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O-Linked N-acetylglucosaminyltransferase (OGT) catalyzes the transfer of a single GlcNAc to the Ser or Thr of nucleocytoplasmic proteins. OGT activity, which may compete with that of kinases, is involved in signaling in animals and plants, and abnormalities in OGT activities have been associated with type 2 diabetes. Here, we show that ogt genes that predict enzymes with characteristic tetratricopeptide repeats and a spindly domain are present in some protists (Giardia, Cryptosporidium, Toxoplasma, and Dictyostelium) but are absent from the majority of protists examined (e.g., Plasmodium, Trypanosoma, Entamoeba, and Trichomonas). Similarly, ogt genes are present in some fungi but are absent from numerous other fungi, suggesting that secondary loss is an important contributor to the evolution of ogt genes. Nucleocytosolic extracts of Giardia and Cryptosporidium show OGT activity, and recombinant Giardia and Cryptosporidium OGTs are active in yeast and bacteria, respectively. These results suggest the possibility that O-GlcNAc modification of nucleocytosolic proteins also has function(s) in simple eukaryotes.
O-Linked N-acetylglucosaminyltransferase (OGT) is a nucleocytoplasmic glycosyl transferase, which catalyzes the addition of a single β-O-linked N-acetylglucosamine (GlcNAc) to the serine or threonine of a polypeptide chain (reviewed in Love and Hanover (2005); Hart et al. (2007)). The C-terminus of the OGT contains the catalytic domain (also known as “spindly” or “spy” domain), which has a weak homology with glycogen phosphorylase (Roos and Hanover 2000; Wrabl and Grishin 2001). The N-terminus of the OGT contains a series of tetratricopeptide repeats (TPRs), which are involved in substrate recognition (Lubas and Hanover 2000; Martinez-Fleites et al. 2008).
In metazoa, OGT, which is essential, modifies transcription factors, nuclear pore proteins, kinases, and many other proteins (Hanover et al. 1987; Jackson and Tjian 1988; Shafi et al. 2000; Love and Hanover 2005; Hart et al. 2007). Numerous sites on nucleocytosolic proteins may be modified by either O-GlcNAc or O-phosphate, suggesting a possible role for the OGT in cellular signaling (Cheng et al. 2000; Hart et al. 2007). OGT activity, as well as O-GlcNAcase activity, has also been associated with hexosamine signaling in metazoa and has been implicated in human type 2 diabetes and in a Caenorhabditis elegans model of diabetes (Hanover et al. 2005; Lehman et al. 2005; Love and Hanover 2005; Forsythe et al. 2006; Whelan et al. 2008; Yang et al. 2008).
Plant OGTs, which are referred to as “spindly” and “secret agent,” negatively regulate the gibberellin signaling pathway and are important in gamete formation and embryogenesis (Hartweck et al. 2002; Silverstone et al. 2007). Recently, a Listeria OGT was shown to add O-GlcNAc to flagellar proteins, as well as down-regulate synthesis of flagellar proteins (Shen et al. 2006). OGT genes and activities are absent from Saccharomyces cerevisiae and Schizosaccharomyces pombe.
Giardia lamblia and Cryptosporidium parvum, which are the focus of the present study, are protists (single-cell eukaryotes) spread by the fecal-oral route that cause diarrhea (Steiner et al. 1997). However, Giardia and Cryptosporidium are unrelated in their morphology and phylogeny. Giardia, which has two similar nuclei, eight flagellae, and an adherence disc, is a deeply divergent and minimal protist (Morrison et al. 2007). For example, Giardia makes an N-glycan precursor containing just two GlcNAc and has a single nucleotide-sugar transporter (NST) that transports UDP-GlcNAc from the cytosol to secretory compartments (Samuelson et al. 2005; Banerjee et al. 2008). In contrast, metazoa have N-glycan precursors with 14 sugars and 18–22 NSTs.
Cryptosporidium has a single nucleus and is related in its appearance and phylogeny to Plasmodium and Toxoplasma (Abrahamsen et al. 2004). Cryptosporidium makes an N-glycan precursor containing eight sugars and has numerous NSTs that transport activated Gal, GalNAc, Man, and Fuc (Samuelson et al. 2005 and our unpublished data).
