Biosynthesis of sulfated saccharides N-glycosidically linked to the protein via glucose. Purification and identification of sulfated dolichyl monophosphoryl tetrasaccharides from halobacteria

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.

Identifying Components of a Halobacterium salinarum N-Glycosylation Pathway

Whereas N-glycosylation is a seemingly universal process in Archaea, pathways of N-glycosylation have only been experimentally verified in a mere handful of species. Toward expanding the number of delineated archaeal N-glycosylation pathways, the involvement of the putative Halobacterium salinarum glycosyltransferases VNG1067G, VNG1066C, and VNG1062G in the assembly of an N-linked tetrasaccharide decorating glycoproteins in this species was addressed. Following deletion of each encoding gene, the impact on N-glycosylation of the S-layer glycoprotein and archaellins, major glycoproteins in this organism, was assessed by mass spectrometry. Likewise, the pool of dolichol phosphate, the lipid upon which this glycan is assembled, was also considered in each deletion strain. Finally, the impacts of such deletions were characterized in a series of biochemical, structural and physiological assays. The results revealed that VNG1067G, VNG1066C, and VNG1062G, renamed Agl25, Agl26, and Agl27 according to the nomenclature used for archaeal N-glycosylation pathway components, are responsible for adding the second, third and fourth sugars of the N-linked tetrasaccharide decorating Hbt. salinarum glycoproteins. Moreover, this study demonstrated how compromised N-glycosylation affects various facets of Hbt. salinarum cell behavior, including the transcription of archaellin-encoding genes.

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 tetrasacchride 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 tetrasacchride. Knowing the composition of this glycan at both the lipid- and protein-bound stages, together with the availability of gene deletion approaches for manipulating Halobacterium salinarum, will allow delineation of the N-glycosylation pathway in this organism.

Investigation of the conserved reentrant membrane helix in the monotopic phosphoglycosyl transferase superfamily supports key molecular interactions with polyprenol phosphate substrates

Long-chain polyprenol phosphates feature in membrane-associated glycoconjugate biosynthesis pathways across domains of life. These unique amphiphilic molecules are best known as substrates of polytopic membrane proteins, including polyprenol-phosphate phosphoglycosyl and glycosyl transferases, and as components of more complex substrates. The linear polyprenols are constrained by double bond geometry and lend themselves well to interactions with polytopic membrane proteins, in which multiple transmembrane helices form a rich landscape for interactions. Recently, a new superfamily of monotopic phosphoglycosyl transferase enzymes has been identified that interacts with polyprenol phosphate substrates via a single reentrant membrane helix. Intriguingly, despite the dramatic differences in their membrane-interaction domains, both polytopic and monotopic enzymes similarly favor a unique cis/trans geometry in their polyprenol phosphate substrates. Herein, we present a multipronged biochemical and biophysical study of PglC, a monotopic phosphoglycosyl transferase that catalyzes the first membrane-committed step in N-linked glycoprotein biosynthesis in Campylobacter jejuni. We probe the significance of polyprenol phosphate geometry both in mediating substrate binding to PglC and in modulating the local membrane environment. Geometry is found to be important for binding to PglC; a conserved proline residue in the reentrant membrane helix is determined to drive polyprenol phosphate recognition and specificity. Pyrene fluorescence studies show that polyprenol phosphates at physiologically-relevant levels increase the disorder of the local lipid bilayer; however, this effect is confined to polyprenol phosphates with specific isoprene geometries. The molecular insights from this study may shed new light on the interactions of polyprenol phosphates with diverse membrane-associated proteins in glycoconjugate biosynthesis.

N-Glycosylation Is Important for Halobacterium salinarum Archaellin Expression, Archaellum Assembly and Cell Motility

Halobacterium salinarum are halophilic archaea that display directional swimming in response to various environmental signals, including light, chemicals and oxygen. In Hbt. salinarum, the building blocks (archaellins) of the archaeal swimming apparatus (the archaellum) are N-glycosylated. However, the physiological importance of archaellin N-glycosylation remains unclear. Here, a tetrasaccharide comprising a hexose and three hexuronic acids decorating the five archaellins was characterized by mass spectrometry. Such analysis failed to detect sulfation of the hexuronic acids, in contrast to earlier reports. To better understand the physiological significance of Hbt. salinarum archaellin N-glycosylation, a strain deleted of aglB, encoding the archaeal oligosaccharyltransferase, was generated. In this ΔaglB strain, archaella were not detected and only low levels of archaellins were released into the medium, in contrast to what occurs with the parent strain. Mass spectrometry analysis of the archaellins in ΔaglB cultures did not detect N-glycosylation. ΔaglB cells also showed a slight growth defect and were impaired for motility. Quantitative real-time PCR analysis revealed dramatically reduced transcript levels of archaellin-encoding genes in the mutant strain, suggesting that N-glycosylation is important for archaellin transcription, with downstream effects on archaellum assembly and function. Control of AglB-dependent post-translational modification of archaellins could thus reflect a previously unrecognized route for regulating Hbt. salinarum motility.

Stereochemical Divergence of Polyprenol Phosphate Glycosyltransferases

In the three domains of life, lipid-linked glycans contribute to various cellular processes ranging from protein glycosylation to glycosylphosphatidylinositol anchor biosynthesis to peptidoglycan assembly. In generating many of these glycoconjugates, phosphorylated polyprenol-based lipids are charged with single sugars by polyprenol phosphate glycosyltransferases. The resultant substrates serve as glycosyltransferase donors, complementing the more common nucleoside diphosphate sugars. It had been accepted that these polyprenol phosphate glycosyltransferases acted similarly, given their considerable sequence homology. Recent findings, however, suggest that matters may not be so simple. In this Opinion we propose that the stereochemistry of sugar addition by polyprenol phosphate glycosyltransferases is not conserved across evolution, even though the GT-A fold that characterizes such enzymes is omnipresent.

