Identification of a glycosylphosphatidylinositol anchor-modifying β1-3 galactosyltransferase in Trypanosoma brucei

Loss of complex-type N-linked glycans attenuates maximum cell density and susceptibility to human serum of Trypanosoma brucei brucei

Trypanosoma brucei brucei is a parasitic protist that expresses cell surface proteins modified with complex-type N-linked glycan (NLG), like multicellular organisms. However, little is known about the role of complex-type NLG. In T. b. brucei, it has been shown that either one of the glycosyltransferases, TbGT11 or TbGT15, is sufficient to initiate the synthesis of complex-type NLG. To clarify the role of complex-type NLG, it is necessary to generate cells lacking both enzymes. Therefore, we deleted TbGT11 and TbGT15 from the genome of T. b. brucei for the phenotypic examination. The mutant strain grew in culture, with reduced maximum cell density; showed decreased susceptibility to normal human serum, which contains trypanolytic factors; and lacked uptake of the haptoglobin-hemoglobin complex. These data indicate that protein modification by complex-type NLG is not essential but is required for receptor function.

Generation of a bloodstream form Trypanosoma brucei double glycosyltransferase null mutant competent in receptor-mediated endocytosis of transferrin

The bloodstream form of Trypanosoma brucei expresses large poly-N-acetyllactosamine (pNAL) chains on complex N-glycans of a subset of glycoproteins. It has been hypothesised that pNAL may be required for receptor-mediated endocytosis. African trypanosomes contain a unique family of glycosyltransferases, the GT67 family. Two of these, TbGT10 and TbGT8, have been shown to be involved in pNAL biosynthesis in bloodstream form Trypanosoma brucei, raising the possibility that deleting both enzymes simultaneously might abolish pNAL biosynthesis and provide clues to pNAL function and/or essentiality. In this paper, we describe the creation of a TbGT10 null mutant containing a single TbGT8 allele that can be excised upon the addition of rapamycin and, from that, a TbGT10 and TbGT8 double null mutant. These mutants were analysed by lectin blotting, glycopeptide methylation linkage analysis and flow cytometry. The data show that the mutants are defective, but not abrogated, in pNAL synthesis, suggesting that other GT67 family members can compensate to some degree for loss of TbGT10 and TbGT8. Despite there being residual pNAL synthesis in these mutants, certain glycoproteins appear to be particularly affected. These include the lysosomal CBP1B serine carboxypeptidase, cell surface ESAG2 and the ESAG6 subunit of the essential parasite transferrin receptor (TfR). The pNAL deficient TfR in the mutants continued to function normally with respect to protein stability, transferrin binding, receptor mediated endocytosis of transferrin and subcellular localisation. Further the pNAL deficient mutants were as viable as wild type parasites in vitro and in in vivo mouse infection experiments. Although we were able to reproduce the inhibition of transferrin uptake with high concentrations of pNAL structural analogues (N-acetylchito-oligosaccharides), this effect disappeared at lower concentrations that still inhibited tomato lectin uptake, i.e., at concentrations able to outcompete lectin-pNAL binding. Based on these findings, we recommend revision of the pNAL-dependent receptor mediated endocytosis hypothesis.

Common and unique features of glycosylation and glycosyltransferases in African trypanosomes

Eukaryotic protein glycosylation is mediated by glycosyl- and oligosaccharyl-transferases. Here, we describe how African trypanosomes exhibit both evolutionary conservation and significant divergence compared with other eukaryotes in how they synthesise their glycoproteins. The kinetoplastid parasites have conserved components of the dolichol-cycle and oligosaccharyltransferases (OSTs) of protein N-glycosylation, and of glycosylphosphatidylinositol (GPI) anchor biosynthesis and transfer to protein. However, some components are missing, and they process and decorate their N-glycans and GPI anchors in unique ways. To do so, they appear to have evolved a distinct and functionally flexible glycosyltransferases (GT) family, the GT67 family, from an ancestral eukaryotic β3GT gene. The expansion and/or loss of GT67 genes appears to be dependent on parasite biology. Some appear to correlate with the obligate passage of parasites through an insect vector, suggesting they were acquired through GT67 gene expansion to assist insect vector (tsetse fly) colonisation. Others appear to have been lost in species that subsequently adopted contaminative transmission. We also highlight the recent discovery of a novel and essential GT11 family of kinetoplastid parasite fucosyltransferases that are uniquely localised to the mitochondria of Trypanosoma brucei and Leishmania major. The origins of these kinetoplastid FUT1 genes, and additional putative mitochondrial GT genes, are discussed.