The first goal here was to determine whether any protists have a predicted OGT, and if so, use phylogenetic methods to determine the ancestry of the protist OGTs. The second goal was to determine whether cytosolic extracts of Giardia and Cryptosporidium, which have ogt genes, have OGT activity. The third goal was to determine whether recombinant Giardia and Cryptosporidium OGTs have the predicted activities.
The Giardia genome predicts a single 1480-aa OGT, which contains just two TPRs at its N-terminus and a “spy” (catalytic) domain that has a large insert in the C-terminus (Figure (Figure11 and supplementary Figure 1). The Cryptosporidium genome predicts a single 1032-aa long OGT, which also contains just two N-terminal TPRs but has an uninterrupted C-terminal “spy” domain. In contrast, human OGT, which is 1046-aa long, has 11 TPRs.
Toxoplasma and Dictyostelium have predicted OGTs, which are absent from other protists examined (Eimeria, Entamoeba, Leishmania, Plasmodium, Spironucleus, Theileria, Trichomonas, and Trypanosoma). Predicted OGTs, which are present in all metazoa and plants examined, are also present in Yarrowia, Aspergillus, Magnaporthea, and Neurospora but are absent from Saccharomyces, Schizosaccharomyces, Candida, and Cryptococcus.
We looked for but did not find protist homologs of the O-GlcNAcase, which removes O-linked GlcNAc from nucleocytosolic proteins of metazoan (Gao et al. 2001). Homologs of this O-GlcNAcase are present in numerous bacteria but are absent from plants and fungi (with the exception of Histoplasma), which contain OGTs.
Phylogenetic analyses were performed to determine the origins of the Giardia and Cryptosporidium OGTs with the following results (Figure (Figure2).2). First, the OGT tree, which was drawn by maximum likelihood methods, is star shaped, so that it is not possible to determine the ancestry of the Giardia OGT. Second, the OGTs of the other protists (Cryptosporidium, Toxoplasma, and Dictyostelium) appear to share a common ancestor with each other and more distant common ancestry with some plants (e.g., Lycopersicon and Petunia) and some bacteria (Bradyrhizobium and Rhodopseudomonas). Third, metazoan OGTs are very tightly clustered and appear to share more distant common ancestry with fungal OGTs and OGTs of some plants (Arabidopsis and Oryza).
The absence of ogt genes in other protists and fungi strongly suggests secondary loss as an explanation for the present diversity of OGTs. In particular, OGT is present in Dictyostelium but absent from the closely related ameba Entamoeba; OGT is present in Giardia but absent from the closely related diplomonad Spironucleus; and OGTs are present in Toxoplasma and Cryptosporidium but are absent from the closely related apicomplexans Plasmodium and Theileria. The present diversity of N-glycan precursors has also been explained by secondary loss of Alg enzymes, which synthesize the N-glycan precursors (Samuelson et al. 2005).
The presence of OGTs in a mixed clade composed of protists, bacteria, and selected plants or in a mixed clade composed of metazoa, fungi, and selected plants suggests the possibility of lateral gene transfer of ogt genes. The possible common ancestry of plant and Rhodobacter OGTs, subsequent to lateral gene transfer, has previously been proposed (Roos and Hanover 2000). While lateral gene transfer is an important contributor to evolution of bacteria and some protists (Giardia, Entamoeba, and Trichomonas), it is not usually an important contributor to evolution of plants or metazoa (De Koning et al. 2000; Loftus et al. 2005; Morrison et al. 2007).
Messenger RNAs encoding Giardia lamblia OGT (GlOGT) are present in Giardia trophozoites and increase by ~4-fold when Giardia encyst in vitro (Banerjee et al. 2008). Nucleocytosolic extracts of Giardia have OGT activity in the presence of radiolabeled UDP-GlcNAc (Figure (Figure3A),3A), and this result was confirmed by the release of a single GlcNAcitol from nucleocytosolic glycoproteins by reductive β-elimination (data not shown). A recombinant His-tagged catalytic domain of GlOGT, which was expressed in Saccharomyces and purified on Ni-NTA beads, failed to glycosylate casein kinase (a test substrate for OGTs) but autoglycosylated with kinetics similar to that of the OGT in Giardia nucleocytosolic extracts (Figure (Figure3B).3B). A single GlcNAcitol was released by reductive β-elimination of the glycosylated protein (Figure (Figure3C).3C). In addition, β-hexosaminidase released ~75% of the radiolabeled GlcNAc from the autoglycosylated GlOGT, confirming the expected β-O-GlcNAc linkage (Figure (Figure4).4). A recombinant human OGT, which is missing in the TPRs, also autoglycosylates but does not glycosylate casein kinase (Lubas and Hanover 2000).
