Lipid sugar carriers at the extremes: The phosphodolichols Archaea use in N-glycosylation

Abstract

N-glycosylation, a post-translational modification whereby glycans are covalently linked to select Asn residues of target proteins, occurs in all three domains of life. Across evolution, the N-linked glycans are initially assembled on phosphorylated cytoplasmically-oriented polyisoprenoids, with polyprenol (mainly C55 undecaprenol) fulfilling this role in Bacteria and dolichol assuming this function in Eukarya and Archaea. The eukaryal and archaeal versions of dolichol can, however, be distinguished on the basis of their length, degree of saturation and by other traits. As is true for many facets of their biology, Archaea, best known in their capacity as extremophiles, present unique approaches for synthesizing phosphodolichols. At the same time, general insight into the assembly and processing of glycan-bearing phosphodolichols has come from studies of the archaeal enzymes responsible. In this review, these and other aspects of archaeal phosphodolichol biology are addressed.

Discovery of novel geranylgeranyl reductases and characterization of their substrate promiscuity

“…This observation additionally applies to their monophosphate counterparts in this work, FP and GGP. All enzymes tested to date seem to conserve this characteristic of avoiding reduction at the α-position on phosphate intermediates, aligning with current paradigms that auxiliary prenyl reductases are responsible for reducing this group in archaea and eukaryotes [40].…”

Stereochemical Divergence of Polyprenol Phosphate Glycosyltransferases

“…Such enzymes are important for the synthesis of substrates for N-glycosylation, a post-translational modification in which select asparagine residues in target proteins are modified by glycans originally assembled on a phosphorylated polyprenol carrier [8–10]. The membrane-resident carriers are the uniformly unsaturated polyprenol phosphates in Bacteria and the dolichol monophosphates and/or dolichol diphosphates in Eukarya and Archaea [5,11–13] (Figure 1).…”

“…In Archaea, N-glycosylation is an almost universal post-translational protein modification that culminates in an extremely diverse set of protein-bound glycans [25–27]. Although current understanding of archaeal N-glycosylation is limited, it is known that both dolichol monophosphate and dolichol diphosphate serve as the lipid carriers upon which glycans for N-linked protein glycosylation are assembled [13]. Indeed, in Archaea, sugar-charged dolichol monophosphates serve as both sugar donors and as carriers for further sugar addition during N-linked glycan assembly [28,29].…”

“…While it is conceivable that the dolichol diphosphate-linked tetradecasaccharide transferred to nascent proteins could be fully assembled using available NDP-sugars in the cytoplasm, and not just the heptasaccharide core, it may be difficult to translocate such an elaborate glycan across the ER membrane. Indeed, across Eukarya, the longest dolichol diphosphate-linked polysaccharide translocated across the ER membrane contains seven sugars [13, 48]; the same is true in the bacterial N-glycosylation system exemplified by C. jejuni [9, 46, 49]. At the same time, it is noteworthy that the protist Trichomonas vaginalis , a lower eukaryote, encodes Alg5 and Dpm1 homologues, yet N-linked glycans in this organism are not elaborated with Glc and/or Man derived from the corresponding dolichol monophosphate carriers [50].…”

“…In most Eukarya (top panel), n = 2 and m = 12–17 (C 75 –C 100 ), with the exception of plants, where n = 3 and m = 6–37 [11,12]. Archaea (middle panel) contain dolichols of varying lengths (C 30 –C 70 ; n = 2–4) and degrees of unsaturation (indicated by the dashed bond) beyond a saturated α- and ω-isoprene unit [13]. Most Bacteria (lower panel) use fully unsaturated polyprenols (n = 2 and m = 7–9).…”