Abstract:
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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. |