Messenger RNAs encoding CpOGT are present in Cryptosporidium sporozoites (data not shown), and nucleocytosolic extracts of Cryptosporidia have OGT activity in the presence of radiolabeled UDP-GlcNAc (Figure (Figure5A).5A). A recombinant full-length CpOGT, which was expressed as a GST-fusion enzyme in Escherichia coli and purified on glutathione-agarose, glycosylated casein kinase (a test target for OGTs) with kinetics similar to that of the OGT in nucleocytosolic extracts of Cryptosporidia (Figure (Figure5B).5B). A single GlcNAcitol was released by reductive β-elimination of the glycosylated proteins (Figure (Figure5C).5C). In addition, β-hexosaminidase released ~83% of the radiolabeled GlcNAc from the glycosylated casein kinase, confirming the expected β-O-GlcNAc linkage (Figure (Figure44).
To our knowledge, the presence of putative ogt genes in some protists and fungi has not previously been shown, and the OGT activities of protist membranes and of recombinant enzymes have not previously been demonstrated. These results suggest the possibility that O-GlcNAc modification of nucleocytosolic proteins also has function(s) in simple eukaryotes. Conversely, the absence of ogt genes (likely by secondary loss) in so many protists and fungi indicates that these organisms have alternative methods for regulating activities of nucleocytosolic proteins.
Future studies, which might help determine the function of the protist OGTs, include the following. First, knockout studies of the OGTs, which are not presently possible in Giardia or Cryptosporidium, may be performed in Dictyostelium and Toxoplasma, where these technologies are readily available. Second, identification of the “OGTome” (nucleocytosolic proteins modified by O-linked GlcNAc) may suggest numerous possible functions for O-GlcNAc modification in these protists. Third, in the context of the hexosamine hypothesis (the idea that OGT and O-GlcNAcase are involved in glucose sensing and signaling) (Love and Hanover 2005), it would be interesting to examine the role of O-GlcNAc modification of nucleocytosolic proteins during encystation by Giardia. This is because OGT mRNAs are increased 4-fold during encystation and UDP-GlcNAc (the limiting substrate for the OGT) is also increased during encystation (Sener et al. 2004; Banerjee et al. 2008).
The predicted proteins of Giardia lamblia and Cryptosporidium parvum, which are present at GiardiaDB and CryptoDB, respectively, were searched with PSI-BLAST using the OGT and O-GlcNAcase (also known as MGEA5) protein sequences from Homo sapiens (Altschul et al. 1997; Gao et al. 2001). Similar methods were used to search the predicted proteins of representative protists (Dictyostelium discoideum, Eimeria tenella, Entamoeba histolytica, Leishmania major, Plasmodium falciparum, Spironucleus vortens, Theileria annulata, Toxoplasma gondii, Trypanosoma brucei, and Trypanosoma cruzi), metazoa, fungi, and plants in the NR database at the NCBI or at specific databases (e.g., PlasmoDB or GeneDB). Trichomonas vaginalis proteins were searched at websites maintained by the J. Craig Venter Institute, Rockville, MD. Predicted OGTs were examined for conserved domains using the CD search at NCBI (Marchler-Bauer et al. 2007). OGTs were aligned using multiple sequence comparison by log-expectation (MUSCLE) (Edgar 2004). The alignment was manually refined, and gaps were removed using BioEdit. The finished alignment was used to construct the phylogenetic tree using TREE-PUZZLE, a program to reconstruct phylogenetic trees from molecular sequence data by the maximum likelihood method (Schmidt et al. 2002).
Trophozoites of the genome project WB strain of Giardia were grown axenically in TYI-S media supplemented with 10% serum and 1 mg/mL bile. Giardia were chilled on ice for 20 min and concentrated at 2000 rpm for 5 min, and genomic DNA and total RNA were isolated using Promega and Invitrogen kits. Alternatively, Giardia cells were lysed by sonication in 10 mM Hepes, pH 7, 10 mM MgCl2, and 25 mM NaCl, supplemented with protease inhibitor cocktail (Sigma). The lysate was centrifuged at 6000 rpm for 5 min to remove unbroken cells and nuclei, and the supernatant was centrifuged at 37,000 rpm in a SW60 rotor (Beckman) for 1 h. The residual pellet was discarded, and the supernatant (nucleocytosolic extract) was used for assays of the endogenous OGT.
