The archaellum: an old motility structure with a new name

Perturbed N-glycosylation of Halobacterium salinarum archaellum filaments leads to filament bundling and compromised cell motility

The swimming device of archaea-the archaellum-presents asparagine (N)-linked glycans. While N-glycosylation serves numerous roles in archaea, including enabling their survival in extreme environments, how this post-translational modification contributes to cell motility remains under-explored. Here, we report the cryo-EM structure of archaellum filaments from the haloarchaeon Halobacterium salinarum, where archaellins, the building blocks of the archaellum, are N-glycosylated, and the N-glycosylation pathway is well-resolved. We further determined structures of archaellum filaments from two N-glycosylation mutant strains that generate truncated glycans and analyzed their motility. While cells from the parent strain exhibited unidirectional motility, the N-glycosylation mutant strain cells swam in ever-changing directions within a limited area. Although these mutant strain cells presented archaellum filaments that were highly similar in architecture to those of the parent strain, N-linked glycan truncation greatly affected interactions between archaellum filaments, leading to dramatic clustering of both isolated and cell-attached filaments. We propose that the N-linked tetrasaccharides decorating archaellins act as physical spacers that minimize the archaellum filament aggregation that limits cell motility.

Mechanisms of microbial co-aggregation in mixed anaerobic cultures

Co-aggregation of anaerobic microorganisms into suspended microbial biofilms (aggregates) serves ecological and biotechnological functions. Tightly packed aggregates of metabolically interdependent bacteria and archaea play key roles in cycling of carbon and nitrogen. Additionally, in biotechnological applications, such as wastewater treatment, microbial aggregates provide a complete metabolic network to convert complex organic material. Currently, experimental data explaining the mechanisms behind microbial co-aggregation in anoxic environments is scarce and scattered across the literature. To what extent does this process resemble co-aggregation in aerobic environments? Does the limited availability of terminal electron acceptors drive mutualistic microbial relationships, contrary to the commensal relationships observed in oxygen-rich environments? And do co-aggregating bacteria and archaea, which depend on each other to harvest the bare minimum Gibbs energy from energy-poor substrates, use similar cellular mechanisms as those used by pathogenic bacteria that form biofilms? Here, we provide an overview of the current understanding of why and how mixed anaerobic microbial communities co-aggregate and discuss potential future scientific advancements that could improve the study of anaerobic suspended aggregates. KEY POINTS: • Metabolic dependency promotes aggregation of anaerobic bacteria and archaea • Flagella, pili, and adhesins play a role in the formation of anaerobic aggregates • Cyclic di-GMP/AMP signaling may trigger the polysaccharides production in anaerobes.

Relevance of extracellular electron uptake mechanisms for electromethanogenesis applications

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

Adhesion pilus retraction powers twitching motility in the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius

Type IV pili are filamentous appendages found in most bacteria and archaea, where they can support functions such as surface adhesion, DNA uptake, aggregation, and motility. In most bacteria, PilT-family ATPases disassemble adhesion pili, causing them to rapidly retract and produce twitching motility, important for surface colonization. As archaea do not possess PilT homologs, it was thought that archaeal pili cannot retract and that archaea do not exhibit twitching motility. Here, we use live-cell imaging, automated cell tracking, fluorescence imaging, and genetic manipulation to show that the hyperthermophilic archaeon Sulfolobus acidocaldarius exhibits twitching motility, driven by retractable adhesion (Aap) pili, under physiologically relevant conditions (75 °C, pH 2). Aap pili are thus capable of retraction in the absence of a PilT homolog, suggesting that the ancestral type IV pili in the last universal common ancestor (LUCA) were capable of retraction.

Sulfolobus acidocaldarius adhesion pili power twitching motility in the absence of a dedicated retraction ATPase

Type IV pili are ancient and widespread filamentous organelles found in most bacterial and archaeal phyla where they support a wide range of functions, including substrate adhesion, DNA uptake, self aggregation, and cell motility. In most bacteria, PilT-family ATPases disassemble adhesion pili, causing them to rapidly retract and produce twitching motility, important for surface colonization. As archaea do not possess homologs of PilT, it was thought that archaeal pili cannot retract. Here, we employ live-cell imaging under native conditions (75°C and pH 2), together with automated single-cell tracking, high-temperature fluorescence imaging, and genetic manipulation to demonstrate that S. acidocaldarius exhibits bona fide twitching motility, and that this behavior depends specifically on retractable adhesion pili. Our results demonstrate that archaeal adhesion pili are capable of retraction in the absence of a PilT retraction ATPase and suggests that the ancestral type IV pilus machinery in the last universal common ancestor (LUCA) relied on such a bifunctional ATPase for both extension and retraction.

The Hypothesis of a "Living Pulse" in Cells

Motility is a great biosignature and its pattern is characteristic for specific microbes. However, motion does also occur within the cell by the myriads of ongoing processes within the cell and the exchange of gases and nutrients with the outside environment. Here, we propose that the sum of these processes in a microbial cell is equivalent to a pulse in complex organisms and suggest a first approach to measure the "living pulse" in microorganisms. We emphasize that if a "living pulse" can be shown to exist, it would have far-reaching applications, such as for finding life in extreme environments on Earth and in extraterrestrial locations, as well as making sure that life is not present where it should not be, such as during medical procedures and in the food processing industry.

Manesh MJ, Sivabalasarma S, Albers SV, Kelly RM. Stay or Go: Sulfolobales Biofilm Dispersal Is Dependent on a Bifunctional VapB Antitoxin

A type II VapB14 antitoxin regulates biofilm dispersal in the archaeal thermoacidophile Sulfolobus acidocaldarius through traditional toxin neutralization but also through noncanonical transcriptional regulation. Type II VapC toxins are ribonucleases that are neutralized by their proteinaceous cognate type II VapB antitoxin. VapB antitoxins have a flexible tail at their C terminus that covers the toxin's active site, neutralizing its activity. VapB antitoxins also have a DNA-binding domain at their N terminus that allows them to autorepress not only their own promoters but also distal targets. VapB14 antitoxin gene deletion in S. acidocaldarius stunted biofilm and planktonic growth and increased motility structures (archaella). Conversely, planktonic cells were devoid of archaella in the ΔvapC14 cognate toxin mutant. VapB14 is highly conserved at both the nucleotide and amino acid levels across the Sulfolobales, extremely unusual for type II antitoxins, which are typically acquired through horizontal gene transfer. Furthermore, homologs of VapB14 are found across the Crenarchaeota, in some Euryarchaeota, and even bacteria. S. acidocaldarius vapB14 and its homolog in the thermoacidophile Metallosphaera sedula (Msed_0871) were both upregulated in biofilm cells, supporting the role of the antitoxin in biofilm regulation. In several Sulfolobales species, including M. sedula, homologs of vapB14 and vapC14 are not colocalized. Strikingly, Sulfuracidifex tepidarius has an unpaired VapB14 homolog and lacks a cognate VapC14, illustrating the toxin-independent conservation of the VapB14 antitoxin. The findings here suggest that a stand-alone VapB-type antitoxin was the product of selective evolutionary pressure to influence biofilm formation in these archaea, a vital microbial community behavior. IMPORTANCE Biofilms allow microbes to resist a multitude of stresses and stay proximate to vital nutrients. The mechanisms of entering and leaving a biofilm are highly regulated to ensure microbial survival, but are not yet well described in archaea. Here, a VapBC type II toxin-antitoxin system in the thermoacidophilic archaeon Sulfolobus acidocaldarius was shown to control biofilm dispersal through a multifaceted regulation of the archaeal motility structure, the archaellum. The VapC14 toxin degrades an RNA that causes an increase in archaella and swimming. The VapB14 antitoxin decreases archaella and biofilm dispersal by binding the VapC14 toxin and neutralizing its activity, while also repressing the archaellum genes. VapB14-like antitoxins are highly conserved across the Sulfolobales and respond similarly to biofilm growth. In fact, VapB14-like antitoxins are also found in other archaea, and even in bacteria, indicating an evolutionary pressure to maintain this protein and its role in biofilm formation.