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

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.

N-glycosylation in the thermoacidophilic archaeon Sulfolobus acidocaldarius involves a short dolichol pyrophosphate carrier

N-glycosylation is a post-translational modification that occurs across evolution. In the thermoacidophilic archaea Sulfolobus acidocaldarius, glycoproteins are modified by an N-linked tribranched hexasaccharide reminiscent of the N-glycans assembled in Eukarya. Previously, hexose-bearing dolichol phosphate was detected in a S. acidocaldarius Bligh-Dyer lipid extract. Here, we used a specialized protocol for extracting lipid-linked oligosaccharides to detect a dolichol pyrophosphate bearing the intact hexasaccharide, as well as its biosynthetic intermediates. Furthermore, evidence for N-glycosylation of two S. acidocaldarius proteins by the same hexasaccharide and its derivatives was collected. These findings thus provide novel insight into archaeal N-glycosylation.

Identification of the S-layer glycoproteins and their covalently linked glycans in the halophilic archaeon Haloarcula hispanica

Haloarcula hispanica is one of members of the Halobacteriaceae, which displays particularly low restriction activity and is therefore important as one of the most tractable haloarchaea for archaeal genetic research. Although the Har. hispanica S-layer protein has been reported glycosylated, the S-layer glycoprotein and its glycosylation have not been investigated yet. In this study, the S-layer proteins of Har. hispanica were extracted and characterized. The S-layer was found containing two different glycoproteins which shared highly similar amino acid sequences. The genes coding for these two S-layer glycoproteins were found next to each other in the genome. Moreover, the N- and O-linked glycans were released from these two S-layer glycoproteins for structural determination. Based on the mass spectrometry and nuclear magnetic resonance, the N-glycan was determined as a branched trisaccharide containing a 225 Da residue corresponded to a 2-amino-6-sulfo-2, 6-dideoxy-quinovose, which was the first time that a naturally occurring form of sulfoquinovosamine was identified. Besides, the O-glycan was characterized as a Glcα-1,4-Gal disaccharide by mass spectrometry combined with monosaccharide composition analysis and glycosidase treatment. The determination of the N- and O-glycan structure will be helpful for studying the diverse protein glycosylation pathways in archaea utilizing H. hispanica as a new model.

Structural elucidation of an asparagine-linked oligosaccharide from the hyperthermophilic archaeon, Archaeoglobus fulgidus

The genome of the hyperthermophilic archaeon, Archaeoglobus fulgidus, contains three paralogous AglB genes that encode oligosaccharyltransferase (OST) proteins. The OST enzymes catalyze the transfer of an oligosaccharide chain from lipid-linked oligosaccharides (LLO) to asparagine residues in proteins. The detergent-solubilized membrane fractions prepared from cultured A. fulgidus cells contain both OST and LLO. The addition of a peptide containing the glycosylation sequon produced oligosaccharide chains attached to a structurally defined peptide. To facilitate the NMR analysis, the cells were grown in rich medium supplemented with (13)C-glucose, to label the LLOs metabolically. The MS analysis of the glycopeptide revealed that the glucose and galactose residues were nearly fully (13)C-labeled, but the mannose residues were fractionally labeled with about 20% efficiency. An immunodetection experiment revealed that the longest AglB paralog (AfAglB-L) was expressed in the membrane fractions under our cell culture conditions, while the other two shorter AglB paralogs (AfAglB-S1 and AfAglB-S2) were not. Thus, the oligosaccharide chain analyzed in this study was the product of AfAglB-L. The N-glycan consists of eight hexose residues, as follows: The α1,3-linked glucose is an optional residue branching from the distal mannose residue. The MS analysis of the minor HPLC peak of the in vitro oligosaccharyl transfer products also revealed an optional sulfate modification on the glucose residue directly linked to the Asn residue. The present data will be useful for structural and functional studies of the N-glycosylation system of A. fulgidus.

Methanothermobacter thermautotrophicus modulates its membrane lipids in response to hydrogen and nutrient availability

Methanothermobacter thermautotrophicus strain ΔH is a model hydrogenotrophic methanogen, for which extensive biochemical information, including the complete genome sequence, is available. Nevertheless, at the cell membrane lipid level, little is known about the responses of this archaeon to environmental stimuli. In this study, the lipid composition of M. thermautotrophicus was characterized to verify how this archaeon modulates its cell membrane components during growth phases and in response to hydrogen depletion and nutrient limitation (potassium and phosphate). As opposed to the higher abundance of phospholipids in the stationary phase of control experiments, cell membranes under nutrient, and energy stress were dominated by glycolipids that likely provided a more effective barrier against ion leakage. We also identified particular lipid regulatory mechanisms in M. thermautotrophicus, which included the accumulation of polyprenols under hydrogen-limited conditions and an increased content of sodiated adducts of lipids in nutrient-limited cells. These findings suggest that M. thermautotrophicus intensely modulates its cell membrane lipid composition to cope with energy and nutrient availability in dynamic environments.

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.