Cryptosporidium parvum (Iowa strain) oocysts, which were passaged through newborn calves, were obtained from Bunch Grass Farm (Dury, Idaho) and stored at 4°C. Contaminating fecal bacteria were lysed by 5–10 pulses of sonication in phosphate buffered saline (PBS) and intact oocysts were concentrated by centrifugation at 1100 × g. Cryptosporidium sporozoites were induced to excyst by incubating oocysts in a RPMI medium with 0.75% Na-taurocholate, pH 7.5, for 2 h at 37°C. DNA, RNA, and nucleocytosol of Cryptosporidia were isolated as described for Giardia.
The 3′ end of the Giardia ogt gene, which encodes the catalytic “spy” domain, was amplified from the genomic DNA by PCR (forward primer: GAATTCATGCCCTACCACTGTTATCTTTA and reverse primer: CTCGAGCTGTGCCCCCGT ACTCTTTA) and cloned into in the pYES2.1/V5-His-TOPO vector (Invitrogen), which makes polyHis-tagged proteins under the Saccharomyces Gal1 promoter. The GlOGT construct was transformed into Saccharomyces cerevisiae strain BY4741 (MATa his3Δ1 leu2Δ0 met15Δ0 ura3Δ0), and yeast were induced with 2% Gal for 16 h at 30°C. Yeast were lysed with glass-beads, and the recombinant GlOGT was purified using Ni-NTA beads (Invitrogen). Expression and purification of GlOGT was confirmed on Western blots of purified GlOGT, using a horse-radish peroxidase-labeled antibody to the polyHis epitope-tag.
The full-length Cryptosporidium ogt gene was amplified from the genomic DNA by PCR (forward primer: ATGCTGAAAGATGGTGGAGTT and reverse primer: CTCGAGTTATTTGCATGCAGATAAA) and cloned into the pGEX-6P-1 vector as a glutathione-S-transferase (GST) fusion protein. The recombinant CpOGT was induced by IPTG and purified using glutathione agarose (GE Life Sciences). Expression and purification of CpOGT in bacteria was confirmed by Western blots, using an anti-GST antibody (GE Life Sciences).
The OGT activities were determined for Giardia and Cryptosporidium nucleocytosolic extracts, as well as for recombinant GlOGT and CpOGT, which were expressed in yeast or bacteria, respectively. For assaying endogenous OGT activity, 25–50 μg of protist nucleocytosolic proteins were incubated with 0.1 μCi of tritiated UDP-GlcNAc in the OGT assay buffer (50 mM Tris–HCl, pH 7.5, 12.5 mM MgCl2 and 1 mM DTT) for 30 min. The reaction was stopped with an equal volume of ice cold 20% TCA, and precipitated proteins were washed three times in water, resuspended in the Eoscint scintillation fluid, and counted in a liquid scintillation counter. Each assay was performed on 3 separate days.
The radiolabeled product was further characterized by reductive β-elimination and chromatography on a Bio-Gel P2 column. Briefly, the TCA pellets were resuspended in 50 mM NaOH containing 1 M NaBH4 at room temperature for 16 h. The reaction was stopped with acetic acid and freed of boric acid by repeated evaporation with methanol containing 1% acetic acid in a rotary evaporator. The residue was passed through a mixed bed AG501-X8 resin (BioRad), and the flow-through was dried, resuspended in water, chromatographed, and counted.
For assaying recombinant GlOGT, 1–2 μg of the purified recombinant protein was used, and the transfer of O-GlcNAc to the enzyme itself was assayed (Lubas and Hanover 2000). For assaying recombinant CpOGT, 1–2 μg of purified recombinant protein was used, and the transfer of O-GlcNAc to casein kinase was determined. The radiolabeled product was characterized by β-elimination, as described above. In addition, 3 μg each of radiolabeled GlOGT and casein kinase was digested for 1 h at 37°C with 1 unit of β-hexosaminidase (New England Biolabs) in 100 μL of buffer that was supplied by the manufacturer. The reaction was stopped by adding an equal volume of chilled methanol, and the released products were cleaned using the GlycocleanR (Prozyme) and counted, as described above.
National Institutes of Health grants AI048082 (to J.S.) and GM31318 (to P.W.R.).
Supplementary data for this article is available online at http://glycob.oxfordjournals.org/.