Mechanism of assembly of type 4 filaments: everything you always wanted to know (but were afraid to ask)

Type 4 filaments (T4F) are a superfamily of filamentous nanomachines - virtually ubiquitous in prokaryotes and functionally versatile - of which type 4 pili (T4P) are the defining member. T4F are polymers of type 4 pilins, assembled by conserved multi-protein machineries. They have long been an important topic for research because they are key virulence factors in numerous bacterial pathogens. Our poor understanding of the molecular mechanisms of T4F assembly is a serious hindrance to the design of anti-T4F therapeutics. This review attempts to shed light on the fundamental mechanistic principles at play in T4F assembly by focusing on similarities rather than differences between several (mostly bacterial) T4F. This holistic approach, complemented by the revolutionary ability of artificial intelligence to predict protein structures, led to an intriguing mechanistic model of T4F assembly.

Comparative Genomic Insights into Chemoreceptor Diversity and Habitat Adaptation of Archaea

Archaea are capable of sensing and responding to environmental changes by several signal transduction systems with different mechanisms. Much attention is paid to model organisms with complex signaling networks to understand their composition and function, but general principles regarding how an archaeal species organizes its chemoreceptor diversity and habitat adaptation are poorly understood.

Convergent evolution in the supercoiling of prokaryotic flagellar filaments

The supercoiling of bacterial and archaeal flagellar filaments is required for motility. Archaeal flagellar filaments have no homology to their bacterial counterparts and are instead homologs of bacterial type IV pili. How these prokaryotic flagellar filaments, each composed of thousands of copies of identical subunits, can form stable supercoils under torsional stress is a fascinating puzzle for which structural insights have been elusive. Advances in cryoelectron microscopy (cryo-EM) make it now possible to directly visualize the basis for supercoiling, and here, we show the atomic structures of supercoiled bacterial and archaeal flagellar filaments. For the bacterial flagellar filament, we identify 11 distinct protofilament conformations with three broad classes of inter-protomer interface. For the archaeal flagellar filament, 10 protofilaments form a supercoil geometry supported by 10 distinct conformations, with one inter-protomer discontinuity creating a seam inside of the curve. Our results suggest that convergent evolution has yielded stable superhelical geometries that enable microbial locomotion.

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.

Comparative Genomics of Thaumarchaeota From Deep-Sea Sponges Reveal Their Niche Adaptation

Thaumarchaeota account for a large portion of microbial symbionts in deep-sea sponges and are even dominant in some cases. In this study, we investigated three new sponge-associated Thaumarchaeota from the deep West Pacific Ocean. Thaumarchaeota were found to be the most dominant phylum in this sponge by both prokaryotic 16S rRNA amplicons and metagenomic sequencing. Fifty-seven published Thaumarchaeota genomes from sponges and other habitats were included for genomic comparison. Similar to shallow sponge-associated Thaumarchaeota, those Thaumarchaeota in deep-sea sponges have extended genome sizes and lower coding density compared with their free-living lineages. Thaumarchaeota in deep-sea sponges were specifically enriched in genes related to stress adapting, symbiotic adhesion and stability, host–microbe interaction and protein transportation. The genes involved in defense mechanisms, such as the restriction-modification system, clustered regularly interspaced short palindromic repeats (CRISPR)/Cas system, and toxin-antitoxin system were commonly enriched in both shallow and deep sponge-associated Thaumarchaeota. Our study demonstrates the significant effects of both depth and symbiosis on forming genomic characteristics of Thaumarchaeota, and provides novel insights into their niche adaptation in deep-sea sponges.

Deeper insights into the effects of substrate to inoculum ratio selection on the relationship of kinetic parameters, microbial communities, and key metabolic pathways during the anaerobic digestion of food waste

The substrate to inoculum ratio (S/I) is a crucial factor that affects not only the stability of the anaerobic digestion (AD) of food waste (FW) but also the methanogenic capacity of the substrate. This is of great significance for the start-up of small-scale batch reactors and the directional regulation of methanogenesi and organic acid production. Most studies have merely clarified the optimal S/I ratio for methane production and revealed the basic composition of microbial communities. However, the mechanism of microbial interactions and the metabolic pathways behind the optimal S/I ratio still remain unclear. Herein, the effects of different S/I ratios (VS basis) on the relationship of kinetic parameters, microbial communities, and metabolic pathways during the AD process of FW were holistically explored. The results revealed that high S/I ratios (4:1, 3:1, 2:1, and 1:1) were prone to irreversible acidification, while low S/I ratios (1:2, 1:3, and 1:4) were favorable for methanogenesis. Moreover, a kinetic analysis demonstrated that the methane yield of S/I = 1:3 were the highest. A bioinformatics analysis found that the diversity of bacteria and archaea of S/I = 1:3 were the most abundant, and the enrichment of Bacteroides and Synergistetes could help to establish a syntrophic relationship with hydrogenotrophic methanogens, which could aid in the fulfillment of a unique niche in the system. In contrast to the findings with the other S/I ratios, the cooperation among microbes in S/I = 1:3 was more apparent. Notably, the abundances of genes encoding key enzymes involved in the methanogenesis pathway under S/I = 1:3 were all the highest. This knowledge will be helpful for revealing the influence mechanism of the ratio relationship between microorganisms and substrates on the biochemical metabolic process of anaerobic digestion, thereby providing effective guidance for the directional regulation of FW batch anaerobic reactors.

