N‐linked glycosylation in Archaea: two paths to the same glycan

Summary N‐linked protein glycosylation occurs in all three branches of life, eukaryotes, bacteria and archaea. The simplest system is that of the bacterium, Campylobacter jejuni , in which a heptasaccharide glycan is added to multiple proteins from a single lipid carrier molecule. In the eukaryotic system a conserved tetradecasaccharide modification is first added to target proteins, but is then modified by trimming and addition of other glycans from additional carrier molecules resulting in a diverse array of glycans of distinct functionality. In the halophilic Archaea from the Dead Sea, Haloferax volcanii , the surface array or S‐layer protein is glycosylated with a pentasaccharide. This glycan is synthesized from two separate carrier molecules, one that carries a tetrasaccharide and another that carries the terminal mannose, in a process that is analogous to that of eukaryotes. In this issue of Molecular Microbiology the glycosylation of the S‐layer of another halophilic Archaea from the Dead Sea, Haloarcula marismortui is characterized (Calo et al. , 2011). This S‐layer is glycosylated with the same pentasaccharide as that of Hfx.  volcanii , but the intact pentasaccharide is synthesized on a single carrier molecule in Har. marismortui in a process that more closely resembles that of the bacterial N‐linked system.