Addition of α-O-GlcNAc to threonine residues define the post-translational modification of mucin-like molecules in Trypanosoma cruzi

Trypanosoma cruzi, an intracellular protozoan etiologic agent of Chagas disease is covered by a dense coat of mucin-type glycoproteins, which is important to promote the parasite entry and persistence in the mammalian host cells. The O-glycosylation of T. cruzi mucins (Tc-mucins) is initiated by enzymatic addition of alpha-O-N-acetylglucosamine (GlcNAc) to threonine (Thr) by the UDP-GlcNAc:polypeptide alpha-N-acetylglucosaminyltransferase (pp-alpha-GlcNAcT) in the Golgi. The Tc-mucin is characterized by the presence of a high structural diversity of O-linked oligosaccharides found among different parasite strains, comprising two O-glycan Cores. In the Core 1, from strains principally associated with the domestic transmission cycle of Chagas disease, the GlcNAc O-4 is substituted with a beta-galactopyranose (beta Galp) unit, and in the most complex oligosaccharides the GlcNAc O-6 is further processed by the addition of beta 1 -> aEuro parts per thousand 2-linked Galp residues creating a short linear Galp-containing chain. In the Core 2 structures, expressed by strains isolated from T. cruzi sylvatic hosts, the GlcNAc O-4 carries a beta-galactofuranose (beta Galf) unit and the GlcNAc O-6 can carry a branched Galp beta 1 -> aEuro parts per thousand 3[Galp beta 1 -> aEuro parts per thousand 2]Galp beta 1 -> aEuro parts per thousand 6 motif. The O-glycans carrying nonreducing terminal beta Galp are available for sialylation by a surface T. cruzi trans-sialidase activity. Based on structural results, this review summarizes available data on the highly conserved process, which adds the GlcNAc unit in alpha-linkage to Thr residues the basis of the post-translational modification system in T. cruzi mucins. In addition, a mechanism unique employed by the parasite to transfer exogenous sialic acid residues to Tc-mucins is presented.