Characterizing the N- and O-linked glycans of the PGF-CTERM sorting domain-containing S-layer protein of Methanoculleus marisnigri

The glycosylation of structural proteins is a widespread posttranslational modification in Archaea. Although only a handful of archaeal N-glycan structures have been determined to date, it is evident that the diversity of structures expressed is greater than in the other domains of life. Here, we report on our investigation of the N- and O-glycan modifications expressed by Methanoculleus marisnigri, a mesophilic methanogen from the Order Methanomicrobiales. Unusually, mass spectrometry (MS) analysis of purified archaella revealed no evidence for N- or O-glycosylation of the constituent archaellins, In contrast, the S-layer protein, identified as a PGF-CTERM sorting domain-containing protein encoded by MEMAR_RS02690, is both N- and O-glycosylated. Two N-glycans were identified by NMR and MS analysis: a trisaccharide α-GlcNAc-4-β-GlcNAc3NGaAN-4-β-Glc-Asn where the second residue is 2-N-acetyl, 3-N-glyceryl-glucosamide and a disaccharide β-GlcNAc3NAcAN-4-β-Glc-Asn, where the terminal residue is 2,3 di-N-acetyl-glucosamide. The same trisaccharide was also found N-linked to a type IV pilin. The S-layer protein is also extensively modified in the threonine-rich region near the C-terminus with O-glycans composed exclusively of hexoses. While the S-layer protein has a predicted PGF-CTERM processing site, no evidence of a truncated and lipidated C-terminus, the expected product of processing by an archaeosortase, was found. Finally, NMR also identified a polysaccharide expressed by M. marisnigri and composed of a repeating tetrasaccharide unit of [-2-β-Ribf-3-α-Rha2OMe-3-α-Rha - 2-α-Rha-]. This is the first report of N- and O-glycosylation in an archaeon from the Order Methanomicrobiales.

The importance of biofilm formation for cultivation of a Micrarchaeon and its interactions with its Thermoplasmatales host

Micrarchaeota is a distinctive lineage assigned to the DPANN archaea, which includes poorly characterised microorganisms with reduced genomes that likely depend on interactions with hosts for growth and survival. Here, we report the enrichment of a stable co-culture of a member of the Micrarchaeota (Ca. Micrarchaeum harzensis) together with its Thermoplasmatales host (Ca. Scheffleriplasma hospitalis), as well as the isolation of the latter. We show that symbiont-host interactions depend on biofilm formation as evidenced by growth experiments, comparative transcriptomic analyses and electron microscopy. In addition, genomic, metabolomic, extracellular polymeric substances and lipid content analyses indicate that the Micrarchaeon symbiont relies on the acquisition of metabolites from its host. Our study of the cell biology and physiology of a Micrarchaeon and its host adds to our limited knowledge of archaeal symbioses.

The Micrarchaeota lineage includes poorly characterized archaea with reduced genomes that likely depend on host interactions for survival. Here, the authors report a stable co-culture of a member of the Micrarchaeota and its host, and use multi-omic and physiological analyses to shed light on this symbiosis.

Methods to Analyze Motility in Eury- and Crenarchaea

Many archaea display swimming motility in liquid medium, which is empowered by the archaellum. Directional movement requires a functional archaellum and a sensing system, such as the chemotaxis system that is used by Euryarchaea. Two well-studied models are the euryarchaeon Haloferax volcanii and the crenarchaeon Sulfolobus acidocaldarius. In this chapter we describe two methods to analyze their swimming behavior and directional movement: (a) time-lapse microscopy under native temperatures and (b) spotting on semi-solid agar or gelrite plates. Whereas the first method allows for deep analysis of swimming behavior, the second method is suited for high throughput comparison of multiple strains.

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.

Evolution of Archaellum Rotation Involved Invention of a Stator Complex by Duplicating and Modifying a Core Component

Novelty in biology can arise from opportunistic repurposing of nascent characteristics of existing features. Understanding how this process happens at the molecular scale, however, suffers from a lack of case studies. The evolutionary emergence of rotary motors is a particularly clear example of evolution of a new function. The simplest of rotary motors is the archaellum, a molecular motor that spins a helical propeller for archaeal motility analogous to the bacterial flagellum. Curiously, emergence of archaellar rotation may have pivoted on the simple duplication and repurposing of a pre-existing component to produce a stator complex that anchors to the cell superstructure to enable productive rotation of the rotor component. This putative stator complex is composed of ArlF and ArlG, gene duplications of the filament component ArlB, providing an opportunity to study how gene duplication and neofunctionalization contributed to the radical innovation of rotary function. Toward understanding how this happened, we used electron cryomicroscopy to determine the structure of isolated ArlG filaments, the major component of the stator complex. Using a hybrid modeling approach incorporating structure prediction and validation, we show that ArlG filaments are open helices distinct to the closed helical filaments of ArlB. Curiously, further analysis reveals that ArlG retains a subset of the inter-protomer interactions of homologous ArlB, resulting in a superficially different assembly that nevertheless reflects the common ancestry of the two structures. This relatively simple mechanism to change quaternary structure was likely associated with the evolutionary neofunctionalization of the archaellar stator complex, and we speculate that the relative deformable elasticity of an open helix may facilitate elastic energy storage during the transmission of the discrete bursts of energy released by ATP hydrolysis to continuous archaellar rotation, allowing the inherent properties of a duplicated ArlB to be co-opted to fulfill a new role. Furthermore, agreement of diverse experimental evidence in our work supports recent claims to the power of new structure prediction techniques.

Bacterial motility: machinery and mechanisms

Bacteria have developed a large array of motility mechanisms to exploit available resources and environments. These mechanisms can be broadly classified into swimming in aqueous media and movement over solid surfaces. Swimming motility involves either the rotation of rigid helical filaments through the external medium or gyration of the cell body in response to the rotation of internal filaments. On surfaces, bacteria swarm collectively in a thin layer of fluid powered by the rotation of rigid helical filaments, they twitch by assembling and disassembling type IV pili, they glide by driving adhesins along tracks fixed to the cell surface and, finally, non-motile cells slide over surfaces in response to outward forces due to colony growth. Recent technological advances, especially in cryo-electron microscopy, have greatly improved our knowledge of the molecular machinery that powers the various forms of bacterial motility. In this Review, we describe the current understanding of the physical and molecular mechanisms that allow bacteria to move around.

The biology of thermoacidophilic archaea from the order Sulfolobales

Thermoacidophilic archaea belonging to the order Sulfolobales thrive in extreme biotopes, such as sulfuric hot springs and ore deposits. These microorganisms have been model systems for understanding life in extreme environments, as well as for probing the evolution of both molecular genetic processes and central metabolic pathways. Thermoacidophiles, such as the Sulfolobales, use typical microbial responses to persist in hot acid (e.g. motility, stress response, biofilm formation), albeit with some unusual twists. They also exhibit unique physiological features, including iron and sulfur chemolithoautotrophy, that differentiate them from much of the microbial world. Although first discovered >50 years ago, it was not until recently that genome sequence data and facile genetic tools have been developed for species in the Sulfolobales. These advances have not only opened up ways to further probe novel features of these microbes but also paved the way for their potential biotechnological applications. Discussed here are the nuances of the thermoacidophilic lifestyle of the Sulfolobales, including their evolutionary placement, cell biology, survival strategies, genetic tools, metabolic processes and physiological attributes together with how these characteristics make thermoacidophiles ideal platforms for specialized industrial processes.

