56869) (Fleischmann et al., 1995), Yersinia enterocolitica (Q56869, Q57022), and TRSB protein (Q56914). The latter is responsible for the biosynthesis of LPS O:3 antigen (homopolymer of 6-deoxy-l-altrose repeating units linked by 1,2 linkages) (Skurnik et al., 1995); Bacillus subtilis (P71059 and P71057) (Glaser et al., 1993), E.coli (P11290), Synechocystis sp. spore coat polysaccharide biosynthesis protein SPSA (P73983), and Pseudomonas aeruginosa mucosa-induced protein MIGA (P95448).
Group 5 of the NRD2 glycosyltransferases consists of enzymes involved in lipopolysaccharide (LPS) synthesis. The LGTA glycosyltransferase from Neisseria meningitidis (Q51115) is responsible for the synthesis of meningococcal LPS oligosaccharide of L3 immunotype. It transfers an N-acetylglucosamine residue to the terminal galactose (4th sugar) of the inner core lacto-N-neotetraose (the last four sugars): [beta]-d-Gal-(1->4)-[beta]-d-GlcNAc-(1->3)-[beta]-d-Gal-(1->4)-[beta]-d-Glc-(1->4)-[alpha]-Hep-(1->5)-[alpha]-KDO (Jennings et al., 1995). Its counterpart in Neisseria gonorrhoeaerhea (Q50946) is encoded by the lsi-2 gene (Danaher et al., 1995). Other members of the group 5 glycosyltransferases include putative proteins from Haemophilus influenzae (Q57287), Bacillus subtilis (P71054) and LGTD protein from Neisseria gonorrhoeaerhea (Q50949) and meningitidis (P96946) that transfer GlcNAc to the terminal Gal of the lacto-N-neotetraose portion of the lipo-oligosaccharide (LOS) inner core (Gotschlich, 1994). Q50951 from Neisseria gonorrhoeae is a putative protein. In the fish pathogen Vibrio anguillarum VIRB protein takes part in the synthesis of the LPS, which is a major surface flagella sheath antigen (Norqvist and Wolf-Watz, 1993).
Group 6 of the NRD2 glycosyltransferases includes RFBV, an abequose transferase from Salmonella enterica, serogroup C2 (P26401), which takes part in the formation of the O-antigen repeat unit (Reeves, 1993):
RFBN is a rhamnosyltransferase of Salmonella enterica serogroup B (Q54129) that participates in the formation of O-antigen: ->2)-[beta]-Man-[-(1->3)-[alpha]-2OAcAbe]-(1->4)-[alpha]-l-Rha-(1->3)-[alpha]-Gal-[-(1->4)-[alpha]-Glc]-(1->. The same function is performed in the Salmonella enterica serogroup E1 by the orf11.9 product (Liu et al., 1993). Other enzymes in this group include the product orf14.1 of Salmonella enterica (Q99192), which probably performs the same role as that encoded by orf11.9 of serogroup, two proteins from Mycoplasma genitalium (P47306 and P47271) (Fraser et al., 1995), one protein from Yersinia enterocolitica (Q56866), and two proteins from Synechocystis sp. (P72899 and P73996).
Rhamnosyltransferases RFBQ (Q03581) and RFBR (Q03582) from Shigella dysenteria and RFBF from Shigella flexneri (P37782) belong to Group 7 of the NRD2 family. RFBQ transfers the 2nd sugar residue (Rha I) and RFBR transfers the 3rd sugar residue (Rha II) to the O-antigen repeating unit of the cell surface LPS: ->2)-[alpha]-l-Rha II-(1->3)-[alpha]-l-RhaI-(1->2)-[alpha]-d-Gal-(1->3)-[alpha]-d-GlcNAc-(1->-Acyl Carrier Lipid (ACL). RFBF transfers a Rha III residue in the Y serotype biosynthesis reaction to form O-antigen: ->2)-[alpha]-l-RhaIII-(1->2)-[alpha]-l-RhaII-(1->3)-[alpha]-l-RhaI-(1->3)-[beta]-d-GlcNAc-(1->-ACL (Morona et al., 1995). Other glycosyltransferases in this group are the following: RFBC from Yersinia enterocolitica (S35296) (Zhang et al., 1993) that participates in 6-deoxy-l-altrose (C3 epimer of rhamnose) polymerization, RFBF rhamnosyltransferase from Leptospira interrogans (P71441) (Mitchison et al., 1997), rhamnosyltransferase from Streptococcus pneumoniae (O07868), and RFBE glycosyltransferase from Klebsiella pneumoniae (Q48482) (Kelly and Whitfield, 1996). EXPE2 from Synorhizobium meliloti participates in the formation of EPS II, which consists of alternating glucose and galactose residues joined by [alpha]-1,3 and [beta]-1,3 linkages.