A Trypanosoma brucei β3 glycosyltransferase superfamily gene encodes a β1-6 GlcNAc-transferase mediating N-glycan and GPI anchor modification

The parasite Trypanosoma brucei exists in both a bloodstream form (BSF) and a procyclic form (PCF), which exhibit large carbohydrate extensions on the N-linked glycans and glycosylphosphatidylinositol (GPI) anchors, respectively. The parasite's glycoconjugate repertoire suggests at least 38 glycosyltransferase (GT) activities, 16 of which are currently uncharacterized. Here, we probe the function(s) of the uncharacterized GT67 glycosyltransferase family and a β3 glycosyltransferase (β3GT) superfamily gene, TbGT10. A BSF-null mutant, created by applying the diCre/loxP method in T. brucei for the first time, showed a fitness cost but was viable in vitro and in vivo and could differentiate into the PCF, demonstrating nonessentiality of TbGT10. The absence of TbGT10 impaired the elaboration of N-glycans and GPI anchor side chains in BSF and PCF parasites, respectively. Glycosylation defects included reduced BSF glycoprotein binding to the lectin ricin and monoclonal antibodies mAb139 and mAbCB1. The latter bind a carbohydrate epitope present on lysosomal glycoprotein p67 that we show here consists of (-6Galβ1-4GlcNAcβ1-)≥4 poly-N-acetyllactosamine repeats. Methylation linkage analysis of Pronase-digested glycopeptides isolated from BSF wild-type and TbGT10 null parasites showed a reduction in 6-O-substituted- and 3,6-di-O-substituted-Gal residues. These data define TbGT10 as a UDP-GlcNAc:βGal β1-6 GlcNAc-transferase. The dual role of TbGT10 in BSF N-glycan and PCF GPI-glycan elaboration is notable, and the β1-6 specificity of a β3GT superfamily gene product is unprecedented. The similar activities of trypanosome TbGT10 and higher-eukaryote I-branching enzyme (EC 2.4.1.150), which belong to glycosyltransferase families GT67 and GT14, respectively, in elaborating N-linked glycans, are a novel example of convergent evolution.

Elimination of GPI2 suppresses glycosylphosphatidylinositol GlcNAc transferase activity and alters GPI glycan modification in Trypanosoma brucei

Many eukaryotic cell-surface proteins are post-translationally modified by a glycosylphosphatidylinositol (GPI) moiety that anchors them to the cell membrane. The biosynthesis of GPI anchors is initiated in the endoplasmic reticulum by transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol. This reaction is catalyzed by GPI GlcNAc transferase, a multisubunit complex comprising the catalytic subunit Gpi3/PIG-A as well as at least five other subunits, including the hydrophobic protein Gpi2, which is essential for the activity of the complex in yeast and mammals, but the function of which is not known. To investigate the role of Gpi2, we exploited Trypanosoma brucei (Tb), an early diverging eukaryote and important model organism that initially provided the first insights into GPI structure and biosynthesis. We generated insect-stage (procyclic) trypanosomes that lack TbGPI2 and found that in TbGPI2-null parasites, (i) GPI GlcNAc transferase activity is reduced, but not lost, in contrast with yeast and human cells, (ii) the GPI GlcNAc transferase complex persists, but its architecture is affected, with loss of at least the TbGPI1 subunit, and (iii) the GPI anchors of procyclins, the major surface proteins, are underglycosylated when compared with their WT counterparts, indicating the importance of TbGPI2 for reactions that occur in the Golgi apparatus. Immunofluorescence microscopy localized TbGPI2 not only to the endoplasmic reticulum but also to the Golgi apparatus, suggesting that in addition to its expected function as a subunit of the GPI GlcNAc transferase complex, TbGPI2 may have an enigmatic noncanonical role in Golgi-localized GPI anchor modification in trypanosomes.

ANALYSIS OF CARBOHYDRATES AND GLYCOCONJUGATES BY MATRIX-ASSISTED LASER DESORPTION/IONIZATION MASS SPECTROMETRY: AN UPDATE FOR 2015-2016

This review is the ninth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2016. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented over 30 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show no sign of deminishing. © 2020 Wiley Periodicals, Inc.

