The Iho670 fibers of Ignicoccus hospitalis: a new type of archaeal cell surface appendage

Versatile cell surface structures of archaea

Archaea are ubiquitously present in nature and colonize environments with broadly varying growth conditions. Several surface appendages support their colonization of new habitats. A hallmark of archaea seems to be the high abundance of type IV pili (T4P). However, some unique non T4 filaments are present in a number of archaeal species. Archaeal surface structures can mediate different processes such as cellular surface adhesion, DNA exchange, motility and biofilm formation and represent an initial attachment site for infecting viruses. In addition to the functionally characterized archaeal T4P, archaeal genomes encode a large number of T4P components that might form yet undiscovered surface structures with novel functions. In this review, we summarize recent advancement in structural and functional characterizations of known archaeal surface structures and highlight the diverse processes in which they play a role.

Structure and in situ organisation of the Pyrococcus furiosus archaellum machinery

The archaellum is the macromolecular machinery that Archaea use for propulsion or surface adhesion, enabling them to proliferate and invade new territories. The molecular composition of the archaellum and of the motor that drives it appears to be entirely distinct from that of the functionally equivalent bacterial flagellum and flagellar motor. Yet, the structure of the archaellum machinery is scarcely known. Using combined modes of electron cryo-microscopy (cryoEM), we have solved the structure of the Pyrococcus furiosus archaellum filament at 4.2 Å resolution and visualise the architecture and organisation of its motor complex in situ. This allows us to build a structural model combining the archaellum and its motor complex, paving the way to a molecular understanding of archaeal swimming motion.

Cryo-EM of bacterial pili and archaeal flagellar filaments

Recent advances in cryo-electron microscopy (cryo-EM) have opened up the possibility that a large class of biological structures, helical polymers, may now be readily reconstructed at near-atomic resolution. This will have a huge impact, since most of these structures are unlikely to be crystallized. This review focuses on new cryo-EM studies involving three classes of bacterial pili (chaperone-usher, mating, and Type IV) as well as on archaeal flagellar filaments. While it has long been known that one domain within archaeal flagellar filaments is homologous to a domain within bacterial Type IV pilins, the new studies shed light on how homologous and even highly conserved subunits can pack together in different ways with only small changes in sequence.

Archaeal flagellin combines a bacterial type IV pilin domain with an Ig-like domain

The bacterial flagellar apparatus, which involves ∼40 different proteins, has been a model system for understanding motility and chemotaxis. The bacterial flagellar filament, largely composed of a single protein, flagellin, has been a model for understanding protein assembly. This system has no homology to the eukaryotic flagellum, in which the filament alone, composed of a microtubule-based axoneme, contains more than 400 different proteins. The archaeal flagellar system is simpler still, in some cases having ∼13 different proteins with a single flagellar filament protein. The archaeal flagellar system has no homology to the bacterial one and must have arisen by convergent evolution. However, it has been understood that the N-terminal domain of the archaeal flagellin is a homolog of the N-terminal domain of bacterial type IV pilin, showing once again how proteins can be repurposed in evolution for different functions. Using cryo-EM, we have been able to generate a nearly complete atomic model for a flagellar-like filament of the archaeon Ignicoccus hospitalis from a reconstruction at ∼4-Å resolution. We can now show that the archaeal flagellar filament contains a β-sandwich, previously seen in the FlaF protein that forms the anchor for the archaeal flagellar filament. In contrast to the bacterial flagellar filament, where the outer globular domains make no contact with each other and are not necessary for either assembly or motility, the archaeal flagellin outer domains make extensive contacts with each other that largely determine the interesting mechanical properties of these filaments, allowing these filaments to flex.

Pyrococcus furiosus flagella: biochemical and transcriptional analyses identify the newly detected flaB0 gene to encode the major flagellin

