Structural characterization of the N-linked pentasaccharide decorating glycoproteins of the halophilic archaeon Haloferax volcanii.

N-Glycosylation is a post-translational modification performed in all three domains of life. In the halophilic archaea Haloferax volcanii, glycoproteins such as the S-layer glycoprotein are modified by an N-linked pentasaccharide assembled by a series of Agl (archaeal glycosylation) proteins. In the present study, mass spectrometry (MS) and nuclear magnetic resonance spectroscopy were used to define the structure of this glycan attached to at least four of the seven putative S-layer glycoprotein N-glycosylation sites, namely Asn-13, Asn-83, Asn-274 and Asn-279. Such approaches detected a trisaccharide corresponding to glucuronic acid (GlcA)-ß1,4-GlcA-ß1,4-glucose-ß1-Asn, a tetrasaccharide corresponding to methyl-O-4-GlcA-ß-1,4-galacturonic acid-α1,4-GlcA-ß1,4-glucose-ß1-Asn, and a pentasaccharide corresponding to hexose-1,2-[methyl-O-4-]GlcA-ß-1,4-galacturonic acid-α1,4-GlcA-ß1,4-glucose-ß1-Asn, with previous MS and radiolabeling experiments showing the hexose at the non-reducing end of the pentasaccharide to be mannose. The present analysis thus corrects the earlier assignment of the penultimate sugar as a methyl ester of a hexuronic acid, instead revealing this sugar to be a methylated GlcA. The assignments made here are in good agreement with what was already known of the Hfx. volcanii N-glycosylation pathway from previous genetic and biochemical efforts while providing new insight into the process.

Lipid modification gives rise to two distinct Haloferax volcanii S-layer glycoprotein populations.

The S-layer glycoprotein is the sole component of the protein shell surrounding Haloferax volcanii cells. The deduced amino acid sequence of the S-layer glycoprotein predicts the presence of a C-terminal membrane-spanning domain. However, several earlier observations, including the ability of EDTA to selectively solubilize the protein, are inconsistent with the presence of a trans-membrane sequence. In the present report, sequential solubilization of the S-layer glycoprotein by EDTA and then with detergent revealed the existence of two distinct populations of the S-layer glycoprotein. Whereas both S-layer glycoprotein populations underwent signal peptide cleavage and N-glycosylation, base hydrolysis followed by mass spectrometry revealed that a lipid, likely archaetidic acid, modified only the EDTA-solubilized version of the protein. These observations are consistent with the S-layer glycoprotein being initially synthesized as an integral membrane protein and subsequently undergoing a processing event in which the extracellular portion of the protein is separated from the membrane-spanning domain and transferred to a waiting lipid moiety.

Diversity in prokaryotic glycosylation: an archaeal-derived N-linked glycan contains legionaminic acid.

VP4, the major structural protein of the haloarchaeal pleomorphic virus, HRPV-1, is glycosylated. To define the glycan structure attached to this protein, oligosaccharides released by ß-elimination were analysed by mass spectrometry and nuclear magnetic resonance spectroscopy. Such analyses showed that the major VP4-derived glycan is a pentasaccharide comprising glucose, glucuronic acid, mannose, sulphated glucuronic acid and a terminal 5-N-formyl-legionaminic acid residue. This is the first observation of legionaminic acid, a sialic acid-like sugar, in an archaeal-derived glycan structure. The importance of this residue for viral infection was demonstrated upon incubation with N-acetylneuraminic acid, a similar monosaccharide. Such treatment reduced progeny virus production by half 4 h post infection. LC-ESI/MS analysis confirmed the presence of pentasaccharide precursors on two different VP4-derived peptides bearing the N-glycosylation signal, NTT. The same sites modified by the native host, Halorubrum sp. strain PV6, were also recognized by the Haloferax volcanii N-glycosylation apparatus, as determined by LC-ESI/MS of heterologously expressed VP4. Here, however, the N-linked pentasaccharide was the same as shown to decorate the S-layer glycoprotein in this species. Hence, N-glycosylation of the haloarchaeal viral protein, VP4, is host-specific. These results thus present additional examples of archaeal N-glycosylation diversity and show the ability of Archaea to modify heterologously expressed proteins.

