Common and unique features of glycosylation and glycosyltransferases in African trypanosomes

Abstract

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.

Generation of a bloodstream formTrypanosoma bruceidouble glycosyltransferase null mutant competent in receptor-mediated endocytosis of transferrin

“…Based on the glycosidic linkages catalysed by TbGT10 and TbGT8 [12,17], and previous structural studies on N -glycans from BSF T. brucei [1], we reasoned that cells simultaneously lacking TbGT10 and TbGT8 might be restricted to synthesising of tri-antennary oligomannose (Man 5-9 GlcNAc 2 ) structures, bi-antennary paucimannose structures and small bi-antennary (Galβ1-4GlcNAcβ1-2) 2 Man 3 GlcNAc 2 complex N -glycans (Fig 4A), some of which might be capped on either or both arms with an αGal residue in 1-3 linkage [25].…”

“…Specifically, there is a reduction in t-Gal, 3-Gal, 6-Gal, 3,6-Gal and 4-GlcNAc residues consistent with a reduction in the UDP-GlcNAc : βGal β1-6 and β1-3 transferase activities of TbGT10 and TbGT8, respectively. However, the residual levels of 3-Gal, 6-Gal, 3,6-Gal residues make it clear that other TbGTs, most likely encoded within the CAZy GT67 family [1], can substitute to some degree for TbGT10 and TbGT8 and that the TbGT10 -/-/TbGT8 -/double null mutant is, therefore, not devoid of pNAL containing Nlinked glycans (S6 Fig) . pNAL expression on the BSF transferrin receptor is greatly reduced in the TbGT10 -/and TbGT10 -/-/TbGT8 -/null mutants but transferrin-binding, localisation and receptor-mediated endocytosis are not affected.…”

“…The elaboration of the N -linked paucimannose Man 3 GlcNAc 2 structure into complex structures is performed by members of a large glycosyltransferase (GT) family located in the Golgi apparatus. This kinetoplastid-specific CAZy [10] GT67 family evolved from a single ancestral eukaryotic β3-GT gene to around 20 genes in T. brucei [1,11]. Despite their common origin, GT67 enzymes vary in their sugar nucleotide donor specificity (UDP-Gal or UDP-GlcNAc), their aglycone acceptors and the inter-sugar glycosidic linkages they catalyse (β1-2, β1-3, β1-4 or β1-6).…”

Identification of the glycosylphosphatidylinositol-specific phospholipase A2 (GPI-PLA2) that mediates GPI fatty acid remodeling in Trypanosoma brucei

“…One explanation could be that with an extra fatty acid now present on the GPI anchor, the procyclins and the GPI anchor core present different aspects of the membrane. This might, in turn, reduce the accessibility of GPI anchor to some of the GPI sidechain processing glycosyltransferases, some or all of which are in the Golgi apparatus, reviewed in ( 29 ), reducing overall glycosylation. Another, not mutually exclusive, explanation could be that, with the additional fatty acid present, the transit time of the procyclins through the Golgi might be reduced, leading to less processing.…”

Identification of the glycosylphosphatidylinositol-specific phospholipase A2 (GPI-PLA2) of GPI fatty acid remodelling inTrypanosoma brucei

“…One explanation could be that, with an extra fatty acid now present on the GPI anchor, the procyclins and the GPI anchor core present different aspect to the membrane. This might, in turn, reduce accessibility GPI anchor to some of the GPI sidechain processing glycosyltransferases, some or all of which are in the Golgi apparatus, reviewed in (29), reducing overall glycosylation. Another, not mutually exclusive, explanation could be that, with the additional fatty acid present, the transit time of the procyclins through the Golgi might be reduced, leading to less processing.…”