Functional characterization of bacterial oligosaccharyltransferases involved in O-linked protein glycosylation

Protein glycosylation is an important posttranslational modification that occurs in all domains of life. Pilins, the structural components of type IV pili, are O glycosylated in Neisseria meningitidis, Neisseria gonorrhoeae, and some strains of Pseudomonas aeruginosa. In this work, we characterized the P. aeruginosa 1244 and N. meningitidis MC58 O glycosylation systems in Escherichia coli. In both cases, sugars are transferred en bloc by an oligosaccharyltransferase (OTase) named PgIL in N. meningitidis and PiIO in P. aeruginosa. We show that, like PiIO, PgIL has relaxed glycan specificity. Both OTases are sufficient for glycosylation, but they require translocation of the undecaprenol-pyrophosphate-linked oligosaccharide substrates into the periplasm for activity. Whereas PiIO activity is restricted to short oligosaccharides, PgIL is able to transfer diverse oligo- and polysaccharides. This functional characterization supports the concept that despite their low sequence similarity, PiIO and PgIL belong to a new family of "O-OTases" that transfer oligosaccharides from lipid carriers to hydroxy lated amino acids in proteins. To date, such activity has not been identified for eukaryotes. To our knowledge, this is the first report describing recombinant O glycoproteins synthesized in E. coli.