Cellulose synthases from Acetobacter xylinum (P19449 and P21877) (Wong et al., 1990; Standal et al., 1994) and Paramecium bursaria Chlorella virus 1 (U42580) (Kutish et al., 1996) belong to the 8th group of NRD2 glycosyltransferases. These enzymes are responsible for the synthesis and crystallization of cellulose (Figure
The last and the most distant group of the NRD2 glycosyltransferases is formed by ceramide glucosyltransferases from humans, two putative proteins from C.elegans, and one putative protein from Synechocystis. The existence of two proteins in C.elegans may indicate that these enzymes utilize different substrates, such as hydroxy ceramide and nonhydroxy ceramide.
Similar to members of the NRD1 family, members of the NRD2 family of glycosyltransferases are found in eukaryotes, prokaryotes, bacteria, and archaebacteria, indicating that this domain appeared prior to separation of these three kingdoms. All NRD2 members are membrane-bound enzymes that can potentially span the membrane several times. It is likely that the evolution of this family of enzymes was closely connected with the evolution of the membrane structure. In addition, we have identified a conserved region of these proteins that is located near the N-terminus or immediately following the N-terminal transmembrane domain (if such a domain is present).
Results of our computer analysis allow us to predict that it is possible to identify new members of the membrane-bound NRD2 family of glycosyltransferases through the following strategies: (1) using degenerate primers based on the conserved amino acid sequence of the NRD domains, (2) hybridization with the NRD portion of known genes, or (3) screening expression libraries (e.g., the [lambda]gt11 library with polyclonal antibodies against the NRD2 domain). We can also speculate that glycosyltransferases needed for eukaryotic glycolipid and bacterial LPS biosynthesis evolved from a common predecessor. Therefore, based on the sequence homology with glycosyltransferases involved in the later steps of LPS biosynthesis, one can expect to discover new glycosyltransferase members of the NRD1 or NRD2 family that are membrane-bound and catalyze the addition of sugar residues to glycoconjugates. Since glycoconjugates play a crucial role in embryonic development, future research should be directed toward identifying those glycoconjugates by isolating NRD1 and NRD2 glycosyltransferases from higher eukaryotes and, in particular, mammals, and analyzing their function.
When we analyzed the NRD1[alpha] family of glycosyltransferases, we determined that all the members of that family transfer carbohydrate residues with the retention of configuration. Comparison of the NRD1[beta] and NRD1[alpha] glycosyltransferase family members shows that one of the distinctive features of the two families is the presence of two conserved glutamic acid residues (marked with numbers 1 and 2 in Figure
Figure 11. Proposed reaction mechanisms for the glycosyltransferase reaction with retention of the sugar donor configuration (A), or inversion (B).