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.

Autophosphorylation of the KaiC-like protein ArlH inhibits oligomerization and interaction with ArlI, the motor ATPase of the archaellum

Motile archaea are propelled by the archaellum, whose motor complex consists of the membrane protein ArlJ, the ATPase ArlI, and the ATP-binding protein ArlH. Despite its essential function and the existence of structural and biochemical data on ArlH, the role of ArlH in archaellum assembly and function remains elusive. ArlH is a structural homolog of KaiC, the central component of the cyanobacterial circadian clock. Since autophosphorylation and dephosphorylation of KaiC are central properties for the function of KaiC, we asked whether autophosphorylation is also a property of ArlH proteins. We observed that both ArlH from the euryarchaeon Pyrococcus furiosus (PfArlH) and from the crenarchaeon Sulfolobus acidocaldarius (SaArlH) have autophosphorylation activity. Using a combination of single-molecule fluorescence measurements and biochemical assays, we show that autophosphorylation of ArlH is closely linked to its oligomeric state when bound to hexameric ArlI. These experiments also strongly suggest that ArlH is a hexamer in its ArlI-bound state. Mutagenesis of the putative catalytic residue (Glu-57 in SaArlH) in ArlH results in a reduced autophosphorylation activity and abolished archaellation and motility in S. acidocaldarius, indicating that optimum phosphorylation activity of ArlH is essential for archaellation and motility.

Functional trait relationships demonstrate life strategies in terrestrial prokaryotes

Functional, physiological traits are the underlying drivers of niche differentiation. A common framework related to niches occupied by terrestrial prokaryotes is based on copiotrophy or oligotrophy, where resource investment is primarily in either rapid growth or stress tolerance, respectively. A quantitative trait-based approach sought relationships between taxa, traits and niche in terrestrial prokaryotes. With 175 taxa from 11 Phyla and 35 Families (n = 5 per Family), traits were considered as discrete counts of shared genome-encoded proteins. Trait composition strongly supported non-random functional distributions as preferential clustering of related taxa via unweighted pair-group method with arithmetic mean. Trait similarity between taxa increased as taxonomic rank decreased. A suite of Random Forest models identified traits significantly enriched or depleted in taxonomic groups. These traits conveyed functions related to rapid growth, nutrient acquisition and stress tolerance consistent with their presence in copiotroph-oligotroph niches. Hierarchical clustering of traits identified a clade of competitive, copiotrophic Families resilient to oxidative stress versus glycosyltransferase-enriched oligotrophic Families resistant to antimicrobials and environmental stress. However, the formation of five clades suggested a more nuanced view to describe niche differentiation in terrestrial systems is necessary. We suggest considering traits involved in both resource investment and acquisition when predicting niche.

A comprehensive history of motility and Archaellation in Archaea

Each of the three Domains of life, Eukarya, Bacteria and Archaea, have swimming structures that were all originally called flagella, despite the fact that none were evolutionarily related to either of the other two. Surprisingly, this was true even in the two prokaryotic Domains of Bacteria and Archaea. Beginning in the 1980s, evidence gradually accumulated that convincingly demonstrated that the motility organelle in Archaea was unrelated to that found in Bacteria, but surprisingly shared significant similarities to type IV pili. This information culminated in the proposal, in 2012, that the 'archaeal flagellum' be assigned a new name, the archaellum. In this review, we provide a historical overview on archaella and motility research in Archaea, beginning with the first simple observations of motile extreme halophilic archaea a century ago up to state-of-the-art cryo-tomography of the archaellum motor complex and filament observed today. In addition to structural and biochemical data which revealed the archaellum to be a type IV pilus-like structure repurposed as a rotating nanomachine (Beeby et al. 2020), we also review the initial discoveries and subsequent advances using a wide variety of approaches to reveal: complex regulatory events that lead to the assembly of the archaellum filaments (archaellation); the roles of the various archaellum proteins; key post-translational modifications of the archaellum structural subunits; evolutionary relationships; functions of archaella other than motility and the biotechnological potential of this fascinating structure. The progress made in understanding the structure and assembly of the archaellum is highlighted by comparing early models to what is known today.

Putative Extracellular Electron Transfer in Methanogenic Archaea

It has been suggested that a few methanogens are capable of extracellular electron transfers. For instance, Methanosarcina barkeri can directly capture electrons from the coexisting microbial cells of other species. Methanothrix harundinacea and Methanosarcina horonobensis retrieve electrons from Geobacter metallireducens via direct interspecies electron transfer (DIET). Recently, Methanobacterium, designated strain YSL, has been found to grow via DIET in the co-culture with Geobacter metallireducens. Methanosarcina acetivorans can perform anaerobic methane oxidation and respiratory growth relying on Fe(III) reduction through the extracellular electron transfer. Methanosarcina mazei is capable of electromethanogenesis under the conditions where electron-transfer mediators like H₂ or formate are limited. The membrane-bound multiheme c-type cytochromes (MHC) and electrically-conductive cellular appendages have been assumed to mediate the extracellular electron transfer in bacteria like Geobacter and Shewanella species. These molecules or structures are rare but have been recently identified in a few methanogens. Here, we review the current state of knowledge for the putative extracellular electron transfers in methanogens and highlight the opportunities and challenges for future research.

Autophosphorylation of the KaiC-like protein ArlH inhibits oligomerisation and interaction with ArlI, the motor ATPase of the archaellum.. [DOI: 10.1101/2021.03.19.436134] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Motile archaea are propelled by the archaellum, whose motor complex consists of the membrane protein ArlJ, the ATPase ArlI, and the ATP-binding protein ArlH. Despite its essential function and the existence of structural and biochemical data on ArlH, the role of ArlH in archaellum assembly and function remains elusive. ArlH is a structural homolog of KaiC, the central component of the cyanobacterial circadian clock. Similar to KaiC, ArlH exhibits autophosphorylation activity, which was observed for both ArlH of the euryarchaeonPyrococcus furiosus (PfArlH)and the crenarchaeonSulfolobus acidocaldarius(SaArlH). Using a combination of single molecule fluorescence measurements and biochemical assays, it is shown that autophosphorylation of ArlH is closely linked to the oligomeric state of ArlH bound to ArlI. These experiments also strongly suggest that ArlH is a hexamer in its functional ArlI bound state. Mutagenesis of the putative catalytic residue Glu-57 inSaArlH results in a reduced autophosphorylation activity and abolished archaellation and motility, suggesting that optimum phosphorylation activity of ArlH is essential for both archaellation and motility.

