Sweet to the extreme: Protein glycosylation in Archaea

Post-translational modifications account for much of the biological diversity generated at the proteome level. Of these, glycosylation is the most prevalent. Long thought to be unique to Eukarya, it is now clear that both Bacteria and Archaea are also capable of N-glycosylation, namely the covalent linkage of oligosaccharides to select target asparagine residues. However, while the eukaryal and bacterial N-glycosylation pathways are relatively well defined, little is known of the parallel process in Archaea. Of late, however, major advances have been made in describing the process of archaeal N-glycosylation. Such efforts have shown, as is often the case in archaeal biology, that protein N-glycosylation in Archaea combines particular aspects of the eukaryal and bacterial pathways along with traits unique to this life form. For instance, while the oligosaccharides of archaeal glycoproteins include nucleotide-activated sugars formed by bacterial pathways, the lipid carrier on which such oligosaccharides are assembled is the same as used in eukaryal N-glycosylation. By contrast, transfer of assembled oligosaccharides to their protein targets shows Archaea-specific properties. Finally, addressing N-glycosylation from an archaeal perspective is providing new general insight into this event, as exemplified by the solution of the first crystal structure of an oligosaccharide transferase from an archaeal source.