Protein glycosylation in Trypanosoma cruzi. The mechanism of glycosylation and structure of protein-bound oligosaccharides

As reported previously, label was incorporated first to the glucose residues of protein-bound Glc₁Man₉GlcNAc₂, Glc₁Man₈GlcNAc₂, and Glc₁Man₇GlcNAc₂ when T. cruzi cells, the causative agent of Chagas disease, were incubated with [U-¹⁴C] glucose. It is now reported that the glucose residues are removed from the oligosaccharides after a chase period. The relative proportion of Man₉GlcNAc₂, Man₈GlcNAc₂, Man₇GlcNAc₂, and Man₆GlcNAc₂ appeared to be the same after 120 and 180 min of chase, thus indicating that these compounds were the fully processed protein-bound oligosaccharides. No complex type protein-bound oligosaccharides were detected. Evidence is presented indicating that Glc₁Man₇GlcNAc₂ was formed mainly by glucosylation of Man₇GlcNAc₂ and not by demannosylation of Glc₁Man₉GlcNAc₂. Man₉GlcNAc₂ was the first oligosaccharide to be labeled when cells were incubated with [2-³H] mannose. Based on these and previous results, an overall mechanism of protein N-glycosylation is proposed. The structure of the oligosaccharides appeared to be similar to some of those present in human glycoproteins. T. cruzi cells isolated from distant locations in South America were found to share a common mechanism of protein glycosylation.