Cellular and Genomic Properties of Haloferax gibbonsii LR2-5, the Host of Euryarchaeal Virus HFTV1

Hypersaline environments are the source of many viruses infecting different species of halophilic euryarchaea. Information on infection mechanisms of archaeal viruses is scarce, due to the lack of genetically accessible virus–host models. Recently, a new archaeal siphovirus, Haloferax tailed virus 1 (HFTV1), was isolated together with its host belonging to the genus Haloferax, but it is not infectious on the widely used model euryarcheon Haloferax volcanii. To gain more insight into the biology of HFTV1 host strain LR2-5, we studied characteristics that might play a role in its virus susceptibility: growth-dependent motility, surface layer, filamentous surface structures, and cell shape. Its genome sequence showed that LR2-5 is a new strain of Haloferax gibbonsii. LR2-5 lacks obvious viral defense systems, such as CRISPR-Cas, and the composition of its cell surface is different from Hfx. volcanii, which might explain the different viral host range. This work provides first deep insights into the relationship between the host of halovirus HFTV1 and other members of the genus Haloferax. Given the close relationship to the genetically accessible Hfx. volcanii, LR2-5 has high potential as a new model for virus–host studies in euryarchaea.

Insights into gene expression changes under conditions that facilitate horizontal gene transfer (mating) of a model archaeon

Horizontal gene transfer is a means by which bacteria, archaea, and eukaryotes are able to trade DNA within and between species. While there are a variety of mechanisms through which this genetic exchange can take place, one means prevalent in the archaeon Haloferax volcanii involves the transient formation of cytoplasmic bridges between cells and is referred to as mating. This process can result in the exchange of very large fragments of DNA between the participating cells. Genes governing the process of mating, including triggers to initiate mating, mechanisms of cell fusion, and DNA exchange, have yet to be characterized. We used a transcriptomic approach to gain a more detailed knowledge of how mating might transpire. By examining the differential expression of genes expressed in cells harvested from mating conditions on a filter over time and comparing them to those expressed in a shaking culture, we were able to identify genes and pathways potentially associated with mating. These analyses provide new insights into both the mechanisms and barriers of mating in Hfx. volcanii.

SaUspA, the Universal Stress Protein of Sulfolobus acidocaldarius Stimulates the Activity of the PP2A Phosphatase and Is Involved in Growth at High Salinity

In Sulfolobus acidocaldarius, the protein phosphatase PP2A plays important regulatory roles in many cellular processes, including cell growth, cell shape and synthesis of the archaellum. A conserved prokaryotic protein, designated as SaUspA, was identified as an interaction partner of the phosphatase PP2A. SaUspA belongs to the universal stress protein (USP) superfamily, members of which are found in bacteria, archaea, plants and invertebrates. Biochemical analysis showed that SaUspA is a homodimeric ATP-binding protein, which also in vitro binds to PP2A. SaUspA did not hydrolyze ATP, but stimulated the phosphatase activity of PP2A and might in this manner affect many other processes. Interestingly, binding of ATP further enhanced SaUspA's interaction with PP2A. In contrast to bacterial usp genes, environmental stress conditions including stationary phase, starvation stress, high salinity stress and UV stress did not stimulate expression of saUspA. Deletion of saUspA led to premature production of the archaellin FlaB in S. acidocaldarius although motility was not affected. The ΔsaUspA mutant showed a significant growth defect under high salinity stress and complementation of ATP-binding deficient mutant SaUspA^G97A failed to restore this growth defect. Compared with the wild type strain, its growth or survival was not affected under heavy metal stress and UV stress. To date, this is the first study in which the physiological role of USP homologs in archaea have been reported.

The switch complex ArlCDE connects the chemotaxis system and the archaellum

Cells require a sensory system and a motility structure to achieve directed movement. Bacteria and archaea possess rotating filamentous motility structures that work in concert with the sensory chemotaxis system. This allows microorganisms to move along chemical gradients. The central response regulator protein CheY can bind to the motor of the motility structure, the flagellum in bacteria, and the archaellum in archaea. Both motility structures have a fundamentally different protein composition and structural organization. Yet, both systems receive input from the chemotaxis system. So far, it was unknown how the signal is transferred from the archaeal CheY to the archaellum motor to initiate motor switching. We applied a fluorescent microscopy approach in the model euryarchaeon Haloferax volcanii and shed light on the sequence order in which signals are transferred from the chemotaxis system to the archaellum. Our findings indicate that the euryarchaeal-specific ArlCDE are part of the archaellum motor and that they directly receive input from the chemotaxis system via the adaptor protein CheF. Hence, ArlCDE are an important feature of the archaellum of euryarchaea, are essential for signal transduction during chemotaxis and represent the archaeal switch complex.

Identification of a novel N-linked glycan on the archaellins and S-layer protein of the thermophilic methanogen, Methanothermococcus thermolithotrophicus

Motility in archaea is facilitated by a unique structure termed the archaellum. N-Glycosylation of the major structural proteins (archaellins) is important for their subsequent incorporation into the archaellum filament. The identity of some of these N-glycans has been determined, but archaea exhibit extensive variation in their glycans, meaning that further investigations can shed light not only on the specific details of archaellin structure and function, but also on archaeal glycobiology in general. Here we describe the structural characterization of the N-linked glycan modifications on the archaellins and S-layer protein of Methanothermococcus thermolithotrophicus, a methanogen that grows optimally at 65 °C. SDS-PAGE and MS analysis revealed that the sheared archaella are composed principally of two of the four predicted archaellins, FlaB1 and FlaB3, which are modified with a branched, heptameric glycan at all N-linked sequons except for the site closest to the N termini of both proteins. NMR analysis of the purified glycan determined the structure to be α-d-glycero-d-manno-Hep3OMe6OMe-(1-3)-[α-GalNAcA3OMe-(1-2)-]-β-Man-(1-4)-[β-GalA3OMe4OAc6CMe-(1-4)-α-GalA-(1-2)-]-α-GalAN-(1-3)-β-GalNAc-Asn. A detailed investigation by hydrophilic interaction liquid ion chromatography-MS discovered the presence of several, less abundant glycan variants, related to but distinct from the main heptameric glycan. In addition, we confirmed that the S-layer protein is modified with the same heptameric glycan, suggesting a common N-glycosylation pathway. The M. thermolithotrophicus archaellin N-linked glycan is larger and more complex than those previously identified on the archaellins of related mesophilic methanogens, Methanococcus voltae and Methanococcus maripaludis This could indicate that the nature of the glycan modification may have a role to play in maintaining stability at elevated temperatures.