N-linked glycosylation in Archaea: a structural, functional, and genetic analysis

N-glycosylation of proteins is one of the most prevalent posttranslational modifications in nature. Accordingly, a pathway with shared commonalities is found in all three domains of life. While excellent model systems have been developed for studying N-glycosylation in both Eukarya and Bacteria, an understanding of this process in Archaea was hampered until recently by a lack of effective molecular tools. However, within the last decade, impressive advances in the study of the archaeal version of this important pathway have been made for halophiles, methanogens, and thermoacidophiles, combining glycan structural information obtained by mass spectrometry with bioinformatic, genetic, biochemical, and enzymatic data. These studies reveal both features shared with the eukaryal and bacterial domains and novel archaeon-specific aspects. Unique features of N-glycosylation in Archaea include the presence of unusual dolichol lipid carriers, the use of a variety of linking sugars that connect the glycan to proteins, the presence of novel sugars as glycan constituents, the presence of two very different N-linked glycans attached to the same protein, and the ability to vary the N-glycan composition under different growth conditions. These advances are the focus of this review, with an emphasis on N-glycosylation pathways in Haloferax, Methanococcus, and Sulfolobus.

Substrate promiscuity: AglB, the archaeal oligosaccharyltransferase, can process a variety of lipid-linked glycans

Across evolution, N-glycosylation involves oligosaccharyltransferases that transfer lipid-linked glycans to selected Asn residues of target proteins. While these enzymes catalyze similar reactions in each domain, differences exist in terms of the chemical composition, length and degree of phosphorylation of the lipid glycan carrier, the sugar linking the glycan to the lipid carrier, and the composition and structure of the transferred glycan. To gain insight into how oligosaccharyltransferases cope with such substrate diversity, the present study analyzed the archaeal oligosaccharyltransferase AglB from four haloarchaeal species. Accordingly, it was shown that despite processing distinct lipid-linked glycans in their native hosts, AglB from Haloarcula marismortui, Halobacterium salinarum, and Haloferax mediterranei could readily replace their counterpart from Haloferax volcanii when introduced into Hfx. volcanii cells deleted of aglB. As the four enzymes show significant sequence and apparently structural homology, it appears that the functional similarity of the four AglB proteins reflects the relaxed substrate specificity of these enzymes. Such demonstration of AglB substrate promiscuity is important not only for better understanding of N-glycosylation in Archaea and elsewhere but also for efforts aimed at transforming Hfx. volcanii into a glycoengineering platform.

Hot and sweet: protein glycosylation in Crenarchaeota

Every living cell is covered with a dense and complex array of covalently attached sugars or sugar chains. The majority of these glycans are linked to proteins via the so-called glycosylation process. Protein glycosylation is found in all three domains of life: Eukarya, Bacteria and Archaea. However, on the basis of the limit in analytic tools for glycobiology and genetics in Archaea, only in the last few years has research on archaeal glycosylation pathways started mainly in the Euryarchaeota Haloferax volcanii, Methanocaldococcus maripaludis and Methanococcus voltae. Recently, major steps of the crenarchaeal glycosylation process of the thermoacidophilic archaeon Sulfolobus acidocaldarius have been described. The present review summarizes the proposed N-glycosylation pathway of S. acidocaldarius, describing the phenotypes of the mutants disrupted in N-glycan biosynthesis as well as giving insights into the archaeal O-linked and glycosylphosphatidylinositol anchor glycosylation process.

Biochemical evidence for an alternate pathway in N-linked glycoprotein biosynthesis

Asparagine-linked glycosylation is a complex protein modification conserved among all three domains of life. Herein we report the in vitro analysis of N-linked glycosylation from the methanogenic archaeon Methanococcus voltae. Using a suite of synthetic and semisynthetic substrates, we show that AglK initiates N-linked glycosylation in M. voltae through the formation of α-linked dolichyl monophosphate N-acetylglucosamine (Dol-P-GlcNAc), which contrasts with the polyprenyl-diphosphate intermediates that feature in both eukaryotes and bacteria. Intriguingly, AglK exhibits high sequence homology to dolichyl-phosphate β-glucosyltransferases, including Alg5 in eukaryotes, suggesting a common evolutionary origin. The combined action of the first two enzymes, AglK and AglC, afforded an α-linked Dol-P-glycan that serves as a competent substrate for the archaeal oligosaccharyl transferase AglB. These studies provide the first biochemical evidence revealing that despite the apparent similarity of the overall pathways, there are actually two general strategies to achieve N-linked glycoproteins across the domains of life.

Post-translation modification in Archaea: lessons from Haloferax volcanii and other haloarchaea

As an ever-growing number of genome sequences appear, it is becoming increasingly clear that factors other than genome sequence impart complexity to the proteome. Of the various sources of proteomic variability, post-translational modifications (PTMs) most greatly serve to expand the variety of proteins found in the cell. Likewise, modulating the rates at which different proteins are degraded also results in a constantly changing cellular protein profile. While both strategies for generating proteomic diversity are adopted by organisms across evolution, the responsible pathways and enzymes in Archaea are often less well described than are their eukaryotic and bacterial counterparts. Studies on halophilic archaea, in particular Haloferax volcanii, originally isolated from the Dead Sea, are helping to fill the void. In this review, recent developments concerning PTMs and protein degradation in the haloarchaea are discussed.

AglS, a novel component of the Haloferax volcanii N-glycosylation pathway, is a dolichol phosphate-mannose mannosyltransferase

In Haloferax volcanii, a series of Agl proteins mediates protein N-glycosylation. The genes encoding all but one of the Agl proteins are sequestered into a single gene island. The same region of the genome includes sequences also suspected but not yet verified as serving N-glycosylation roles, such as HVO_1526. In the following, HVO_1526, renamed AglS, is shown to be necessary for the addition of the final mannose subunit of the pentasaccharide N-linked to the surface (S)-layer glycoprotein, a convenient reporter of N-glycosylation in Hfx. volcanii. Relying on bioinformatics, topological analysis, gene deletion, mass spectrometry, and biochemical assays, AglS was shown to act as a dolichol phosphate-mannose mannosyltransferase, mediating the transfer of mannose from dolichol phosphate to the tetrasaccharide corresponding to the first four subunits of the pentasaccharide N-linked to the S-layer glycoprotein.