All known NRD1[beta] glycosyltransferases transfer sugar residue with the inversion of configuration that would require one nucleophile. We noticed that E residue at position 2 and the [alpha]-helical domain are conserved in the NRD1[beta] (Figure
Since NRD1[alpha] glycosyltransferases can accept both purine- and pyrimidine-containing sugar nucleotides, they are lacking conserved nucleotide recognition motifs. If our model is correct, substitution of E residue at position 2 for D in any NRD1[beta] or NRD1[alpha] family member should cause a dramatic reduction in the reaction rate, while substitution for any other amino acid should completely eliminate the enzyme activity. Substitution of H residue for R/K at position 1 in NRD1 family members could possibly be tolerated with some changes in the reaction rate. Substitution of H/R/K residues at the position 1 for E may convert inverting glycosyltransferase into retaining and convert resulting anomeric configuration from [beta] to [alpha]. Glycosyltransferases with S/T residues located two amino acid residues before residue E at position 2 could be potentially regulated by phosphorylation since placing a strong negative charge in the catalytic domain between two amino acid residues involved in the reaction should affect enzyme activity. Some microbial NRD1 glycosyltransferases apparently do not have a conserved E residue at position 2 (Figure
The NRD2 family includes inverting glycosyltransferases that can accept both purine- and pyrimidine-containing sugar nucleotides. At this point we are unable to propose any particular amino acid residue as a candidate for the nucleophile or for nucleotide recognition, but the YN...E motif, E at the position 1 and two of the DD motifs (at positions 2 and 3) are good candidates for the beginning of site-directed mutagenesis experiments (Figure
Based on our analysis, the most logical way to classify the glycosyltransferases that we have analyzed is according to the type of bond they form and the reaction mechanisms they carry out their catalytic function. Thus, all glycosyltransferases analyzed fall into two types: retaining glycosyltransferases that transfer sugar with the retention of configuration of anomeric carbon, and inverting glycosyltransferases that transfer sugar with the inversion of configuration of anomeric carbon. The retaining glycosyltransferases that we have analyzed are characterized by the presence of two E residues in the proposed catalytic domain and are further divided into the class of prokaryotic glycosyltransferases and eukaryotic glycosyltransferases within two subclasses of animal and plant glycosyltransferases. The inverting glycosyltransferases that we have analyzed are divided into 2 subclasses: H/(R/K)-E subclass with H or R/K residue at position 1 and E residue at position 2 and NRD2 subclass. Each of these subtypes is further divided into the class of prokaryotic and eukaryotic glycosyltransferases. The latter is further divided into two subclasses of plant and animal enzymes. The division of NRD1[beta] glycosyltransferases into five classes described in Figure
In conclusion, we have described glycosyltransferases that belong to three different families according to the similarity and proposed catalytic domain based on the most conserved amino acid residues. Using computer alignment of known glycosyltransferases, we generated patterns that we used to search the database for additional putative glycosyltransferases. Based on the amino acid residues present in the catalytic domain and the type of bond these glycosyltransferases catalyze, we have proposed reaction mechanisms and classified glycosyltransferases. Further studies by site-directed mutagenesis and/or x-ray structure analysis are required for the confirmation of our model. The crystallographic information can be used for the design of enzyme inhibitors. Because some of the NRD1 and NRD2 enzymes are indispensable proteins in bacteria, these inhibitors could lead to a new class of antibiotics. Since microbial glycosyltransferases are much closer to each other than to eukaryotic glycosyltransferases (Figures
This work was supported by a USPHS Grant NS11853-25. We thank Drs. Cara-Lynne Schengrund and H.Tonie Wright for their helpful discussion of the manuscript.
NRD, nucleotide recognition domain; CGT, ceramide galactosyltransferase; UGT, glucuronosyltransferase; DAG, diacylglycerol; DAGGalT, diacylglycerol galactosyltransferase; GlcNAcT, N-acetylglucosaminyltransferase; GalNAcT, N-acetylgalactosaminyltransferase; EPS, exopolysaccharide; LPS, lipopolysaccharide; CPS, capsular polysaccharide structures; ManT, mannosyltransferase; LOS, lipooligosaccharide; GalT, galactosyltransferase; GlcAT, glucuronosyltransferase; CGlcT, ceramide glucosyltransferase; DPM, dolichol phosphate mannose; DPManT, dolichol-phosphate mannosyltransferase; ER, endoplasmic reticulum.
