Only in Halobacterium salinarum: Sugar modifications unique to an archaeal N-linked glycan

Halobacterium salinarum are halophilic archaea that grow at or near saturating salt conditions. In addition to providing insight into how life copes with extreme conditions, studying Hbt. salinarum has been central to advances made in numerous fields, including bioenergetics, membrane protein structure determination and optogenetics. From the perspective of carbohydrate research, Hbt. salinarum provided the first example of N-glycosylation outside Eukarya. Yet, even 50 years after the first report of such post-translational modification in these haloarchaea, various aspects of Hbt. salinarum N-glycosylation seemingly unique to this organism remain largely unaddressed. These include questions related to the incorporation of iduronic acid in an N-linked glycan decorating Hbt. salinarum glycoproteins and the sulfation of this sugar at the O-3 position, as well as the transient methylation of this glycan at the lipid carrier- but not the protein-bound stage. In this review, recent progress on each of these unusual and unique aspects of Hbt. salinarum N-glycosylation is discussed.