At the membrane frontier: a prospectus on the remarkable evolutionary conservation of polyprenols and polyprenyl-phosphates

Long-chain polyprenols and polyprenyl-phosphates are ubiquitous and essential components of cellular membranes throughout all domains of life. Polyprenyl-phosphates, which include undecaprenyl-phosphate in bacteria and the dolichyl-phosphates in archaea and eukaryotes, serve as specific membrane-bound carriers in glycan biosynthetic pathways responsible for the production of cellular structures such as N-linked protein glycans and bacterial peptidoglycan. Polyprenyl-phosphates are the only form of polyprenols with a biochemically-defined role; however, unmodified or esterified polyprenols often comprise significant percentages of the cellular polyprenol pool. The strong evolutionary conservation of unmodified polyprenols as membrane constituents and polyprenyl-phosphates as preferred glycan carriers in biosynthetic pathways is poorly understood. This review surveys the available research to explore why unmodified polyprenols have been conserved in evolution and why polyprenyl-phosphates are universally and specifically utilized for membrane-bound glycan assembly.

Different routes to the same ending: comparing the N-glycosylation processes of Haloferax volcanii and Haloarcula marismortui, two halophilic archaea from the Dead Sea

Recent insight into the N-glycosylation pathway of the haloarchaeon, Haloferax volcanii, is helping to bridge the gap between our limited understanding of the archaeal version of this universal post-translational modification and the better-described eukaryal and bacterial processes. To delineate as yet undefined steps of the Hfx. volcanii N-glycosylation pathway, a comparative approach was taken with the initial characterization of N-glycosylation in Haloarcula marismortui, a second haloarchaeon also originating from the Dead Sea. While both species decorate the reporter glycoprotein, the S-layer glycoprotein, with the same N-linked pentasaccharide and employ dolichol phosphate as lipid glycan carrier, species-specific differences in the two N-glycosylation pathways exist. Specifically, Har. marismortui first assembles the complete pentasaccharide on dolichol phosphate and only then transfers the glycan to the target protein, as in the bacterial N-glycosylation pathway. In contrast, Hfx. volcanii initially transfers the first four pentasaccharide subunits from a common dolichol phosphate carrier to the target protein and only then delivers the final pentasaccharide subunit from a distinct dolichol phosphate to the N-linked tetrasaccharide, reminiscent of what occurs in eukaryal N-glycosylation. This study further indicates the extraordinary diversity of N-glycosylation pathways in Archaea, as compared with the relatively conserved parallel processes in Eukarya and Bacteria.

The expanding horizons of asparagine-linked glycosylation

Asparagine-linked glycosylation involves the sequential assembly of an oligosaccharide onto a polyisoprenyl donor, followed by the en bloc transfer of the glycan to particular asparagine residues within acceptor proteins. These N-linked glycans play a critical role in a wide variety of biological processes, such as protein folding, cellular targeting and motility, and the immune response. In the past decade, research in the field of N-linked glycosylation has achieved major advances, including the discovery of new carbohydrate modifications, the biochemical characterization of the enzymes involved in glycan assembly, and the determination of the biological impact of these glycans on target proteins. It is now firmly established that this enzyme-catalyzed modification occurs in all three domains of life. However, despite similarities in the overall logic of N-linked glycoprotein biosynthesis among the three kingdoms, the structures of the appended glycans are markedly different and thus influence the functions of elaborated proteins in various ways. Though nearly all eukaryotes produce the same nascent tetradecasaccharide (Glc(3)Man(9)GlcNAc(2)), heterogeneity is introduced into this glycan structure after it is transferred to the protein through a complex series of glycosyl trimming and addition steps. In contrast, bacteria and archaea display diversity within their N-linked glycan structures through the use of unique monosaccharide building blocks during the assembly process. In this review, recent progress toward gaining a deeper biochemical understanding of this modification across all three kingdoms will be summarized. In addition, a brief overview of the role of N-linked glycosylation in viruses will also be presented.

Distinct glycan-charged phosphodolichol carriers are required for the assembly of the pentasaccharide N-linked to the Haloferax volcanii S-layer glycoprotein

In Archaea, dolichol phosphates have been implicated as glycan carriers in the N-glycosylation pathway, much like their eukaryal counterparts. To clarify this relation, highly sensitive liquid chromatography/mass spectrometry was employed to detect and characterize glycan-charged phosphodolichols in the haloarchaeon Haloferax volcanii. It is reported that Hfx. volcanii contains a series of C(55) and C(60) dolichol phosphates presenting saturated isoprene subunits at the α and ω positions and sequentially modified with the first, second, third and methylated fourth sugar subunits comprising the first four subunits of the pentasaccharide N-linked to the S-layer glycoprotein, a reporter of N-glycosylation. Moreover, when this glycan-charged phosphodolichol pool was examined in cells deleted of agl genes encoding glycosyltransferases participating in N-glycosylation and previously assigned roles in adding pentasaccharide residues one to four, the composition of the lipid-linked glycans was perturbed in the identical manner as was S-layer glycoprotein N-glycosylation in these mutants. In contrast, the fifth sugar of the pentasaccharide, identified as mannose in this study, is added to a distinct dolichol phosphate carrier. This represents the first evidence that in Archaea, as in Eukarya, the oligosaccharides N-linked to glycoproteins are sequentially assembled from glycans originating from distinct phosphodolichol carriers.