We have described previously that the flagella of the Euryarchaeon Pyrococcus furiosus are multifunctional cell appendages used for swimming, adhesion to surfaces and formation of cell-cell connections. Here, we characterize these organelles with respect to their biochemistry and transcription. Flagella were purified by shearing from cells followed by CsCl-gradient centrifugation and were found to consist mainly of a ca. 30 kDa glycoprotein. Polymerization studies of denatured flagella resulted in an ATP-independent formation of flagella-like filaments. The N-terminal sequence of the main flagellin was determined by Edman degradation, but none of the genes in the complete genome code for a protein with that N-terminus. Therefore, we resequenced the respective region of the genome, thereby discovering that the published genome sequence is not correct. A total of 771 bp are missing in the data base, resulting in the correction of the previously unusual N-terminal sequence of flagellin FlaB1 and in the identification of a third flagellin. To keep in line with the earlier nomenclature we call this flaB0. Very interestingly, the previously not identified flaB0 codes for the major flagellin. Transcriptional analyses of the revised flagellar operon identified various different cotranscripts encoding only a single protein in case of FlaB0 and FlaJ or up to five proteins (FlaB0-FlaD). Analysing the RNA of cells from different growth phases, we found that the length and number of detected cotranscript increased over time suggesting that the flagellar operon is transcribed mostly in late exponential and stationary growth phase.

The Iho670 fibers of Ignicoccus hospitalis are anchored in the cell by a spherical structure located beneath the inner membrane

The Iho670 fibers of the hyperthermophilic crenarchaeon of Ignicoccus hospitalis were shown to contain several features that indicate them as type IV pilus-like structures. The application of different visualization methods, including electron tomography and the reconstruction of a three-dimensional model, enabled a detailed description of a hitherto undescribed anchoring structure of the cell appendages. It could be identified as a spherical structure beneath the inner membrane. Furthermore, pools of the fiber protein Iho670 could be localized in the inner as well as the outer cellular membrane of I. hospitalis cells and in the tubes/vesicles in the intermembrane compartment by immunological methods.

The temperature gradient-forming device, an accessory unit for normal light microscopes to study the biology of hyperthermophilic microorganisms

To date, the behavior of hyperthermophilic microorganisms in their biotope has been studied only to a limited degree; this is especially true for motility. One reason for this lack of knowledge is the requirement for high-temperature microscopy-combined, in most cases, with the need for observations under strictly anaerobic conditions-for such studies. We have developed a custom-made, low-budget device that, for the first time, allows analyses in temperature gradients up to 40°C over a distance of just 2 cm (a biotope-relevant distance) with heating rates up to ∼5°C/s. Our temperature gradient-forming device can convert any upright light microscope into one that works at temperatures as high as 110°C. Data obtained by use of this apparatus show how very well hyperthermophiles are adapted to their biotope: they can react within seconds to elevated temperatures by starting motility-even after 9 months of storage in the cold. Using the temperature gradient-forming device, we determined the temperature ranges for swimming, and the swimming speeds, of 15 selected species of the genus Thermococcus within a few months, related these findings to the presence of cell surface appendages, and obtained the first evidence for thermotaxis in Archaea.

Life on the edge: functional genomic response of Ignicoccus hospitalis to the presence of Nanoarchaeum equitans

The marine hyperthermophilic crenarchaeon Ignicoccus hospitalis supports the propagation on its surface of Nanoarchaeum equitans, an evolutionarily enigmatic archaeon that resembles highly derived parasitic and symbiotic bacteria. The cellular and molecular mechanisms that enable this interarchaea relationship and the intimate physiologic consequences to I. hospitalis are unknown. Here, we used concerted proteomic and transcriptomic analyses to probe into the functional genomic response of I. hospitalis as N. equitans multiplies on its surface. The expression of over 97% of the genes was detected at mRNA level and over 80% of the predicted proteins were identified and their relative abundance measured by proteomics. These indicate that little, if any, of the host genomic information is silenced during growth in the laboratory. The primary response to N. equitans was at the membrane level, with increases in relative abundance of most protein complexes involved in energy generation as well as that of several transporters and proteins involved in cellular membrane stabilization. Similar upregulation was observed for genes and proteins involved in key metabolic steps controlling nitrogen and carbon metabolism, although the overall biosynthetic pathways were marginally impacted. Proliferation of N. equitans resulted, however, in selective downregulation of genes coding for transcription factors and replication and cell cycle control proteins as I. hospitalis shifted its physiology from its own cellular growth to that of its ectosymbiont/parasite. The combination of these multiomic approaches provided an unprecedented level of detail regarding the dynamics of this interspecies interaction, which is especially pertinent as these organisms are not genetically tractable.

