A gene of the beta3-glycosyltransferase family encodes N-acetylglucosaminyltransferase II function in Trypanosoma brucei

Abstract

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 Manalpha1-3 and Manalpha1-6 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 beta3-glycosyltransferase superfamily (1). Here, we demonstrate that TbGT15, another member of the same beta3-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 Manalpha1-6 arm of the conserved trimannosyl core may carry predominantly linear poly-N-acetyllactosamine chains whereas the Manalpha1-3 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 beta3-glycosyltransferase family members to catalyze beta1-2 glycosidic linkages.