Add salt, add sugar: N-glycosylation in Haloferax volcanii.

Although performed by members of all three domains of life, the archaeal version of N-glycosylation remains the least understood. Studies on Haloferax volcanii have, however, begun to correct this situation. A combination of bioinformatics, molecular biology, biochemical and mass spectrometry approaches have served to delineate the Agl pathway responsible for N-glycosylation of the S-layer glycoprotein, a reporter of this post-translational modification in Hfx. volcanii. More recently, differential N-glycosylation of the S-layer glycoprotein as a function of environmental salinity was demonstrated, showing that this post-translational modification serves an adaptive role in Hfx. volcanii. Furthermore, manipulation of the Agl pathway, together with the capability of Hfx. volcanii to N-glycosylate non-native proteins, forms the basis for establishing this species as a glyco-engineering platform. In the present review, these and other recent findings are addressed.

S-layer glycoproteins and flagellins: reporters of archaeal posttranslational modifications.

Many archaeal proteins undergo posttranslational modifications. S-layer proteins and flagellins have been used successfully to study a variety of these modifications, including N-linked glycosylation, signal peptide removal and lipid modification. Use of these well-characterized reporter proteins in the genetically tractable model organisms, Haloferax volcanii, Methanococcus voltae and Methanococcus maripaludis, has allowed dissection of the pathways and characterization of many of the enzymes responsible for these modifications. Such studies have identified archaeal-specific variations in signal peptidase activity not found in the other domains of life, as well as the enzymes responsible for assembly and biosynthesis of novel N-linked glycans. In vitro assays for some of these enzymes have already been developed. N-linked glycosylation is not essential for either Hfx. volcanii or the Methanococcus species, an observation that allowed researchers to analyze the role played by glycosylation in the function of both S-layers and flagellins, by generating mutants possessing these reporters with only partial attached glycans or lacking glycan altogether. In future studies, it will be possible to consider questions related to the heterogeneity associated with given modifications, such as differential or modulated glycosylation.

AglM and VNG1048G, Two Haloarchaeal UDP-Glucose Dehydrogenases, Show Different Salt-Related Behaviors.

Haloferax volcanii AglM and Halobacterium salinarum VNG1048G are UDP-glucose dehydrogenases involved in N-glycosylation in each species. Despite sharing >60% sequence identity and the ability of VNG1048G to functionally replace AglM in vivo, these proteins behaved differently as salinity changed. Whereas AglM was active in 2-4 M NaCl, VNG1048G lost much of its activity when salinity dropped below 3 M NaCl. To understand the molecular basis of this phenomenon, each protein was examined by size exclusion chromatrography in 2 M NaCl. Whereas AglM appeared as a dodecamer, VNG1048G was essentially detected as a dodecamer and a dimer. The specific activity of the VNG1048G dodecamer was only a sixth of that of AglM, while the dimer was inactive. As such, not only was the oligomeric status of VNG1048G affected by lowered salinity, so was the behavior of the individual dodecamer subunits. Analyzing surface-exposed residues in homology models of the two UDP-glucose dehydrogenases revealed the more acidic and less basic VNG1048G surface, further explaining the greater salt-dependence of the Hbt. salinarum enzyme.

Deciphering a pathway of Halobacterium salinarum N-glycosylation.

