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Chapter 25 : N-Linked Protein Glycosylation in Campylobacter
Category: Bacterial Pathogenesis
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N-linked protein glycosylation is the most common type of protein modification in eukaryotes and is the topic of this chapter. The chapter demonstrates that the Campylobacter jejuni glycome is an excellent toolbox for glycobiologists to understand the fundamentals of this pathway, to develop new techniques for glycobiology, and to exploit this pathway for novel diagnostics and therapeutics. A section summarizes the N-linked proteins identified so far and provides further information on the roles for the posttranslational modification in Campylobacter which involves in cellular function. The importance of CjaA for the in vivo survival of Campylobacter has recently been shown in chicken colonization studies: birds immunized with an avirulent strain of Salmonella expressing plasmid-borne cjaA showed reduced C. jejuni colonization. In addition, gene clusters corresponding to the N-linked protein glycosylation pathway were shown to be present in various isolates of C. jejuni, C. lari RM2100, C. upsaliensis RM3195, C. jejuni subsp. doylei 269.97, C. coli RM2228, C. hominis ATCC BAA-381, C. curvus 525.92, C. concisus 13826, and C. fetus subsp. fetus 82-40, demonstrating that this pathway and potentially the bacillosamine-containing heptasaccharide are conserved among all Campylobacter species. C. jejuni provides researchers with an excellent model system because this organism has both well-characterized O-linked and N-linked protein glycosylation systems.
Schematic pathway of N-linked protein glycosylation at the plasma membrane of Campylobacter jejuni. The assembly of a heptasaccharide takes place on the lipid bactoprenylpyrophosphate at the cytoplasmic side of the membrane by the glycosyltransferases PglC, PglA, PglJ, PglH, and PglI. UDP-bacillosamine is synthesized from UDP-GlcNAc by PglF, PglE, and PglD. The ABC-transporter PglK mediates the translocation of the lipid-linked heptasaccharide across the membrane. The oligosaccharyltransferase catalyzes the transfer of the heptasaccharide from the lipid carrier to selected asparagine residues on nascent polypeptide chains. This pathway is encoded by the pgl operon shown below, with the pglB gene encoding the oligosaccharyltransferase highlighted.
Summary of biological effects caused by disruption of protein N-glycosylation in C. jejuni.
Genetic organization of pgl gene orthologs in Proteobacteria. The commonality of the conserved pgl gene clusters in Delta- and Epsilonproteobacteria is shown. Orthologs of Cj1119c–1132c from C. jejuni NCTC 11168 (NC_002163) were identified by the blastp or tblastx algorithm (http://www.ncbi.nlm.nih.gov/). The arrows indicate the transcriptional orientations of the genes; gaps between pgl genes are indicated by either the number of open reading frames (orfs) or slashed lines indicating orthologs that were found elsewhere in the chromosome. The pgl genes are conserved in all Campylobacter species examined, including C. jejuni subsp. doylei 269.97 (NC-009707), C. coli RM2228 (NZ_AAFL00000000), C. lari RM2100 (NZ_AAFK00000000), C. upsaliensis RM3195 (NZ_AAFJ00000000), C. hominis ATCC BAA-381 (NC_009714), C. curvus 525.92 (NC_009715), C. concisus 13826 (NZ_AAQZ00000000), and C. fetus subsp. fetus 82-40 (NC_008599). Pgl gene orthologs were also found in the related Epsilonproteobacteria: Wolinella succinogenes DSZM 1740 (NC_005090), Sulfurovum sp. NBC37-1 (NC_009663), Nitratiruptor sp. SB55-2 (NC_009662), and in the Deltaproteobacterium Desulfovibrio desulfuricans G20 (NC_007519). Genes encoding the essential oligosaccharyltransferse PglB are depicted in black. Biosynthetic Pgl enzymes (Gne, PglE (E), PglF (F), PglD (D)), glycosyltransferases (PglA (A), PglJ (J), PglH (H), PglI (I), PglC (C)), and the flanking gene products PglG (G) and WlaA were designated according to their orthologs in C. jejuni NCTC 11168 or as glycosyltransferase if no homology to any C. jejuni NCTC 11168 Pgl glycosyltransferase was found. PglC orthologs in Sulfurovum sp. NBC37-1 and Nitratiruptor sp. SB55-2 are indicated with a question mark because other proteins with a higher percentage identity (ID) to PglC of C. jejuni NCTC 11168 can be found in the genome of both species. Note that the PglF ortholog protein in C. jejuni subsp. doylei 269.97 is annotated as a pseudogene that might be due to a sequencing error in the unfinished genome sequencing project. Orthologs to putative ABC-type transporters that show low homologies to PglK (K) of C. jejuni NCTC 11168 are indicated by a question mark.
Putative Campylobacter N-linked glycoproteins
Transcriptional profiling of pgl mutants
C. jejuni NTCT 11168 pgl gene orthologs