Biosynthesis and role of N-linked glycosylation in cell surface structures of archaea with a focus on flagella and s layers

The genetics and biochemistry of the N-linked glycosylation system of Archaea have been investigated over the past 5 years using flagellins and S layers as reporter proteins in the model organisms, Methanococcus voltae, Methanococcus maripaludis, and Haloferax volcanii. Structures of archaeal N-linked glycans have indicated a variety of linking sugars as well as unique sugar components. In M. voltae, M. maripaludis, and H. volcanii, a number of archaeal glycosylation genes (agl) have been identified by deletion and complementation studies. These include many of the glycosyltransferases and the oligosaccharyltransferase needed to assemble the glycans as well as some of the genes encoding enzymes required for the biosynthesis of the sugars themselves. The N-linked glycosylation system is not essential for any of M. voltae, M. maripaludis, or H. volcanii, as demonstrated by the successful isolation of mutants carrying deletions in the oligosaccharyltransferase gene aglB (a homologue of the eukaryotic Stt3 subunit of the oligosaccharyltransferase complex). However, mutations that affect the glycan structure have serious effects on both flagellation and S layer function.

Structure and synthesis of polyisoprenoids used in N-glycosylation across the three domains of life

N-linked protein glycosylation was originally thought to be specific to eukaryotes, but evidence of this post-translational modification has now been discovered across all domains of life: Eucarya, Bacteria, and Archaea. In all cases, the glycans are first assembled in a step-wise manner on a polyisoprenoid carrier lipid. At some stage of lipid-linked oligosaccharide synthesis, the glycan is flipped across a membrane. Subsequently, the completed glycan is transferred to specific asparagine residues on the protein of interest. Interestingly, though the N-glycosylation pathway seems to be conserved, the biosynthetic pathways of the polyisoprenoid carriers, the specific structures of the carriers, and the glycan residues added to the carriers vary widely. In this review we will elucidate how organisms in each basic domain of life synthesize the polyisoprenoids that they utilize for N-linked glycosylation and briefly discuss the subsequent modifications of the lipid to generate a lipid-linked oligosaccharide.

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.

Sweet to the extreme: protein glycosylation in Archaea

Post-translational modifications account for much of the biological diversity generated at the proteome level. Of these, glycosylation is the most prevalent. Long thought to be unique to Eukarya, it is now clear that both Bacteria and Archaea are also capable of N-glycosylation, namely the covalent linkage of oligosaccharides to select target asparagine residues. However, while the eukaryal and bacterial N-glycosylation pathways are relatively well defined, little is known of the parallel process in Archaea. Of late, however, major advances have been made in describing the process of archaeal N-glycosylation. Such efforts have shown, as is often the case in archaeal biology, that protein N-glycosylation in Archaea combines particular aspects of the eukaryal and bacterial pathways along with traits unique to this life form. For instance, while the oligosaccharides of archaeal glycoproteins include nucleotide-activated sugars formed by bacterial pathways, the lipid carrier on which such oligosaccharides are assembled is the same as used in eukaryal N-glycosylation. By contrast, transfer of assembled oligosaccharides to their protein targets shows Archaea-specific properties. Finally, addressing N-glycosylation from an archaeal perspective is providing new general insight into this event, as exemplified by the solution of the first crystal structure of an oligosaccharide transferase from an archaeal source.

Polyisoprenoid alcohols--recent results of structural studies

Polyisoprenoid alcohols (polyprenols and dolichols) are linear polymers of from several up to more than 100 isoprene units identified in almost all living organisms. Studies of their chemical structures have resulted in the discovery of new variants such as the recently described alloprenols with reversed configuration of the double bond in the alpha-isoprene unit. In parallel, structural elucidation of metabolically labeled plant dolichols has indicated that both the mevalonate and methylerythritol phosphate pathways are involved in the biosynthesis of dolichols in roots, leading to the construction of a spatial model of their biosynthesis. According to this model, in root cells, synthesis of the dolichol molecule is initiated in the plastids, and the resulting intermediates, oligoprenyl diphosphates, are exported to the cytoplasm and are elongated up to the desired chain length. The metabolic consequences of this putative model are discussed in the context of the enzymatic machinery involved.

Biosynthesis of D-arabinose in mycobacteria - a novel bacterial pathway with implications for antimycobacterial therapy

Decaprenyl-phospho-arabinose (beta-D-arabinofuranosyl-1-O-monophosphodecaprenol), the only known donor of d-arabinose in bacteria, and its precursor, decaprenyl-phospho-ribose (beta-D-ribofuranosyl-1-O-monophosphodecaprenol), were first described in 1992. En route to D-arabinofuranose, the decaprenyl-phospho-ribose 2'-epimerase converts decaprenyl-phospho-ribose to decaprenyl-phospho-arabinose, which is a substrate for arabinosyltransferases in the synthesis of the cell-wall arabinogalactan and lipoarabinomannan polysaccharides of mycobacteria. The first step of the proposed decaprenyl-phospho-arabinose biosynthesis pathway in Mycobacterium tuberculosis and related actinobacteria is the formation of D-ribose 5-phosphate from sedoheptulose 7-phosphate, catalysed by the Rv1449 transketolase, and/or the isomerization of d-ribulose 5-phosphate, catalysed by the Rv2465 d-ribose 5-phosphate isomerase. d-Ribose 5-phosphate is a substrate for the Rv1017 phosphoribosyl pyrophosphate synthetase which forms 5-phosphoribosyl 1-pyrophosphate (PRPP). The activated 5-phosphoribofuranosyl residue of PRPP is transferred by the Rv3806 5-phosphoribosyltransferase to decaprenyl phosphate, thus forming 5'-phosphoribosyl-monophospho-decaprenol. The dephosphorylation of 5'-phosphoribosyl-monophospho-decaprenol to decaprenyl-phospho-ribose by the putative Rv3807 phospholipid phosphatase is the committed step of the pathway. A subsequent 2'-epimerization of decaprenyl-phospho-ribose by the heteromeric Rv3790/Rv3791 2'-epimerase leads to the formation of the decaprenyl-phospho-arabinose precursor for the synthesis of the cell-wall arabinans in Actinomycetales. The mycobacterial 2'-epimerase Rv3790 subunit is similar to the fungal D-arabinono-1,4-lactone oxidase, the last enzyme in the biosynthesis of D-erythroascorbic acid, thus pointing to an evolutionary link between the D-arabinofuranose- and L-ascorbic acid-related pathways. Decaprenyl-phospho-arabinose has been a lead compound for the chemical synthesis of substrates for mycobacterial arabinosyltransferases and of new inhibitors and potential antituberculosis drugs. The peculiar (omega,mono-E,octa-Z) configuration of decaprenol has yielded insights into lipid biosynthesis, and has led to the identification of the novel Z-polyprenyl diphosphate synthases of mycobacteria. Mass spectrometric methods were developed for the analysis of anomeric linkages and of dolichol phosphate-related lipids. In the field of immunology, the renaissance in mycobacterial polyisoprenoid research has led to the identification of mimetic mannosyl-beta-1-phosphomycoketides of pathogenic mycobacteria as potent lipid antigens presented by CD1c proteins to human T cells.