N-glycosylation in Archaea-New roles for an ancient posttranslational modification

Genome analysis points to N-glycosylation as being an almost universal posttranslational modification in Archaea. Although such predictions have been confirmed in only a limited number of species, such studies are making it increasingly clear that the N-linked glycans which decorate archaeal glycoproteins present diversity in terms of both glycan composition and architecture far beyond what is seen in the other two domains of life. In addition to continuing to decipher pathways of N-glycosylation, recent efforts have revealed how Archaea exploit this variability in novel roles. As well as encouraging glycoprotein synthesis, folding and assembly into properly functioning higher ordered complexes, N-glycosylation also provides Archaea with a strategy to cope with changing environments. Archaea can, moreover, exploit the apparent species-specific nature of N-glycosylation for selectivity in mating, and hence, to maintain species boundaries, and in other events where cell-selective interactions are required. At the same time, addressing components of N-glycosylation pathways across archaeal phylogeny offers support for the concept of an archaeal origin for eukaryotes. In this MicroReview, these and other recent discoveries related to N-glycosylation in Archaea are considered.

Tree of motility - A proposed history of motility systems in the tree of life

Motility often plays a decisive role in the survival of species. Five systems of motility have been studied in depth: those propelled by bacterial flagella, eukaryotic actin polymerization and the eukaryotic motor proteins myosin, kinesin and dynein. However, many organisms exhibit surprisingly diverse motilities, and advances in genomics, molecular biology and imaging have showed that those motilities have inherently independent mechanisms. This makes defining the breadth of motility nontrivial, because novel motilities may be driven by unknown mechanisms. Here, we classify the known motilities based on the unique classes of movement‐producing protein architectures. Based on this criterion, the current total of independent motility systems stands at 18 types. In this perspective, we discuss these modes of motility relative to the latest phylogenetic Tree of Life and propose a history of motility. During the ~4 billion years since the emergence of life, motility arose in Bacteria with flagella and pili, and in Archaea with archaella. Newer modes of motility became possible in Eukarya with changes to the cell envelope. Presence or absence of a peptidoglycan layer, the acquisition of robust membrane dynamics, the enlargement of cells and environmental opportunities likely provided the context for the (co)evolution of novel types of motility.

The archaeal-bacterial lipid divide, could a distinct lateral proton route hold the answer?

The archaea-bacteria lipid divide is one of the big evolutionary enigmas concerning these two domains of life. In short, bacterial membranes are made of fatty-acid esters whereas archaeal ones contain isoprenoid ethers, though at present we do not have a good understanding on why they evolved differently. The lateral proton transfer mode of energy transduction in membranes posits that protons utilize the solvation layer of the membrane interface as the main route between proton pumps and ATPases, avoiding dissipation of energy to the bulk phase. In this article I present the hypothesis on a proton-transport route through the ester groups of bacterial phospholipids as an explanation for the evolutionary divergence seen between bacteria and archaea. REVIEWERS: This article was reviewed by Uri Gophna (Editorial Board member) and Víctor Sojo.

Interaction of two strongly divergent archaellins stabilizes the structure of the Halorubrum archaellum

Halophilic archaea from the genus Halorubrum possess two extraordinarily diverged archaellin genes, flaB1 and flaB2. To clarify roles for each archaellin, we compared two natural Halorubrum lacusprofundi strains: One of them contains both archaellin genes, and the other has the flaB2 gene only. Both strains synthesize functional archaella; however, the strain, where both archaellins are present, is more motile. In addition, we expressed these archaellins in a Haloferax volcanii strain from which the endogenous archaellin genes were deleted. Three Hfx. volcanii strains expressing Hrr. lacusprofundi archaellins produced functional filaments consisting of only one (FlaB1 or FlaB2) or both (FlaB1/FlaB2) archaellins. All three strains were motile, although there were profound differences in the efficiency of motility. Both native and recombinant FlaB1/FlaB2 filaments have greater thermal stability and resistance to low salinity stress than single‐component filaments. Functional supercoiled Hrr. lacusprofundi archaella can be composed of either single archaellin: FlaB2 or FlaB1; however, the two divergent archaellin subunits provide additional stabilization to the archaellum structure and thus adaptation to a wider range of external conditions. Comparative genomic analysis suggests that the described combination of divergent archaellins is not restricted to Hrr. lacusprofundi, but is occurring also in organisms from other haloarchaeal genera.

Effect of changes at the conserved + 3 position of mature archaellins on in vitro cleavage by the pre-archaellin peptidase FlaK of Methanococcus maripaludis

Archaea swim using archaella that are domain-specific rotary type IV pilus-like appendages. The structural components of the archaellum filament are archaellins, initially made as preproteins with type IV pilin-like signal peptides which are removed by signal peptidases that are homologues of prepilin peptidases that remove signal peptides from type IV pilins. N-terminal sequences of archaellins, including the signal peptide cleavage site, are conserved and various positions have been previously shown to be critical for signal peptide removal. Archaellins have an absolute conservation of glycine at the + 3 position from the signal peptide cleavage site. To investigate its role in signal peptide cleavage, I used archaellin variants in which the + 3 glycine was mutated to all other possibilities in in vitro cleavage reactions. Cleavage was observed with ten different amino acids at the + 3 position, indicating that the observed glycine conservation is not required for this essential processing step.

Post-Translational Modifications Aid Archaeal Survival

Since the pioneering work of Carl Woese, Archaea have fascinated biologists of almost all areas given their unique evolutionary status, wide distribution, high diversity, and ability to grow in special environments. Archaea often thrive in extreme conditions such as high temperature, high/low pH, high salinity, and anoxic ecosystems. All of these are threats to the stability and proper functioning of biological molecules, especially proteins and nucleic acids. Post-translational modifications (PTMs), such as phosphorylation, methylation, acetylation, and glycosylation, are reportedly widespread in Archaea and represent a critical adaptive mechanism to extreme habitats. Here, we summarize our current understanding of the contributions of PTMs to aid in extremophile survival, with a particular focus on the maintenance of genome stability.

Species-Specific Recognition of Sulfolobales Mediated by UV-Inducible Pili and S-Layer Glycosylation Patterns

The UV-inducible pili system of Sulfolobales (Ups) mediates the formation of species-specific cellular aggregates. Within these aggregates, cells exchange DNA to repair DNA double-strand breaks via homologous recombination. Substitution of the Sulfolobus acidocaldarius pilin subunits UpsA and UpsB with their homologs from Sulfolobus tokodaii showed that these subunits facilitate species-specific aggregation. A region of low conservation within the UpsA homologs is primarily important for this specificity. Aggregation assays in the presence of different sugars showed the importance of N-glycosylation in the recognition process. In addition, the N-glycan decorating the S-layer of S. tokodaii is different from the one of S. acidocaldarius Therefore, each Sulfolobus species seems to have developed a unique UpsA binding pocket and unique N-glycan composition to ensure aggregation and, consequently, also DNA exchange with cells from only the same species, which is essential for DNA repair by homologous recombination.IMPORTANCE Type IV pili can be found on the cell surface of many archaea and bacteria where they play important roles in different processes. The UV-inducible pili system of Sulfolobales (Ups) pili from the crenarchaeal Sulfolobales species are essential in establishing species-specific mating partners, thereby assisting in genome stability. With this work, we show that different Sulfolobus species have specific regions in their Ups pili subunits, which allow them to interact only with cells from the same species. Additionally, different Sulfolobus species have unique surface-layer N-glycosylation patterns. We propose that the unique features of each species allow the recognition of specific mating partners. This knowledge for the first time gives insights into the molecular basis of archaeal self-recognition.