Extracellular Vesicles in Trypanosomatids: Host Cell Communication

Trypanosoma cruzi, Trypanosoma brucei and Leishmania (Trypanosomatidae: Kinetoplastida) are parasitic protozoan causing Chagas disease, African Trypanosomiasis and Leishmaniases worldwide. They are vector borne diseases transmitted by triatomine bugs, Tsetse fly, and sand flies, respectively. Those diseases cause enormous economic losses and morbidity affecting not only rural and poverty areas but are also spreading to urban areas. During the parasite-host interaction, those organisms release extracellular vesicles (EVs) that are crucial for the immunomodulatory events triggered by the parasites. EVs are involved in cell-cell communication and can act as important pro-inflammatory mediators. Therefore, interface between EVs and host immune responses are crucial for the immunopathological events that those diseases exhibit. Additionally, EVs from these organisms have a role in the invertebrate hosts digestive tracts prior to parasite transmission. This review summarizes the available data on how EVs from those medically important trypanosomatids affect their interaction with vertebrate and invertebrate hosts.

UDP-glycosyltransferase genes in trypanosomatid genomes have diversified independently to meet the distinct developmental needs of parasite adaptations

BACKGROUND

Trypanosomatid parasites such as Trypanosoma spp. and Leishmania spp. are a major source of infectious disease in humans and domestic animals worldwide. Fundamental to the host-parasite interactions of these potent pathogens are their cell surfaces, which are highly decorated with glycosylated proteins and other macromolecules. Trypanosomatid genomes contain large multi-copy gene families encoding UDP-dependent glycosyltransferases (UGTs), the primary role of which is cell-surface decoration. Here we report a phylogenetic analysis of UGTs from diverse trypanosomatid genomes, the aim of which was to understand the origin and evolution of their diversity.

RESULTS

By combining phylogenetics with analyses of recombination, and selection, we compared UGT repertoire, genomic context and sequence evolution across 19 trypanosomatids. We identified a UGT lineage present in stercorarian trypanosomes and a free-living kinetoplastid Bodo saltans that likely represents the ancestral state of this gene family. The phylogeny of parasite-specific genes shows that UGTs repertoire in Leishmaniinae and salivarian trypanosomes has expanded independently and with distinct evolutionary dynamics. In the former, the ancestral UGT repertoire was organised in a tandem array from which sporadic transpositions to telomeric regions occurred, allowing expansion most likely through telomeric exchange. In the latter, the ancestral UGT repertoire was comprised of seven subtelomeric lineages, two of which have greatly expanded potentially by gene transposition between these dynamic regions of the genome.

CONCLUSIONS

The phylogeny of UGTs confirms that they represent a substantial parasite-specific innovation, which has diversified independently in the distinct trypanosomatid lineages. Nonetheless, developmental regulation has been a strong driver of UGTs diversification in both African trypanosomes and Leishmania.

RFT1 Protein Affects Glycosylphosphatidylinositol (GPI) Anchor Glycosylation

The membrane protein RFT1 is essential for normal protein N-glycosylation, but its precise function is not known. RFT1 was originally proposed to translocate the glycolipid Man₅GlcNAc₂-PP-dolichol (needed to synthesize N-glycan precursors) across the endoplasmic reticulum membrane, but subsequent studies showed that it does not play a direct role in transport. In contrast to the situation in yeast, RFT1 is not essential for growth of the parasitic protozoan Trypanosoma brucei, enabling the study of its function in a null background. We now report that lack of T. brucei RFT1 (TbRFT1) not only affects protein N-glycosylation but also glycosylphosphatidylinositol (GPI) anchor side-chain modification. Analysis by immunoblotting, metabolic labeling, and mass spectrometry demonstrated that the major GPI-anchored proteins of T. brucei procyclic forms have truncated GPI anchor side chains in TbRFT1 null parasites when compared with wild-type cells, a defect that is corrected by expressing a tagged copy of TbRFT1 in the null background. In vivo and in vitro labeling experiments using radiolabeled GPI precursors showed that GPI underglycosylation was not the result of decreased formation of the GPI precursor lipid or defective galactosylation of GPI intermediates in the endoplasmic reticulum, but rather due to modifications that are expected to occur in the Golgi apparatus. Unexpectedly, immunofluorescence microscopy localized TbRFT1 to both the endoplasmic reticulum and the Golgi, consistent with the proposal that TbRFT1 plays a direct or indirect role in GPI anchor glycosylation in the Golgi apparatus. Our results implicate RFT1 in a wider range of glycosylation processes than previously appreciated.