Genomic analysis points to N-glycosylation as being a common posttranslational modification in Archaea. To date, however, pathways of archaeal N-glycosylation have only been described for few species. With this in mind, the similarities of N-linked glycans decorating glycoproteins in the haloarchaea Haloferax volcanii and Halobacterium salinarum directed a series of bioinformatics, genetic, and biochemical experiments designed to describe that Hbt. salinarum pathway responsible for biogenesis of one of the two N-linked oligosaccharides described in this species. As in Hfx. volcanii, where agl (archaeal glycosylation) genes that encode proteins responsible for the assembly and attachment of a pentasaccharide to target protein Asn residues are clustered in the genome, Hbt. salinarum also contains a group of clustered homologous genes (VNG1048G-VNG1068G). Introduction of these Hbt. salinarum genes into Hfx. volcanii mutant strains deleted of the homologous sequence restored the lost activity. Moreover, transcription of the Hbt. salinarum genes in the native host, as well as in vitro biochemical confirmation of the predicted functions of several of the products of these genes provided further support for assignments made following bioinformatics and genetic experiments. Based on the results obtained in this study, the first description of an N-glycosylation pathway in Hbt. salinarum is offered.

Archaeal S-layer glycoproteins: post-translational modification in the face of extremes.

Corresponding to the sole or basic component of the surface (S)-layer surrounding the archaeal cell in most known cases, S-layer glycoproteins are in direct contact with the harsh environments that characterize niches where Archaea can thrive. Accordingly, early work examining archaeal S-layer glycoproteins focused on identifying those properties that allow members of this group of proteins to maintain their structural integrity in the face of extremes of temperature, pH, and salinity, as well as other physical challenges. However, with expansion of the list of archaeal strains serving as model systems, as well as growth in the number of molecular tools available for the manipulation of these strains, studies on archaeal S-layer glycoproteins are currently more likely to consider the various post-translational modifications these polypeptides undergo. For instance, archaeal S-layer glycoproteins can undergo proteolytic cleavage, both N- and O-glycosylation, lipid-modification and oligomerization. In this mini-review, recent findings related to the post-translational modification of archaeal S-layer glycoproteins are considered.

Analysis of putative nonulosonic acid biosynthesis pathways in Archaea reveals a complex evolutionary history.

Sialic acids and the other nonulosonic acid sugars, legionaminic acid and pseudaminic acid, are nine carbon-containing sugars that can be detected as components of the glycans decorating proteins and other molecules in Eukarya and Bacteria. Yet, despite the prevalence of N-glycosylation in Archaea and the variety of sugars recruited for the archaeal version of this post-translational modification, only a single report of a nonulosonic acid sugar in an archaeal N-linked glycan has appeared. Hence, to obtain a clearer picture of nonulosonic acid sugar biosynthesis capability in Archaea, 122 sequenced genomes were scanned for the presence of genes involved in the biogenesis of these sugars. The results reveal that while Archaea and Bacteria share a common route of sialic acid biosynthesis, numerous archaeal nonulosonic acid sugar biosynthesis pathway components were acquired from elsewhere via various routes. Still, the limited number of Archaea encoding components involved in the synthesis of nonulosonic acid sugars implies that such saccharides are not major components of glycans in this domain.

N-glycosylation in Haloferax volcanii: adjusting the sweetness.

Long believed to be restricted to Eukarya, it is now known that cells of all three domains of life perform N-glycosylation, the covalent attachment of glycans to select target protein asparagine residues. Still, it is only in the last decade that pathways of N-glycosylation in Archaea have been delineated. In the haloarchaeon Haloferax volcanii, a series of Agl (archaeal glycosylation) proteins is responsible for the addition of an N-linked pentasaccharide to modified proteins, including the surface (S)-layer glycoprotein, the sole component of the surface layer surrounding the cell. The S-layer glycoprotein N-linked glycosylation profile changes, however, as a function of surrounding salinity. Upon growth at different salt concentrations, the S-layer glycoprotein is either decorated by the N-linked pentasaccharide introduced above or by both this pentasaccharide as well as a tetrasaccharide of distinct composition. Recent efforts have identified Agl5-Agl15 as components of a second Hfx. volcanii N-glycosylation pathway responsible for generating the tetrasaccharide attached to S-layer glycoprotein when growth occurs in 1.75 M but not 3.4 M NaCl-containing medium.