Genetic analysis of a type IV pili-like locus in the archaeon Methanococcus maripaludis

Methanococcus maripaludis is a stringently anaerobic archaeon with two studied surface structures, archaella and type IV pili. Previously, it was shown that three pilin genes (mmp0233 [epdA], mmp0236 [epdB] and mmp0237 [epdC]) located within an 11 gene cluster in the genome were necessary for normal piliation. This study focused on analysis of the remaining genes to determine their potential involvement in piliation. Reverse transcriptase PCR experiments demonstrated the 11 genes formed a single transcriptional unit. Deletions were made in all the non-pilin genes except mmp0231. Electron microscopy revealed that all the genes in the locus except mmp0235 and mmp0238 were essential for piliation. Complementation with a plasmid-borne wild-type copy of the deleted gene restored at least some piliation. We identified genes for an assembly ATPase and two versions of the conserved pilin platform forming protein necessary for pili assembly at a separate genetic locus.

Identification of an additional minor pilin essential for piliation in the archaeon Methanococcus maripaludis

Methanococcus maripaludis is an archaeon with two studied surface appendages, archaella and type IV-like pili. Previously, the major structural pilin was identified as MMP1685 and three additional proteins were designated as minor pilins (EpdA, EpdB and EpdC). All of the proteins are likely processed by the pilin-specific prepilin peptidase EppA. Six other genes were identified earlier as likely encoding pilin proteins processed also by EppA. In this study, each of the six genes (mmp0528, mmp0600, mmp0601, mmp0709, mmp0903 and mmp1283) was deleted and the mutants examined by electron microscopy to determine their essentiality for pili formation. While mRNA transcripts of all genes were detected by RT-PCR, only the deletion of mmp1283 led to nonpiliated cells. This strain could be complemented back to a piliated state by supplying a wildtype copy of the mmp1283 gene in trans. This study adds to the complexity of the type IV pili system in M. maripaludis and raises questions about the functions of the remaining five pilin-like genes and whether M. maripaludis under other growth conditions may be able to assemble additional pili-like structures.

Surface appendages of archaea: structure, function, genetics and assembly

Organisms representing diverse subgroupings of the Domain Archaea are known to possess unusual surface structures. These can include ones unique to Archaea such as cannulae and hami as well as archaella (archaeal flagella) and various types of pili that superficially resemble their namesakes in Bacteria, although with significant differences. Major advances have occurred particularly in the study of archaella and pili using model organisms with recently developed advanced genetic tools. There is common use of a type IV pili-model of assembly for several archaeal surface structures including archaella, certain pili and sugar binding structures termed bindosomes. In addition, there are widespread posttranslational modifications of archaellins and pilins with N-linked glycans, with some containing novel sugars. Archaeal surface structures are involved in such diverse functions as swimming, attachment to surfaces, cell to cell contact resulting in genetic transfer, biofilm formation, and possible intercellular communication. Sometimes functions are co-dependent on other surface structures. These structures and the regulation of their assembly are important features that allow various Archaea, including thermoacidophilic, hyperthermophilic, halophilic, and anaerobic ones, to survive and thrive in the extreme environments that are commonly inhabited by members of this domain.

Diversity, assembly and regulation of archaeal type IV pili-like and non-type-IV pili-like surface structures

Archaea have evolved fascinating surface structures allowing rapid adaptation to changing environments. The archaeal surface appendages display such diverse biological roles as motility, adhesion, biofilm formation, exchange of genetic material and species-specific interactions and, in turn, increase fitness of the cells. Intriguingly, despite sharing the same functions with their bacterial counterparts, the assembly mechanism of many archaeal surface structures is rather related to assembly of bacterial type IV pili. This review summarizes our state-of-the-art knowledge about unique structural and biochemical properties of archaeal surface appendages with a particular focus on archaeal type IV pili-like structures. The latter comprise not only widely distributed archaella (formerly known as archaeal flagella), but also different highly specialized archaeal pili, which are often restricted to certain species. Recent findings regarding assembly mechanisms, structural aspects and physiological roles of these type IV pili-like structures will be discussed in detail. Recently, first regulatory proteins involved in transition from both planktonic to sessile lifestyle and in assembly of archaella were identified. To conclude, we provide novel insights into regulatory mechanisms underlying the assembly of archaeal surface structures.