The Remarkable Dual-Level Diversity of Prokaryotic Flagellins

Bacterial and archaeal flagellins are remarkable in having a shared region with variation in housekeeping proteins and a region with extreme diversity, perhaps greater than for any other protein. Analysis of the 113,285 available full-gene sequences of flagellin genes from published bacterial and archaeal sequences revealed the nature and enormous extent of flagellin diversity. There were 35,898 unique amino acid sequences that were resolved into 187 clusters. Analysis of the Escherichia coli and Salmonella enterica flagellins revealed that the variation occurs at two levels. The first is the division of the variable regions into sequence forms that are so divergent that there is no meaningful alignment even within species, and these corresponded to the E. coli or S. enterica H-antigen groups. The second level is variation within these groups, which is extensive in both species. Shared sequence would allow PCR of the variable regions and thus strain-level analysis of microbiome DNA.

Flagellin, the agent of prokaryotic flagellar motion, is very widely distributed and is the H antigen of serology. Flagellin molecules have a variable region that confers serotype specificity, encoded by the middle of the gene, and also conserved regions encoded by the two ends of the gene. We collected all available prokaryotic flagellin protein sequences and found the variable region diversity to be at two levels. In each species investigated, there are hypervariable region (HVR) forms without detectable homology in protein sequences between them. There is also considerable variation within HVR forms, indicating that some have been diverging for thousands of years and that interphylum horizontal gene transfers make a major contribution to the evolution of such atypical diversity.

IMPORTANCE Bacterial and archaeal flagellins are remarkable in having a shared region with variation in housekeeping proteins and a region with extreme diversity, perhaps greater than for any other protein. Analysis of the 113,285 available full-gene sequences of flagellin genes from published bacterial and archaeal sequences revealed the nature and enormous extent of flagellin diversity. There were 35,898 unique amino acid sequences that were resolved into 187 clusters. Analysis of the Escherichia coli and Salmonella enterica flagellins revealed that the variation occurs at two levels. The first is the division of the variable regions into sequence forms that are so divergent that there is no meaningful alignment even within species, and these corresponded to the E. coli or S. enterica H-antigen groups. The second level is variation within these groups, which is extensive in both species. Shared sequence would allow PCR of the variable regions and thus strain-level analysis of microbiome DNA.

The structure of the periplasmic FlaG-FlaF complex and its essential role for archaellar swimming motility

Motility structures are vital in all three domains of life. In Archaea, motility is mediated by the archaellum, a rotating type IV pilus-like structure that is a unique nanomachine for swimming motility in nature. Whereas periplasmic FlaF binds the surface layer (S-layer), the structure, assembly and roles of other periplasmic components remain enigmatic, limiting our knowledge of the archaellum's functional interactions. Here, we find that the periplasmic protein FlaG and the association with its paralogue FlaF are essential for archaellation and motility. Therefore, we determine the crystal structure of Sulfolobus acidocaldarius soluble FlaG (sFlaG), which reveals a β-sandwich fold resembling the S-layer-interacting FlaF soluble domain (sFlaF). Furthermore, we solve the sFlaG₂-sFlaF₂ co-crystal structure, define its heterotetrameric complex in solution by small-angle X-ray scattering and find that mutations that disrupt the complex abolish motility. Interestingly, the sFlaF and sFlaG of Pyrococcus furiosus form a globular complex, whereas sFlaG alone forms a filament, indicating that FlaF can regulate FlaG filament assembly. Strikingly, Sulfolobus cells that lack the S-layer component bound by FlaF assemble archaella but cannot swim. These collective results support a model where a FlaG filament capped by a FlaG-FlaF complex anchors the archaellum to the S-layer to allow motility.

Assembly, Functions and Evolution of Archaella, Flagella and Cilia

Cells from all three domains of life on Earth utilize motile macromolecular devices that protrude from the cell surface to generate forces that allow them to swim through fluid media. Research carried out on archaea during the past decade or so has led to the recognition that, despite their common function, the motility devices of the three domains display fundamental differences in their properties and ancestry, reflecting a striking example of convergent evolution. Thus, the flagella of bacteria and the archaella of archaea employ rotary filaments that assemble from distinct subunits that do not share a common ancestor and generate torque using energy derived from distinct fuel sources, namely chemiosmotic ion gradients and FlaI motor-catalyzed ATP hydrolysis, respectively. The cilia of eukaryotes, however, assemble via kinesin-2-driven intraflagellar transport and utilize microtubules and ATP-hydrolyzing dynein motors to beat in a variety of waveforms via a sliding filament mechanism. Here, with reference to current structural and mechanistic information about these organelles, we briefly compare the evolutionary origins, assembly and tactic motility of archaella, flagella and cilia.

Insights into the mechanism of ATP-driven rotary motors from direct torque measurement

Motor proteins are molecular machines that convert chemical energy into mechanical work. In addition to existing studies performed on the linear motors found in eukaryotic cells, researchers in biophysics have also focused on rotary motors such as F₁-ATPase. Detailed studies on the rotary F₁-ATPase motor have correlated all chemical states to specific mechanical events at the single-molecule level. Recent studies showed that there exists another ATP-driven protein motor in life: the rotary machinery that rotates archaeal flagella (archaella). Rotation speed, stepwise movement, and variable directionality of the motor of Halobacterium salinarum were described in previous studies. Here we review recent experimental work discerning the molecular mechanism underlying how the archaellar motor protein FlaI drives rotation by generation of motor torque. In combination, those studies found that rotation slows as the viscous drag of markers increases, but torque remains constant at 160 pN·nm independent of rotation speed. Unexpectedly, the estimated work done in a single rotation is twice the expected energy that would come from hydrolysis of six ATP molecules in the FlaI hexamer. To reconcile the apparent contradiction, a new and general model for the mechanism of ATP-driven rotary motors is discussed.

