Campylobacter (food poisoning bacteria)
Protein glycosylation is one of the most important posttranslational modifications that occur in all domains of life. In particular, asparagine-linked glycosylation is the most ubiquitous protein modification (Figure 1).
N-glycosylation is a complicated process, coordinated with more than 100 different enzymes. However, sugar chains and proteins are brought together by only one enzyme, called oligosaccharyltransferase (OST). N-glycosylation is thought to occur only in eukarya in principle, but archaea have this modification. In addition, it also occurs in some eubacteria. OST is a membrane protein composed of several subunits in higher eukaryotes, but they have only single subunit in lower eukaryotes, eubacteria, and archaea.
OST works at the center of the N-glycosylation process, and the structure determination has been attempted in the world. However, most of them targeted the human and yeast proteins and thus failed. In addition, OST is a membrane protein and its entire structure is hard to be determined. Therefore, we tested other proteins from non-eukaryotic species, and selected their soluble domains as a target of structure determination. Consequently, we successfully determined the structure of the soluble globular domain of OST from archaeal Pyrococcus AglB. Successively, we determined the structure of the soluble globular domain of OST from eubacterial Campylobacter PglB.
When the two structures are compared, additional beta-sheet subdomains (called as P1 and P2) are found in OST from Pyrococcus AglB (Fig. 2). The P1 and P2 are shown in red.
This is because the amino-acid sequence of OST from Pyrococcus AglB is longer than that from Campylobacter PglB. The rest, common regions are shown in blue, and the beta-barrel folds (IS-domain) can be seen in green. On the basis of the amino-acid sequence homology alone, those common beta-barrel folds were not expected. The two OST proteins are distant homologues, and so it is not surprising that their tertiary structures of the putative active sites are similar, although their sequences are very different (Fig. 3).
From the structural comparison, a new amino-acid motif, which is predicted to be involved in the enzymatic function, has been identified. This structural motif resides in a long helix (a light brown helix in Fig. 3), with the sequence motif of DxxKxxxI (D: Asp, K: Lys, I: Ile, and x: any amino-acids) in Pyrococcus AglB, and MxxIxxxV (M: Met, and V: Val) in Campylobacter PglB. When these amino acid residues were replaced by an alanine residue, the catalytic activities decreased as expected, and those motifs were proven to contribute to the OST activity.
Protein Data Bank (PDB)
Author: Daisuke Kohda Translator: Daisuke Kohda and Haruki Nakamura