Structure and function of the adhesive type IV pilus of Sulfolobus acidocaldarius

Archaea display a variety of type IV pili on their surface and employ them in different physiological functions. In the crenarchaeon Sulfolobus acidocaldarius the most abundant surface structure is the aap pilus (archaeal adhesive pilus). The construction of in frame deletions of the aap genes revealed that all the five genes (aapA, aapX, aapE, aapF, aapB) are indispensible for assembly of the pilus and an impact on surface motility and biofilm formation was observed. Our analyses revealed that there exists a regulatory cross-talk between the expression of aap genes and archaella (formerly archaeal flagella) genes during different growth phases. The structure of the aap pilus is entirely different from the known bacterial type IV pili as well as other archaeal type IV pili. An aap pilus displayed 3 stranded helices where there is a rotation per subunit of ∼138° and a rise per subunit of ∼5.7 Å. The filaments have a diameter of ∼110 Å and the resolution was judged to be ∼9 Å. We concluded that small changes in sequence might be amplified by large changes in higher-order packing. Our finding of an extraordinary stability of aap pili possibly represents an adaptation to harsh environments that S. acidocaldarius encounters.

Breaking through a phylogenetic impasse: a pair of associated archaea might have played host in the endosymbiotic origin of eukaryotes

For over a century, the origin of eukaryotes has been a topic of intense debate among scientists. Although it has become widely accepted that organelles such as the mitochondria and chloroplasts arose via endosymbiosis, the origin of the eukaryotic nucleus remains enigmatic. Numerous models for the origin of the nucleus have been proposed over the years, many of which use endosymbiosis to explain its existence. Proposals of microbes whose ancestors may have served as either a host or a guest in various endosymbiotic scenarios abound, none of which have been able to sufficiently incorporate the cell biological as well as phylogenetic data which links these organisms to the nucleus. While it is generally agreed that eukaryotic nuclei share more features in common with archaea rather than with bacteria, different studies have identified either one or the other of the two major groups of archaea as potential ancestors, leading to somewhat of a stalemate. This paper seeks to resolve this impasse by presenting evidence that not just one, but a pair of archaea might have served as host to the bacterial ancestor of the mitochondria. This pair may have consisted of ancestors of both Ignicoccus hospitalis as well as its ectosymbiont/ectoparasite ‘Nanoarchaeum equitans’.

Control of archaellation in Sulfolobus acidocaldarius: unravelling of the regulation of surface structure biosynthesis in Archaea begins

Archaea have a variety of surface appendages including archaella (archaeal flagella), pili, hami and cannulae. While expected to be energetically expensive to express, studies focused on the regulation of such structures are nevertheless lacking. In the current issue of Molecular Microbiology, Reimann et al. (2012) identified a two-partner system called ArnA and ArnB in Sulfolobus acidocaldarius that interact strongly with each other and are repressors of archaella expression while also having an enhancing effect on the appearance of type IV pili. ArnA is a forkhead-associated domain-containing protein while ArnB is a von Willebrand domain-containing protein. Both proteins can be phosphorylated in vitro by S. acidocaldarius protein kinases. The repression of archaella expression is dependent on dephosphorylation of the Arn proteins. Deletions of arnA or arnB resulted in increased levels of archaella operon proteins and cells that were hypermotile due to increased archaellation. Direct effects of ArnA/ArnB on transcription from fla promoters were demonstrated using arnA and arnB deletion strains but only a modest increase in transcription was demonstrated in each mutant suggesting that the repression effect observed may be due to protein-protein interactions. This paper represents a significant step forward in our understanding of archaeal surface structure biogenesis.

Response to Jarrell and Albers: seven letters less does not say more

Jarrell and Albers [1] argue that motility structures called flagella should be renamed as archaella in one of the three domains of life, namely the Archaea. In their text the authors present an excellent comparison of flagella from the two domains Bacteria and Archaea (the third domain, Eukarya, is covered less completely). The new name archaellum, however, does not give any new information on the motility structure itself. I argue that the substitution of the phrase 'archaeal flagellum' by 'archaellum' does not lead to more clarity, but will lead to confusion: seven letters less does not say more!

