Sulphation of a repetitive saccharide in halobacterial cell wall glycoprotein

An N-linked tetrasaccharide from Halobacterium salinarum presents a novel modification, sulfation of iduronic acid at the O-3 position

Halobacterium salinarum, a halophilic archaeon that grows at near-saturating salt concentrations, provided the first example of N-glycosylation outside Eukarya. Yet, almost 50 years later, numerous aspects of such post-translational protein processing in this microorganism remain to be determined, including the architecture of glycoprotein-bound glycans. In the present report, nuclear magnetic resonance spectroscopy was used to define a tetrasaccharide N-linked to both archaellins, building blocks of the archaeal swimming device (the archaellum), and the S-layer glycoprotein that comprises the protein shell surrounding the Hbt. salinarum cell as β-GlcA(2S)-(1 → 4)-α-IdoA(3S)-(1 → 4)-β-GlcA-(1 → 4)-β-Glc-Asn. The structure of this tetrasaccharide fills gaps remaining from previous studies, including confirmation of the first known inclusion of iduronic acid in an archaeal N-linked glycan. At the same time, the sulfation of this iduronic acid at the O-3 position has not, to the best of our knowledge, been previously seen. As such, this may represent yet another unique facet of N-glycosylation in Archaea.

Utilization of virus φCh1 elements to establish a shuttle vector system for Halo(alkali)philic Archaea via transformation of Natrialba magadii

In the study described here, we successfully developed a transformation system for halo(alkali)philic members of the Archaea. This transformation system comprises a series of Natrialba magadii/Escherichia coli shuttle vectors based on a modified method to transform halophilic members of the Archaea and genomic elements of the N. magadii virus Ch1. The shuttle vector pRo-5, based on the repH-containing region of Ch1, stably replicated in E. coli and N. magadii and in several halophilic and haloalkaliphilic members of the Archaea not transformable so far. The Ch1 operon ORF53/ORF54 (repH) was essential for pRo-5 replication and was thus identified as the minimal replication origin. The plasmid allowed homologous and heterologous gene expression, as exemplified by the expression of Ch1 ORF3452, which encodes a structural protein, and the reporter gene bgaH of Haloferax lucentense in N. magadii. The new transformation/vector system will facilitate genetic studies within N. magadii and other haloalkaliphilic archaea and will allow the detailed characterization of the gene functions of N. magadii virus Ch1 in their extreme environments.

Defining the topology of the N-glycosylation pathway in the halophilic archaeon Haloferax volcanii

In Eukarya, N glycosylation involves the actions of enzymes working on both faces of the endoplasmic reticulum membrane. The steps of bacterial N glycosylation, in contrast, transpire essentially on the cytoplasmic side of the plasma membrane, with only transfer of the assembled glycan to the target protein occurring on the external surface of the cell. For Archaea, virtually nothing is known about the topology of enzymes involved in assembling those glycans that are subsequently N linked to target proteins on the external surface of the cell. To remedy this situation, subcellular localization and topology predictive algorithms, protease accessibility, and immunoblotting, together with cysteine modification following site-directed mutagenesis, were enlisted to define the topology of Haloferax volcanii proteins experimentally proven to participate in the N-glycosylation process. AglJ and AglD, involved in the earliest and latest stages, respectively, of assembly of the pentasaccharide decorating the H. volcanii S-layer glycoprotein, were shown to present their soluble N-terminal domain, likely containing the putative catalytic site of each enzyme, to the cytosol. The same holds true for Alg5-B, Dpm1-A, and Mpg1-D, proteins putatively involved in this posttranslational event. The results thus point to the assembly of the pentasaccharide linked to certain Asn residues of the H. volcanii S-layer glycoprotein as occurring within the cell.

S-layer nanoglycobiology of bacteria

Cell surface layers (S-layers) are common structures of the bacterial cell envelope with a lattice-like appearance that are formed by a self-assembly process. Frequently, the constituting S-layer proteins are modified with covalently linked glycan chains facing the extracellular environment. S-layer glycoproteins from organisms of the Bacillaceae family possess long, O-glycosidically linked glycans that are composed of a great variety of sugar constituents. The observed variations already exceed the display found in eukaryotic glycoproteins. Recent investigations of the S-layer protein glycosylation process at the molecular level, which has lagged behind the structural studies due to the lack of suitable molecular tools, indicated that the S-layer glycoprotein glycan biosynthesis pathway utilizes different modules of the well-known biosynthesis routes of lipopolysaccharide O-antigens. The genetic information for S-layer glycan biosynthesis is usually present in S-layer glycosylation (slg) gene clusters acting in concert with housekeeping genes. To account for the nanometer-scale cell surface display feature of bacterial S-layer glycosylation, we have coined the neologism 'nanoglycobiology'. It includes structural and biochemical aspects of S-layer glycans as well as molecular data on the machinery underlying the glycosylation event. A key aspect for the full potency of S-layer nanoglycobiology is the unique self-assembly feature of the S-layer protein matrix. Being aware that in many cases the glycan structures associated with a protein are the key to protein function, S-layer protein glycosylation will add a new and valuable component to an 'S-layer based molecular construction kit'. In our long-term research strategy, S-layer nanoglycobiology shall converge with other functional glycosylation systems to produce 'functional' S-layer neoglycoproteins for diverse applications in the fields of nanobiotechnology and vaccine technology. Recent advances in the field of S-layer nanoglycobiology have made our overall strategy a tangible aim of the near future.