Diversification of the type IV filament superfamily into machines for adhesion, protein secretion, DNA uptake, and motility

Processes of molecular innovation require tinkering and shifting in the function of existing genes. How this occurs in terms of molecular evolution at long evolutionary scales remains poorly understood. Here, we analyse the natural history of a vast group of membrane-associated molecular systems in Bacteria and Archaea-the type IV filament (TFF) superfamily-that diversified in systems involved in flagellar or twitching motility, adhesion, protein secretion, and DNA uptake. The phylogeny of the thousands of detected systems suggests they may have been present in the last universal common ancestor. From there, two lineages-a bacterial and an archaeal-diversified by multiple gene duplications, gene fissions and deletions, and accretion of novel components. Surprisingly, we find that the 'tight adherence' (Tad) systems originated from the interkingdom transfer from Archaea to Bacteria of a system resembling the 'EppA-dependent' (Epd) pilus and were associated with the acquisition of a secretin. The phylogeny and content of ancestral systems suggest that initial bacterial pili were engaged in cell motility and/or DNA uptake. In contrast, specialised protein secretion systems arose several times independently and much later in natural history. The functional diversification of the TFF superfamily was accompanied by genetic rearrangements with implications for genetic regulation and horizontal gene transfer: systems encoded in fewer loci were more frequently exchanged between taxa. This may have contributed to their rapid evolution and spread across Bacteria and Archaea. Hence, the evolutionary history of the superfamily reveals an impressive catalogue of molecular evolution mechanisms that resulted in remarkable functional innovation and specialisation from a relatively small set of components.

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.

Motor torque measurement of Halobacterium salinarum archaellar suggests a general model for ATP-driven rotary motors

It is unknown how the archaellum-the rotary propeller used by Archaea for motility-works. To further understand the molecular mechanism by which the hexameric ATPase motor protein FlaI drives rotation of the membrane-embedded archaellar motor, we determined motor torque by imposition of various loads on Halobacterium salinarum archaella. Markers of different sizes were attached to single archaella, and their trajectories were quantified using three-dimensional tracking and high-speed recording. We show that rotation slows as the viscous drag of markers increases, but torque remains constant at 160 pN·nm independent of rotation speed. Notably, the estimated work done in a single rotation is twice the expected energy that would come from hydrolysis of six ATP molecules in the hexamer, indicating that more ATP molecules are required for one rotation of archaellum. To reconcile the apparent contradiction, we suggest a new and general model for the mechanism of ATP-driven rotary motors.

Single-molecule pull-out manipulation of the shaft of the rotary motor F1-ATPase

F₁-ATPase is a rotary motor protein in which the central γ-subunit rotates inside the cylinder made of α₃β₃ subunits. To investigate interactions between the γ shaft and the cylinder at the molecular scale, load was imposed on γ through a polystyrene bead by three-dimensional optical trapping in the direction along which the shaft penetrates the cylinder. Pull-out event was observed under high-load, and thus load-dependency of lifetime of the interaction was estimated. Notably, accumulated counts of lifetime were comprised of fast and slow components. Both components exponentially dropped with imposed loads, suggesting that the binding energy is compensated by the work done by optical trapping. Because the mutant, in which the half of the shaft was deleted, showed only one fast component in the bond lifetime, the slow component is likely due to the native interaction mode held by multiple interfaces.

Positioning of the Motility Machinery in Halophilic Archaea

Archaea are ubiquitous single cellular microorganisms that play important ecological roles in nature. The intracellular organization of archaeal cells is among the unresolved mysteries of archaeal biology. With this work, we show that cells of haloarchaea are polarized. The cellular positioning of proteins involved in chemotaxis and motility is spatially and temporally organized in these cells. This suggests the presence of a specific mechanism responsible for the positioning of macromolecular protein complexes in archaea.

Bacteria and archaea exhibit tactical behavior and can move up and down chemical gradients. This tactical behavior relies on a motility structure, which is guided by a chemosensory system. Environmental signals are sensed by membrane-inserted chemosensory receptors that are organized in large ordered arrays. While the cellular positioning of the chemotaxis machinery and that of the flagellum have been studied in detail in bacteria, we have little knowledge about the localization of such macromolecular assemblies in archaea. Although the archaeal motility structure, the archaellum, is fundamentally different from the flagellum, archaea have received the chemosensory machinery from bacteria and have connected this system with the archaellum. Here, we applied a combination of time-lapse imaging and fluorescence and electron microscopy using the model euryarchaeon Haloferax volcanii and found that archaella were specifically present at the cell poles of actively dividing rod-shaped cells. The chemosensory arrays also had a polar preference, but in addition, several smaller arrays moved freely in the lateral membranes. In the stationary phase, rod-shaped cells became round and chemosensory arrays were disassembled. The positioning of archaella and that of chemosensory arrays are not interdependent and likely require an independent form of positioning machinery. This work showed that, in the rod-shaped haloarchaeal cells, the positioning of the archaellum and of the chemosensory arrays is regulated in time and in space. These insights into the cellular organization of H. volcanii suggest the presence of an active mechanism responsible for the positioning of macromolecular protein complexes in archaea.

High-resolution archaellum structure reveals a conserved metal-binding site

Many archaea swim by means of archaella. While the archaellum is similar in function to its bacterial counterpart, its structure, composition, and evolution are fundamentally different. Archaella are related to archaeal and bacterial type IV pili. Despite recent advances, our understanding of molecular processes governing archaellum assembly and stability is still incomplete. Here, we determine the structures of Methanococcus archaella by X‐ray crystallography and cryo‐EM. The crystal structure of Methanocaldococcus jannaschii FlaB1 is the first and only crystal structure of any archaellin to date at a resolution of 1.5 Å, which is put into biological context by a cryo‐EM reconstruction from Methanococcus maripaludis archaella at 4 Å resolution created with helical single‐particle analysis. Our results indicate that the archaellum is predominantly composed of FlaB1. We identify N‐linked glycosylation by cryo‐EM and mass spectrometry. The crystal structure reveals a highly conserved metal‐binding site, which is validated by mass spectrometry and electron energy‐loss spectroscopy. We show in vitro that the metal‐binding site, which appears to be a widespread property of archaellin, is required for filament integrity.

Taxis in archaea

Microorganisms can move towards favorable growth conditions as a response to environmental stimuli. This process requires a motility structure and a system to direct the movement. For swimming motility, archaea employ a rotating filament, the archaellum. This archaea-specific structure is functionally equivalent, but structurally different, from the bacterial flagellum. To control the directionality of movement, some archaea make use of the chemotaxis system, which is used for the same purpose by bacteria. Over the past decades, chemotaxis has been studied in detail in several model bacteria. In contrast, archaeal chemotaxis is much less explored and largely restricted to analyses in halophilic archaea. In this review, we summarize the available information on archaeal taxis. We conclude that archaeal chemotaxis proteins function similarly as their bacterial counterparts. However, because the motility structures are fundamentally different, an archaea-specific docking mechanism is required, for which initial experimental data have only recently been obtained.