Diversity and subcellular distribution of archaeal secreted proteins

Secreted proteins make up a significant percentage of a prokaryotic proteome and play critical roles in important cellular processes such as polymer degradation, nutrient uptake, signal transduction, cell wall biosynthesis, and motility. The majority of archaeal proteins are believed to be secreted either in an unfolded conformation via the universally conserved Sec pathway or in a folded conformation via the Twin arginine transport (Tat) pathway. Extensive in vivo and in silico analyses of N-terminal signal peptides that target proteins to these pathways have led to the development of computational tools that not only predict Sec and Tat substrates with high accuracy but also provide information about signal peptide processing and targeting. Predictions therefore include indications as to whether a substrate is a soluble secreted protein, a membrane or cell wall anchored protein, or a surface structure subunit, and whether it is targeted for post-translational modification such as glycosylation or the addition of a lipid. The use of these in silico tools, in combination with biochemical and genetic analyses of transport pathways and their substrates, has resulted in improved predictions of the subcellular localization of archaeal secreted proteins, allowing for a more accurate annotation of archaeal proteomes, and has led to the identification of potential adaptations to extreme environments, as well as phyla-specific pathways among the archaea. A more comprehensive understanding of the transport pathways used and post-translational modifications of secreted archaeal proteins will also facilitate the identification and heterologous expression of commercially valuable archaeal enzymes.

The unusual cell biology of the hyperthermophilic Crenarchaeon Ignicoccus hospitalis

The Crenarchaeon Ignicoccus hospitalis is an anaerobic, obligate chemolithoautotrophic hyperthermophile, growing by reduction of elemental sulfur using molecular hydrogen as electron donor. Together with Nanoarchaeum equitans it forms a unique, archaeal biocoenosis, in which I. hospitalis serves as host for N. equitans. Both organisms can be cultivated in a stable coculture which is mandatory for N. equitans but not for I. hospitalis. This strong dependence is affirmed by the fact that N. equitans obtains its lipids and amino acids from the host. I. hospitalis cells exhibit several unique features: they can adhere to surfaces by extracellular appendages ('fibers') which are not used for motility; they use a novel CO(2) fixation pathway, the dicarboxylate/4-hydroxybutyrate pathway; and they exhibit a unique cell envelope for Archaea consisting of two membranes but lacking an S-layer. These membranes form two cell compartments, a tightly packed cytoplasm surrounded by a weakly staining intermembrane compartment (IMC) with a variable width from 20 to 1,000 nm. In this IMC, many round or elongated vesicles are found which may function as carriers of lipids or proteins out of the cytoplasm. Based on immuno-EM analyses and immuno-fluorescence experiments it was demonstrated recently that the A(1)A(O) ATP synthase, the H(2):sulfur oxidoreductase complex and the acetyl-CoA synthetase (ACS) of I. hospitalis are located in its outermost membrane. Therefore, this membrane is energized and is here renamed as "outer cellular membrane" (OCM). Among all prokaryotes possessing two membranes in their cell envelope, I. hospitalis is the first organism with an energized outermost membrane and ATP synthesis outside the cytoplasm. Since DNA and ribosomes are localized in the cytoplasm, energy conservation is separated from information processing and protein biosynthesis in I. hospitalis. This raises questions concerning the function and characterization of the two membranes, the two cell compartments and of a possible ATP transfer to N. equitans.

Filaments from Ignicoccus hospitalis show diversity of packing in proteins containing N-terminal type IV pilin helices

Bacterial motility is driven by the rotation of flagellar filaments that supercoil. The supercoiling involves the switching of coiled-coil protofilaments between two different states. In archaea, the flagellar filaments responsible for motility are formed by proteins with distinct homology in their N-terminal portion to bacterial Type IV pilins. The bacterial pilins have a single N-terminal hydrophobic α-helix, not the coiled coil found in flagellin. We have used electron cryo-microscopy to study the adhesion filaments from the archaeon Ignicoccus hospitalis. While I. hospitalis is non-motile, these filaments make transitions between rigid stretches and curved regions and appear morphologically similar to true archaeal flagellar filaments. A resolution of ~7.5Å allows us to unambiguously build a model for the packing of these N-terminal α-helices, and this packing is different from several bacterial Type IV pili whose structure has been analyzed by electron microscopy and modeling. Our results show that the mechanism responsible for the supercoiling of bacterial flagellar filaments cannot apply to archaeal filaments.

