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.

Keywords: evolutionary biology; glycosylation; glycosyltransferases; trypanosomes.

Figure 1.

Summary of the glycan and GT repertoires of T. brucei. The tsetse midgut-dwelling procyclic form (PCF) cells express GPI-anchored and N-glycosylated procyclin glycoproteins with simple Man₅GlcNAc₂ oligomannose N-glycans and GPI anchors with extensively modified GPI-anchor glycans. The mammalian host-dwelling bloodstream form (BSF) cells express variant surface glycoproteins (VSGs) that can contain oligomannose (triantennary Man₉GlcNAc₂ to Man₅GlcNAc₂), paucimannose (biantennary Man₅GlcNAc₂ to Man₃GlcNAc₂) and small complex N-glycans. Some VSGs are also O-glycosylated, as indicated. In addition, other flagellar pocket and endosomal/lysosomal glycoproteins, such as p67, bear giant poly-LacNAc-containing N-glycans in the BSF lifecycle stage. In contrast, the BSF GPI-anchor sidechains are smaller than those of PCF cells, containing up to 6 Gal residues.

Figure 2.

GT repertoire of T. brucei. Phylogenetic analysis of T. b. brucei, T. b. gambiense, T. evansi and T. vivax GT genes. The lineages within the GT67 family are according to [66] and the T. brucei GT sub-families within those lineages (e.g. TbGT1 to TbGT15) are according to [15,18]. Those TbGTs that appear in proteomics data are shown in Figure 3.

Figure 3.

Protein abundance data for enzymes and proteins involved in protein glycosylation in T. brucei. The subset of gene products detected by proteomics associated with oligosaccharide transfer (purple), ER quality control (green) together with the T. brucei GT67 glycosyltransferases (orange), GT11 and GT25 mitochondrial glycosyltransferases (yellow) and GT60 glycosyltransferase (cyan) are shown. Gene IDs, gene names and encoded activities (and abbreviated names) are shown in the table. The latter are plotted according to quantitative proteomics values (iBAQ values) in the BSF (x-axis) and PCF (y-axis) total cell proteomes [68] using the tool described in [69].