1. Protein glycosylation.
Protein glycosylation. (A) En bloc N-protein glycosylation in the model organism C. jejuni. The type example of en bloc protein glycosylation is the general N-protein glycosylation system of C. jejuni. Prior to the start of the glycosylation process, UDP-N-acetylglucosamine (GlcNAc) is modified to N-diacetyl bacillosamine (diNAcBac) by the action of PglF, PglE, and PglD. The priming glycosyltransferase (GT) PglC transfers this sugar moiety to an undecaprenyl phosphate (UndP) carrier, followed by the successive actions of the PglA, PglJ, and PglH GTs adding N-acetylgalactosamine (GalNAc) and glucose (Glc) residues. Once finished, the glycan moiety is transferred across the inner membrane (IM) by the PglK flippase. Finally, the PglB oligosaccharyltransferase (OST) then N-glycosylates the protein (see references 77, 78, and 293 and the references mentioned in the text). OM, outer membrane. (B) The peculiar sequential N-protein glycosylation system of H. influenzae. In H. influenzae, the glycan moieties are directly attached to serines and threonines in the protein by HMW1C. Transportation across the inner membrane relies on the Sec pathway, followed by further export to the cell surface by HMW1B (73, 296, 335). (C) Sequential O-protein glycosylation of pili, flagella, and other proteins (illustrated for Fap1 of S. parasanguinis). Glycosylation of the Fap1 fimbrial protein of S. parasanguinis remains to be fully elucidated. It is currently known that the first step of the glycosylation process is catalyzed by a heterodimer of Gtf1 and Gtf2, while Gtf3 regulates the latter glycosylation steps. The glycoprotein secretion mechanism is especially of note here, as it involves a SecA2-SecY2 heterodimer. SecY2 forms a translocation channel, with SecA2 being an accessory protein. Gap1 and Gap3 are crucial accessory proteins for fimbrial biogenesis (399, 400, 402, 403, 406, 407, 409, 411, 412). (D) The general en bloc protein glycosylation system of Neisseria spp. In Neisseria spp., the PglD, PglC, and PglB or PglB2 enzymes modify a nucleotide-activated GlcNAc residue. The action of PglB results in the formation of 2,4-diacetamido-2,4,6-trideoxyhexose (DATDH), and the action of PglB2 results in the formation of 2-acetamido-4-glyceramido-2,4,6-trideoxyhexose (GATDH). These enzymes are also the priming GTs, adding these sugar residues to a UndP carrier. The successive action of PglA and PglE results in the addition of two Gal residues. The glycan is then transferred to the periplasm by a PglF flippase and ligated to the protein by the PglL/PglO OST (66, 67, 476, 480, 496). Gal, galactose.
Hanne L. P. Tytgat, and Sarah Lebeer Microbiol. Mol. Biol. Rev. 2014; doi: /MMBR