Proteomic characterization of cellular and molecular processes that enable the Nanoarchaeum equitans--Ignicoccus hospitalis relationship

Nanoarchaeum equitans, the only cultured representative of the Nanoarchaeota, is dependent on direct physical contact with its host, the hyperthermophile Ignicoccus hospitalis. The molecular mechanisms that enable this relationship are unknown. Using whole-cell proteomics, differences in the relative abundance of >75% of predicted protein-coding genes from both Archaea were measured to identify the specific response of I. hospitalis to the presence of N. equitans on its surface. A purified N. equitans sample was also analyzed for evidence of interspecies protein transfer. The depth of cellular proteome coverage achieved here is amongst the highest reported for any organism. Based on changes in the proteome under the specific conditions of this study, I. hospitalis reacts to N. equitans by curtailing genetic information processing (replication, transcription) in lieu of intensifying its energetic, protein processing and cellular membrane functions. We found no evidence of significant Ignicoccus biosynthetic enzymes being transported to N. equitans. These results suggest that, under laboratory conditions, N. equitans diverts some of its host's metabolism and cell cycle control to compensate for its own metabolic shortcomings, thus appearing to be entirely dependent on small, transferable metabolites and energetic precursors from I. hospitalis.

The mode of cell wall growth in selected archaea is similar to the general mode of cell wall growth in bacteria as revealed by fluorescent dye analysis

The surfaces of 8 bacterial and 23 archaeal species, including many hyperthermophilic Archaea, could be stained using succinimidyl esters of fluorescent dyes. This allowed us for the first time to analyze the mode of cell wall growth in Archaea by subculturing stained cells. The data obtained show that incorporation of new cell wall material in Archaea follows the pattern observed for Bacteria: in the coccoid species Pyrococcus furiosus incorporation was in the region of septum formation while for the rod-shaped species Methanopyrus kandleri and Methanothermus sociabilis, a diffuse incorporation of cell wall material over the cell length was observed. Cell surface appendages like fimbriae/pili, fibers, or flagella were detectable by fluorescence staining only in a very few cases although their presence was proven by electron microscopy. Our data in addition prove that Alexa Fluor dyes can be used for in situ analyses at temperatures up to 100°C.

Genetic and mass spectrometry analyses of the unusual type IV-like pili of the archaeon Methanococcus maripaludis

The structure of pili from the archaeon Methanococcus maripaludis is unlike that of any bacterial pili. However, genetic analysis of the genes involved in the formation of these pili has been lacking until this study. Pili were isolated from a nonflagellated (ΔflaK) mutant and shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to consist primarily of subunits with an apparent molecular mass of 17 kDa. In-frame deletions were created in three genes, MMP0233, MMP0236, and MMP0237, which encode proteins with bacterial type IV pilin-like signal peptides previously identified by in silico methodology as likely candidates for pilus structural proteins. Deletion of MMP0236 or MMP0237 resulted in mutant cells completely devoid of pili on the cell surface, while deletion of the third pilin-like gene, MMP0233, resulted in cells greatly reduced in the number of pili on the surface. Complementation with the deleted gene in each case returned the cells to a piliated state. Surprisingly, mass spectrometry analysis of purified pili identified the major structural pilin as another type IV pilin-like protein, MMP1685, whose gene is located outside the first pilus locus. This protein was found to be glycosylated with an N-linked branched pentasaccharide glycan. Deletion and complementation analysis confirmed that MMP1685 is required for piliation.

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.

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.

Shaping the archaeal cell envelope

Although archaea have a similar cellular organization as other prokaryotes, the lipid composition of their membranes and their cell surface is unique. Here we discuss recent developments in our understanding of the archaeal protein secretion mechanisms, the assembly of macromolecular cell surface structures, and the release of S-layer-coated vesicles from the archaeal membrane.

Analysis of the ultrastructure of archaea by electron microscopy

The ultrastructural characterization of archaeal cells is done with both types of electron microscopy, transmission electron microscopy, and scanning electron microscopy. Depending on the scientific question, different preparation methods have to be employed and need to be optimized, according to the special cultivation conditions of these-in many cases extreme-microorganisms. Recent results using various electron microscopy techniques show that archaeal cells have a variety of cell appendages, used for motility as well as for establishing cell-cell and cell-surface contacts. Cryo-preparation methods, in particular high-pressure freezing and freeze-substitution, are crucial for obtaining results: (1) showing the cells in ultrathin sections in a good structural preservation, often with unusual shapes and subcellular complexity, and (2) enabling us to perform immunolocalization studies. This is an important tool to make a link between biochemical and ultrastructural studies.