Revisiting N-glycosylation in Halobacterium salinarum: Characterizing a dolichol phosphate- and glycoprotein-bound tetrasaccharide

Although Halobacterium salinarum provided the first example of N-glycosylation outside the Eukarya, much regarding such post-translational modification in this halophilic archaea remains either unclear or unknown. The composition of an N-linked glycan decorating both the S-layer glycoprotein and archaellins offers one such example. Originally described some 40 years ago, reports from that time on have presented conflicted findings regarding the composition of this glycan, as well as differences between the protein-bound glycan and that version of the glycan attached to the lipid upon which it is assembled. To clarify these points, liquid chromatography-electrospray ionization mass spectrometry was employed here to revisit the composition of this glycan both when attached to selected asparagine residues of target proteins and when bound to the lipid dolichol phosphate upon which the glycan is assembled. Such efforts revealed the N-linked glycan as corresponding to a tetrasaccharide comprising a hexose, a sulfated hexuronic acid, a hexuronic acid and a second sulfated hexuronic acid. When attached to dolichol phosphate but not to proteins, the same tetrasaccharide is methylated on the final sugar. Moreover, in the absence of the oligosaccharyltransferase AglB, there is an accumulation of the dolichol phosphate-linked methylated and disulfated tetrasaccharide. Knowing the composition of this glycan at both the lipid- and protein-bound stages, together with the availability of gene deletion approaches for manipulating Hbt. salinarum, will allow delineation of the N-glycosylation pathway in this organism.