Chemoenzymatic synthesis of polyprenyl phosphates

Polyprenyl phosphates, including undecaprenyl phosphate and dolichyl phosphate, are essential intermediates in several important biochemical pathways including N-linked protein glycosylation in eukaryotes and prokaryotes and prokaryotic cell wall biosynthesis. Herein, we describe the evaluation of three potential undecaprenol kinases as agents for the chemoenzymatic synthesis of polyprenyl phosphates. Target enzymes were expressed in crude cell envelope fractions and quantified via the use of luminescent lanthanide-binding tags (LBTs). The Streptococcus mutans diacylglycerol kinase (DGK) was shown to be a very useful agent for polyprenol phosphorylation using ATP as the phosphoryl transfer agent. In addition, the S. mutans DGK can be coupled with two Campylobacter jejuni glycosyltransferases involved in N-linked glycosylation to efficiently biosynthesize the undecaprenyl pyrophosphate-linked disaccharide needed for studies of PglB, the C. jejuni oligosaccharyl transferase.

Identification of AglE, a second glycosyltransferase involved in N glycosylation of the Haloferax volcanii S-layer glycoprotein

Archaea, like Eukarya and Bacteria, are able to N glycosylate select protein targets. However, in contrast to relatively advanced understanding of the eukaryal N glycosylation process and the information being amassed on the bacterial process, little is known of this posttranslational modification in Archaea. Toward remedying this situation, the present report continues ongoing efforts to identify components involved in the N glycosylation of the Haloferax volcanii S-layer glycoprotein. By combining gene deletion together with mass spectrometry, AglE, originally identified as a homologue of murine Dpm1, was shown to play a role in the addition of the 190-Da sugar subunit of the novel pentasaccharide decorating the S-layer glycoprotein. Topological analysis of an AglE-based chimeric reporter assigns AglE as an integral membrane protein, with its N terminus and putative active site facing the cytoplasm. These finding, therefore, contribute to the developing picture of the N glycosylation pathway in Archaea.

Metabolism of halophilic archaea

In spite of their common hypersaline environment, halophilic archaea are surprisingly different in their nutritional demands and metabolic pathways. The metabolic diversity of halophilic archaea was investigated at the genomic level through systematic metabolic reconstruction and comparative analysis of four completely sequenced species: Halobacterium salinarum, Haloarcula marismortui, Haloquadratum walsbyi, and the haloalkaliphile Natronomonas pharaonis. The comparative study reveals different sets of enzyme genes amongst halophilic archaea, e.g. in glycerol degradation, pentose metabolism, and folate synthesis. The carefully assessed metabolic data represent a reliable resource for future system biology approaches as it also links to current experimental data on (halo)archaea from the literature.

Structure-guided identification of a new catalytic motif of oligosaccharyltransferase

Asn-glycosylation is widespread not only in eukaryotes but also in archaea and some eubacteria. Oligosaccharyltransferase (OST) catalyzes the co-translational transfer of an oligosaccharide from a lipid donor to an asparagine residue in nascent polypeptide chains. Here, we report that a thermophilic archaeon, Pyrococcus furiosus OST is composed of the STT3 protein alone, and catalyzes the transfer of a heptasaccharide, containing one hexouronate and two pentose residues, onto peptides in an Asn-X-Thr/Ser-motif-dependent manner. We also determined the 2.7-A resolution crystal structure of the C-terminal soluble domain of Pyrococcus STT3. The structure-based multiple sequence alignment revealed a new motif, DxxK, which is adjacent to the well-conserved WWDYG motif in the tertiary structure. The mutagenesis of the DK motif residues in yeast STT3 revealed the essential role of the motif in the catalytic activity. The function of this motif may be related to the binding of the pyrophosphate group of lipid-linked oligosaccharide donors through a transiently bound cation. Our structure provides the first structural insights into the formation of the oligosaccharide-asparagine bond.

A defect in dolichol phosphate biosynthesis causes a new inherited disorder with death in early infancy

The following study describes the discovery of a new inherited metabolic disorder, dolichol kinase (DK1) deficiency. DK1 is responsible for the final step of the de novo biosynthesis of dolichol phosphate. Dolichol phosphate is involved in several glycosylation reactions, such as N-glycosylation, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, and C- and O-mannosylation. We identified four patients who were homozygous for one of two mutations (c.295T-->A [99Cys-->Ser] or c.1322A-->C [441Tyr-->Ser]) in the corresponding hDK1 gene. The residual activity of mutant DK1 was 2%-4% when compared with control cells. The mutated alleles failed to complement the temperature-sensitive phenotype of DK1-deficient yeast cells, whereas the wild-type allele restored the normal growth phenotype. Affected patients present with a very severe clinical phenotype, with death in early infancy. Two of the patients died from dilative cardiomyopathy.

