Structural biology of oligosaccharyltransferase (ost)

Asparagine-linked glycosylation of proteins is widespread not only in eukaryotes but also in archaea and some eubacteria. The oligosaccharyl transfer reaction is catalyzed by a membrane-bound enzyme, oligosaccharyltransferase (OST). The donor substrate is the lipid-linked oligosaccharide (LLO), and the acceptor is the asparagine residues in the N-glycosylation sequon (Asn-X-Ser/Thr, X 6¼ Pro). The catalytic subunit of OST consists of an N-terminal transmembrane region consisting of 13 TM helices and a C-terminal globular domain. The crystal structures of the eubacterial and archaeal catalytic subunits revealed the structural basis of the sequon recognition and activation. Two conserved motifs, termed DXDs, form a metal ion containing catalytic center that activates the side-chain carboxamide group of the acceptor Asn residue in the sequon. Other conserved motifs, WWDYG and DK/MI motifs, constitute a binding site for the Ser and Thr residues in the sequon. In addition to the crystallography, NMR and biochemical studies suggested that the flexibility of one long loop in the TM region and the Ser/Thr pocket in the C-terminal globular domain were both important for the enzymatic activity. It is likely that their dynamic nature facilitates the efficient scanning of a nascent polypeptide chain for the N-glycosylation sequons when coupled with ribosomal protein synthesis.