Function of NRD1[alpha] family members
[alpha]-Man-(1->3)-[alpha]-Man-(1->3)-Glc-PP-Undecaprenol intermediate. MtfB and MtfA then finish the formation of O9 mannan-repeating unit ->2)-[alpha]-Man-(1->2)-[alpha]-Man-(1->2)-[alpha]-Man-(1->2)-[alpha]-Man-(1->2)-[alpha]-Man-(1-> (Kido et al., 1995). TrsD, E, and H glycosyltransferases participate in the biosynthesis of the O-antigen LPS outer core, serotype O:3 (Skurnik et al., 1995). E.coli K12 GalT is responsible for the formation of an [alpha]-Gal-(1->6) bond in the outer core biosynthesis: [beta]-GlcNAc-(1->6)-[alpha]-GlcIII-(1->2)-[alpha]-GlcII-(1->3)-[-[alpha]-Gal-(1->6)]-[alpha]-GlcI-(1-> inner core (Pradel et al., 1992). CapH of Staphylococcus aureus (Lin et al., 1994; Lee, 1995; Sau and Lee, 1996), Cap1E and Cap1G of Streptococcus pneumonia (Garcia and Lopez, 1997) are glycosyltransferases involved in the biosynthesis of the capsular polysaccharide. Cap1E is a galacturonosyltransferase that catalyzes the formation of an [alpha] (1-3) bond. EpsF and EpsG from Streptococcus thermophilus are involved in the exopolysaccharide (EPS) biosynthesis (Stingele et al., 1996). The family of RfbF galactosyltransferases has three members: GalT from Klebsiella pneumonia serotypes O1 (Q48487; domain starts at aa 277) and O8, and RfbF GalT from Serratia marcescens (Q54481; domain starts at amino acid residue 277). RfbF is capable of forming both [alpha] and [beta] bonds in the LPS core biosynthesis: [beta]-Gal-(1->3)-[alpha]-Gal-(1->3)-[beta]-GlcNAc-(1->lipid (Clarke et al., 1995; Kelly and Whitfield, 1996). CpsF from Proteus vulgaris participates in the formation of capsular polysaccharide (Gygi et al., 1995; Rahman et al., 1997). RfbF from Campylobacter hyoilei is a GalT involved in lipo-oligosaccharide (LOS) biosynthesis (Korolik et al., 1997). RfbP is a GalT that participates in the formation of O:8-antigen repeating unit (Jiang et al., 1991). RfbU is ManT responsible for the synthesis of O-antigen repeating unit of B serogroup: ->2)-[2-O-Ac-[alpha]-Abe-(1->3)]-[alpha]-Man-(1->4)-[alpha]-l-Rha-(1->3)-[[alpha]-Glc-(1->4)]-[beta]-Gal-(1->. RfbW and RfbZ are both ManT responsible for the addition of the second and first mannose to the repeat unit of O-antigen of C2 serogroup correspondingly: ->4-[2-O-Ac-[alpha]-Abe-(1->3)]-[alpha]-l-Rha-(1->2)-[alpha]-Man II-(1->2)-[alpha]-Man I-(1->3)-[alpha]-Gal-(1-> (Brown et al., 1992; Liu et al., 1993; Reeves, 1993; Xiang et al., 1993, 1994). RfbO ManT which catalyzes the transfer of mannose in E1 serogroup O-antigen biosynthesis forming a [beta]-(1->4) bond has no homology within the NRD1[alpha] domain. The remaining enzymes of O-antigen repeat unit biosynthesis RfbV-abequosetransferase and RfbN-rhamnosyltransferase belong to the NRD2 family (see below). AceC from Acetobacter xylinum is a ManT which adds a third carbohydrate residue in the exopolysaccharide acetan biosynthesis (Rha-Glu-Glu-GluA-ManOAc-Glu-Glu)n-PP-Prenol) (Griffin et al., 1994). Not shown are two very closely related ManT from Acetobacter xylinum (Q44571; starts at amino acid residue 272) and GumH from Xanthomonas campestris (Q56774; starts at amino acid residue 279). Among eukaryotes, the same domain is shared by 5 sucrose-phosphate synthases (the one from rice is shown), 17 sucrose synthases from both monocotyledonae and dicotyledonae, 3 putative proteins from C.elegans, and 1 from S.cerevisiae. Not shown in Figure The NRD2 family of glycosyltransferases
Function of NRD2 family members
Catalytic domain and proposed reaction mechanisms
Classification of glycosyltransferases and concluding remarks
Acknowledgments
Abbreviations
References
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