Precise bacterial polyprenol length control fails inSaccharomyces cerevisiae

A comparison of amino acid sequences of yeast Rer2p and Srt1p Z-prenyltransferases shows that the spatial organization of their substrate tunnels agrees with that determined by X-ray for the E. coli undecaprenyl diphosphate synthase (UPPs). The observed trend in the maxima of product length distribution shifted from C(55) in UPPs to C(80) in Rer2p and to C(110) in Srt1p. This suggests a significant increase in the size of the enzyme hydrophobic tunnel from approximately 1000 A(3) of E. coli UPPs to approximately 1300 A(3) required to accommodate C(80) in Rer2p and to 1700 A(3) for C(110) in Srt1p. Moreover, Srt1p products reaching C(290) indicate the failure of a strict bacterial-like chain length control. On the basis of E. coli UPPs crystallographic structure the yeast Rer2p model was constructed. In the model three amino acid residues inserted into the sequence corresponding to the "floor" region of the tunnel extends the bottom loop what results in the required increase of the tunnel volume. Moreover, thermal fluctuations of this loop occasionally create a hole in the tunnel floor, making escape of polyprenol omega end out of the tunnel possible what switches off the control mechanism of product length thereby allowing a practically unlimited elongation process leading to an exponential distribution of longer chain polyprenols.

Identification of genes involved in the biosynthesis and attachment of Methanococcus voltae N-linked glycans: insight into N-linked glycosylation pathways in Archaea

N-linked glycosylation is recognized as an important post-translational modification across all three domains of life. However, the understanding of the genetic pathways for the assembly and attachment of N-linked glycans in eukaryotic and bacterial systems far outweighs the knowledge of comparable processes in Archaea. The recent characterization of a novel trisaccharide [beta-ManpNAcA6Thr-(1-4)-beta-GlcpNAc3NAcA-(1-3)-beta-GlcpNAc]N-linked to asparagine residues in Methanococcus voltae flagellin and S-layer proteins affords new opportunities to investigate N-linked glycosylation pathways in Archaea. In this contribution, the insertional inactivation of several candidate genes within the M. voltae genome and their resulting effects on flagellin and S-layer glycosylation are reported. Two of the candidate genes were shown to have effects on flagellin and S-layer protein molecular mass and N-linked glycan structure. Further examination revealed inactivation of either of these two genes also had effects on flagella assembly. These genes, designated agl (archaeal glycosylation) genes, include a glycosyl transferase (aglA) involved in the attachment of the terminal sugar to the glycan and an STT3 oligosaccharyl transferase homologue (aglB) involved in the transfer of the complete glycan to the flagellin and S-layer proteins. These findings document the first experimental evidence for genes involved in any glycosylation process within the domain Archaea.

Influence of pilin glycosylation on Pseudomonas aeruginosa 1244 pilus function

The opportunistic pathogen Pseudomonas aeruginosa is a leading cause of nosocomial pneumonia. Among its virulence factors, the type IV pili of P. aeruginosa strain 1244 contain a covalently linked, three-sugar glycan of previously unknown significance. The work described in this paper was carried out to determine the influence of the P. aeruginosa 1244 pilin glycan on pilus function, as well as a possible role in pathogenesis. To accomplish this, a deletion was introduced into the pilO gene of this organism. The isogenic knockout strain produced, 1244G7, was unable to glycosylate pilin but could produce pili normal in appearance and quantity. In addition, this strain had somewhat reduced twitching motility, was sensitive to pilus-specific bacteriophages, and could form a normal biofilm. Analysis of whole cells and isolated pili from wild-type P. aeruginosa strain 1244 by transmission electron microscopy with a glycan-specific immunogold label showed that this saccharide was distributed evenly over the fiber surface. The presence of the pilin glycan reduced the hydrophobicity of purified pili as well as whole cells. With regard to pathogenicity, P. aeruginosa strains producing glycosylated pili were commonly found among clinical isolates and particularly among those strains isolated from sputum. Competition index analysis using a mouse respiratory model comparing strains 1244 and 1244G7 indicated that the presence of the pilin glycan allowed for significantly greater survival in the lung environment. These results collectively suggest that the pilin glycan is a significant virulence factor and may aid in the establishment of infection.

Posttranslational protein modification in Archaea

One of the first hurdles to be negotiated in the postgenomic era involves the description of the entire protein content of the cell, the proteome. Such efforts are presently complicated by the various posttranslational modifications that proteins can experience, including glycosylation, lipid attachment, phosphorylation, methylation, disulfide bond formation, and proteolytic cleavage. Whereas these and other posttranslational protein modifications have been well characterized in Eucarya and Bacteria, posttranslational modification in Archaea has received far less attention. Although archaeal proteins can undergo posttranslational modifications reminiscent of what their eucaryal and bacterial counterparts experience, examination of archaeal posttranslational modification often reveals aspects not previously observed in the other two domains of life. In some cases, posttranslational modification allows a protein to survive the extreme conditions often encountered by Archaea. The various posttranslational modifications experienced by archaeal proteins, the molecular steps leading to these modifications, and the role played by posttranslational modification in Archaea form the focus of this review.

