Pathway of protein glycosylation in the trypanosomatid Crithidia fasciculata

Cells of the insect parasite Crithidia fasciculata incubated with [14C]glucose were found to possess only one lipid-bound oligosaccharide with solubility in chloroform/methanol/water mixtures and net charge similar to the charges of dolichol pyrophosphate derivatives. The saccharide moiety could be released from lipid by mild acid hydrolysis. Several enzymatic and chemical treatments of the oligosaccharide indicated that the latter had the structure Man alpha leads to Man alpha leads to Man alpha leads to [Man alpha leads to Man alpha leads to Man (alpha 1 leads to 6)]Man leads to GlcNAc(beta 1 leads to 4)GlcNAc. Two labeled oligosaccharides were liberated from proteins by a sequential treatment with a protease and endo-beta-N-acetylglucosamindase H. One of the protein-bound oligosaccharides had the same structure as the lipid-linked compound, whereas in the second oligosaccharide some mannose residues had been replaced by galactose units, but both compounds migrated as did a Man7GlcNAc standard. These were the largest oligosaccharides obtained even after short labeling periods. It is suggested that glycosylation of proteins in the protozoan Crithidia fasciculata does not involved glucosylated lipid-bound oligosaccharides as intermediates.