Facing extremes: archaeal surface-layer (glyco)proteins

Archaea are best known in their capacities as extremophiles, i.e. micro-organisms able to thrive in some of the most drastic environments on Earth. The protein-based surface layer that envelopes many archaeal strains must thus correctly assemble and maintain its structural integrity in the face of the physical challenges associated with, for instance, life in high salinity, at elevated temperatures or in acidic surroundings. Study of archaeal surface-layer (glyco)proteins has thus offered insight into the strategies employed by these proteins to survive direct contact with extreme environments, yet has also served to elucidate other aspects of archaeal protein biosynthesis, including glycosylation, lipid modification and protein export. In this mini-review, recent advances in the study of archaeal surface-layer (glyco)proteins are discussed.

Protein glycosylation in Haloferax volcanii: partial characterization of a 98-kDa glycoprotein

The plasma membrane of Haloferax volcanii contains several glycoproteins, including a 98-kDa species. Using lectin-based chromatography, the glycoprotein was isolated and partially characterized. Sequence comparison, based on antibody binding as well as one-dimensional peptide maps show that the 98-kDa glycoprotein is distinct from the S-layer glycoprotein, the major glycoprotein in H. volcanii. The 98-kDa glycoprotein can be further distinguished from the S-layer glycoprotein on the basis of membrane attachment. Unlike the S-layer glycoprotein, the 98-kDa glycoprotein is not associated with the membrane in a Mg2+-dependent manner. Both proteins, however, apparently rely on a similar mechanism of glycosylation, since neither was affected by treatment with bacitracin or tunicamycin, agents known to interfere with protein glycosylation in other species. Finally, the pattern of glycosylation of the 98-kDa glycoprotein is not shared by a 95-kDa glycoprotein of the related Haloferax mediterranei strain.

Exchange of Ser-4 for Val, Leu or Asn in the sequon Asn-Ala-Ser does not prevent N-glycosylation of the cell surface glycoprotein from Halobacterium halobium

The archaeon Halobacterium halobium expresses a cell surface glycoprotein (CSG) with a repeating pentasaccharide unit N-glycosidically linked via N-acetylgalactosamine to Asn-2 of the polypeptide (GalNAc(1-N)Asn linkage type). This aspar-agine of the linkage unit is located within the N-terminal sequence Ala-Asn-Ala-Ser-, in accordance with the tripeptide consensus sequence Asn-Xaa-Ser/Thr typical for nearly every N-glycosylation site known so far, which are of the GlcNAc(1-N)-Asn linkage type. By a gene replacement method csg mutants were created which replace the serine residue of the consensus sequence by valine, leucine, and asparagine. Unexpectedly, this elimination of the consensus sequence did not prevent N-glycosylation. All respective mutant cell surface glycoproteins were N-glycosylated at Asn-2 with the same N-glycan chain as the wild type CSG. Asn-479 is N-glyco-sylated via a Glc(1-N)Asn linkage type in the wild type CSG. Replacement of Ser-481 in the sequence Asn-Ser-Ser for valine prevented glycosylation of Asn-479. From these results we postulate the existence of two different N-glycosyltransferases in H.halobium, one of which does not use the typical consensus sequence Asn-Xaa-Ser/Thr necessary for all other N-glycosyltransferases described so far.

Structure and biosynthesis of prokaryotic glycoproteins

Glycoproteins as components of cell surfaces are not restricted to eukaryotes. The prokaryotic glycoprotein studied in greatest detail so far is the cell surface glycoprotein of the archaebacterium Halobacterium halobium. This bacterial glycoprotein contains 3 different types of glycoconjugates, and each type of glycoconjugate involves a different carbohydrate-protein linkage unit: 1) One glycosaminoglycan chain, constructed from a repeating sulfated pentasaccharide block, is linked to one protein molecule via the novel N-glycosyl linkage unit asparaginyl-N-acetylgalactosamine. 2) Ten sulfated oligosaccharides that contain glucose, glucuronic acid and iduronic acid are bound to the protein via the hitherto unknown N-glycosyl linkage unit asparaginylglucose. 3) About 15 disaccharides, glucosylgalactose, are O-glycosyl-linked to a cluster of threonine residues close to the C-terminus of the core protein. The overall structure of the cell surface glycoprotein of halobacteria is thus reminiscent of animal proteoglycans and a functional role of the glycosaminoglycan chain in maintaining the rod shape of halobacteria is discussed. Biosynthesis of the two N-glycosyl linkage units involves dolichol monophosphate and dolicholdiphosphate-linked saccharide precursors. Sulfation and epimerization of the glycoconjugates occur at the lipid-linked level and the mature saccharides are transferred to the protein core on the cell surface. The sulfated oligosaccharides that finally become bound to asparagine via glucose are transiently methylated at their lipid-linked stage and this transient chemical modification seems to be required for the biosynthesis of the corresponding N-glycosyl bond.

The cell wall structure of a magnesium-dependent halobacterium, Halobacterium volcanii CD-2, from the Dead Sea

Cell wall preparations from the magnesium-dependent halophilic bacterium, Halobacterium volcanii, were studied by high-resolution electron microscopy complemented with image analysis and processing. For ultrastructural studies, specimens were prepared by a variety of methods, including negative staining, and metal shadowing after air-drying, freeze-drying, or freeze-fracturing and etching. All methods revealed the cell wall to be composed of a near-hexagonal lattice of unit cells having a center-to-center spacing of 15.5 nm. While negatively stained samples yielded two types variably revealed the unit cell to be composed of six protomers surrounding a central mass depression. This low-resolution unit cell morphology appears very similar to that of other bacterial cell wall S-layers studied to date.