New Helical Binding Domain Mediates a Glycosyltransferase Activity of a Bifunctional Protein

Serine-rich repeat glycoproteins (SRRPs) conserved in streptococci and staphylococci are important for bacterial colonization and pathogenesis. Fap1, a well studied SRRP is a major surface constituent of Streptococcus parasanguinis and is required for bacterial adhesion and biofilm formation. Biogenesis of Fap1 is a multistep process that involves both glycosylation and secretion. A series of glycosyltransferases catalyze sequential glycosylation of Fap1. We have identified a unique hybrid protein dGT1 (dual glycosyltransferase 1) that contains two distinct domains. N-terminal DUF1792 is a novel GT-D-type glycosyltransferase, transferring Glc residues to Glc-GlcNAc-modified Fap1. C-terminal dGT1 (CgT) is predicted to possess a typical GT-A-type glycosyltransferase, however, the activity remains unknown. In this study, we determine that CgT is a distinct glycosyltransferase, transferring GlcNAc residues to Glc-Glc-GlcNAc-modified Fap1. A 2.4-Å x-ray crystal structure reveals that CgT has a unique binding domain consisting of three α helices in addition to a typical GT-A-type glycosyltransferase domain. The helical domain is crucial for the oligomerization of CgT. Structural and biochemical studies revealed that the helix domain is required for the protein-protein interaction and crucial for the glycosyltransferase activity of CgT in vitro and in vivo. As the helix domain presents a novel structural fold, we conclude that CgT represents a new member of GT-A-type glycosyltransferases.

A Gene of the β3-Glycosyltransferase Family Encodes N-Acetylglucosaminyltransferase II Function in Trypanosoma brucei

The bloodstream form of the human pathogen Trypanosoma brucei expresses oligomannose, paucimannose, and complex N-linked glycans, including some exceptionally large poly-N-acetyllactosamine-containing structures. Despite the presence of complex N-glycans in this organism, no homologues of the canonical N-acetylglucosaminyltransferase I or II genes can be found in the T. brucei genome. These genes encode the activities that initiate the elaboration of the Manα1-3 and Manα1-6 arms, respectively, of the conserved trimannosyl-N-acetylchitobiosyl core of N-linked glycans. Previously, we identified a highly divergent T. brucei N-acetylglucosaminyltransferase I (TbGnTI) among a set of putative T. brucei glycosyltransferase genes belonging to the β3-glycosyltransferase superfamily (Damerow, M., Rodrigues, J. A., Wu, D., Güther, M. L., Mehlert, A., and Ferguson, M. A. (2014) J. Biol. Chem. 289, 9328-9339). Here, we demonstrate that TbGT15, another member of the same β3-glycosyltransferase family, encodes an equally divergent N-acetylglucosaminyltransferase II (TbGnTII) activity. In contrast to multicellular organisms, where GnTII activity is essential, TbGnTII null mutants of T. brucei grow in culture and are still infectious to animals. Characterization of the large poly-N-acetyllactosamine containing N-glycans of the TbGnTII null mutants by methylation linkage analysis suggests that, in wild-type parasites, the Manα1-6 arm of the conserved trimannosyl core may carry predominantly linear poly-N-acetyllactosamine chains, whereas the Manα1-3 arm may carry predominantly branched poly-N-acetyllactosamine chains. These results provide further detail on the structure and biosynthesis of complex N-glycans in an important human pathogen and provide a second example of the adaptation by trypanosomes of β3-glycosyltransferase family members to catalyze β1-2 glycosidic linkages.

Aspergillus fumigatus Cap59-like protein A is involved in α1,3-mannosylation of GPI-anchors

Glycosylphosphatidylinositol (GPI) attaches a variety of eukaryotic proteins to the outer leaflet of the plasma membrane. In fungi, these proteins may also be transferred to the cell wall, to which they are covalently linked via a remnant of the GPI-anchor. They play crucial physiological roles in cell-cell interactions, adhesion or cell wall biogenesis. The biosynthesis of GPI-anchors in the endoplasmic reticulum, their transfer to proteins, early remodelling and transport to the Golgi apparatus has been fairly well described. In contrast, almost nothing is known about the genes and enzymes involved in adding glycan side chains to GPI after protein attachment. In this study, we characterized an α1,3-mannosyltransferase involved in maturation of GPI-anchors from the pathogenic fungus Aspergillus fumigatus. This enzyme shows homology to Cryptococcus neoformans Cap59p, a putative glycosyltransferase involved in capsule formation and virulence, and was thus named Cap59-like protein A (ClpA). Targeted deletion of the clpA gene in A. fumigatus led to absence of α1,3-mannose from mature GPI-anchors. The enzyme was further located to the Golgi-like apparatus of A. fumigatus and was shown to be active in the yeast Saccharomyces cerevisiae.