Identification and characterization of the unique N-linked glycan common to the flagellins and S-layer glycoprotein of Methanococcus voltae

The flagellum of Methanococcus voltae is composed of four structural flagellin proteins FlaA, FlaB1, FlaB2, and FlaB3. These proteins possess a total of 15 potential N-linked sequons (NX(S/T)) and show a mass shift on an SDS-polyacrylamide gel indicating significant post-translational modification. We describe here the structural characterization of the flagellin glycan from M. voltae using mass spectrometry to examine the proteolytic digests of the flagellin proteins in combination with NMR analysis of the purified glycan using a sensitive, cryogenically cooled probe. Nano-liquid chromatography-tandem mass spectrometry analysis of the proteolytic digests of the flagellin proteins revealed that they are post-translationally modified with a novel N-linked trisaccharide of mass 779 Da that is composed of three sugar residues with masses of 318, 258, and 203 Da, respectively. In every instance the glycan is attached to the peptide through the asparagine residue of a typical N-linked sequon. The glycan modification has been observed on 14 of the 15 sequon sites present on the four flagellin structural proteins. The novel glycan structure elucidated by NMR analysis was shown to be a trisaccharide composed of beta-ManpNAcA6Thr-(1-4)-beta-Glc-pNAc3NAcA-(1-3)-beta-GlcpNAc linked to Asn. In addition, the same trisaccharide was identified on a tryptic peptide of the S-layer protein from this organism implicating a common N-linked glycosylation pathway.

The HfaB and HfaD adhesion proteins of Caulobacter crescentus are localized in the stalk

The differentiating bacterium Caulobacter crescentus produces two different cell types at each cell division, a motile swarmer cell and an adhesive stalked cell. The stalked cell harbours a stalk, a thin cylindrical extension of the cell surface. The tip of the stalk is decorated with a holdfast, an adhesive organelle composed at least in part of polysaccharides. The synthesis of the stalk and holdfast occur at the same pole during swarmer cell differentiation. Mutations in the hfaABDC gene cluster had been shown to disrupt the attachment of the holdfast to the tip of the stalk, but the role of individual genes was unknown. We used lacZ fusions of various DNA fragments from the hfaABDC region to show that these genes form an operon. In order to analyse the relative contribution of the different genes to holdfast attachment, mutations were constructed for each gene. hfaC was not required for holdfast attachment or binding to surfaces. The hfaA and hfaD mutants shed some holdfast material into the surrounding medium and were partially deficient in binding to surfaces. Unlike hfaA and hfaB mutants, hfaD mutants were still able to form rosettes efficiently. Cells with insertions in hfaB were unable to bind to surfaces, and lectin binding studies indicated that the hfaB mutants had the strongest holdfast shedding phenotype. We determined that HfaB and HfaD are membrane-associated proteins and that HfaB is a lipoprotein. Purification of stalks and cell bodies indicated that both HfaB and HfaD are enriched in the stalk as compared to the cell body. These results suggest that HfaB and HfaD, and probably HfaA, serve to anchor the holdfast to the tip of the stalk.

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.

Polymorphisms in Pilin Glycosylation Locus of Neisseria meningitidis Expressing Class II Pili

We have located a locus, pgl , in Neisseria meningitidis strain NMB required for the glycosylation of class II pili. Between five and eight open reading frames (ORFs) ( pglF, pglB, pglC, pglB2, orf2, orf3, orf8 , and avtA ) were present in the pgl clusters of different meningococcal isolates. The Class I pilus-expressing strains Neisseria gonorrhoeae MS11 and N. meningitidis MC58 each contain a pgl cluster in which orf2 and orf3 have been deleted. Strain NMB and other meningococcal isolates which express class II type IV pili contained pgl clusters in which pglB had been replaced by pglB2 and an additional novel ORF, orf8 , had been inserted between pglB2 and pglC . Insertional inactivation of the eight ORFs of the pgl cluster of strain NMB showed that pglF, pglB2, pglC , and pglD, but not orf2, orf3, orf8 , and avtA , were necessary for pilin glycosylation. Pilin glycosylation was not essential for resistance to normal human serum, as pglF and pglD mutants retained wild-type levels of serum resistance. Although pglB2 and pglC mutants were significantly sensitive to normal human serum under the experimental conditions used, subsequent examination of the encapsulation phenotypes revealed that pglB2 and pglC mutants expressed almost 50% less capsule than wild-type NMB. A mutation in orf3 , which did not affect pilin glycosylation, also resulted in a 10% reduction in capsule expression and a moderately serum sensitive phenotype. On the basis of these results we suggest that pilin glycosylation may proceed via a lipid-linked oligosaccharide intermediate and that blockages in this pathway may interfere with capsular transport or assembly.

The ins(ide) and out(side) of dolichyl phosphate biosynthesis and recycling in the endoplasmic reticulum

The precursor oligosaccharide donor for protein N-glycosylation in eukaryotes, Glc3Man9GlcNAc(2)-P-P-dolichol, is synthesized in two stages on both leaflets of the rough endoplasmic reticulum (ER). There is good evidence that the level of dolichyl monophosphate (Dol-P) is one rate-controlling factor in the first stage of the assembly process. In the current topological model it is proposed that ER proteins (flippases) then mediate the transbilayer movement of Man-P-Dol, Glc-P-Dol, and Man5GlcNAc(2)-P-P-Dol from the cytoplasmic leaflet to the lumenal leaflet. The rate of flipping of the three intermediates could plausibly influence the conversion of Man5GlcNAc(2)-P-P-Dol to Glc3Man(9)GlcNAc(2)-P-P-Dol in the second stage on the lumenal side of the rough ER. This article reviews the current understanding of the enzymes involved in the de novo biosynthesis of Dol-P and other polyisoprenoid glycosyl carrier lipids and speculates about the role of membrane proteins and enzymes that could be involved in the transbilayer movement of the lipid intermediates and the recycling of Dol-P and Dol-P-P discharged during glycosylphosphatidylinositol anchor biosynthesis, N-glycosylation, and O- and C-mannosylation reactions on the lumenal surface of the rough ER.