Functional assignment of multiple ESCRT-III homologs in cell division and budding in Sulfolobus islandicus

Mechanism for Vipp1 spiral formation, ring biogenesis, and membrane repair

The ESCRT-III-like protein Vipp1 couples filament polymerization with membrane remodeling. It assembles planar sheets as well as 3D rings and helical polymers, all implicated in mitigating plastid-associated membrane stress. The architecture of Vipp1 planar sheets and helical polymers remains unknown, as do the geometric changes required to transition between polymeric forms. Here we show how cyanobacterial Vipp1 assembles into morphologically-related sheets and spirals on membranes in vitro. The spirals converge to form a central ring similar to those described in membrane budding. Cryo-EM structures of helical filaments reveal a close geometric relationship between Vipp1 helical and planar lattices. Moreover, the helical structures reveal how filaments twist-a process required for Vipp1, and likely other ESCRT-III filaments, to transition between planar and 3D architectures. Overall, our results provide a molecular model for Vipp1 ring biogenesis and a mechanism for Vipp1 membrane stabilization and repair, with implications for other ESCRT-III systems.

ESCRTs - a multi-purpose membrane remodeling device encoded in all life forms

The ESCRT (endosomal sorting complexes required for transport) membrane remodeling complex, found across all life forms, exhibits a versatility that transcends evolutionary boundaries. From orchestrating the constriction of micron-wide tubes in cell division to facilitating the budding of 50 nm vesicles in receptor degradation, ESCRTs perform diverse functions in animal cells. However, the basis of this functional diversity remains enigmatic. While extensively studied in eukaryotes, the role of ESCRTs in prokaryotes is only beginning to emerge. This review synthesizes data on ESCRT systems across the tree of life, focusing on microorganisms and drawing parallels to their functions in human cells. This comparative approach highlights the remarkable plasticity of the ESCRT system across functional, structural, and genomic levels in both prokaryotes and eukaryotes. This integrated knowledge supports a model in which the ESCRT system evolved as a multipurpose membrane remodeling tool, adaptable to specific functions within and across organisms. Our review not only underscores the significance of ESCRTs in microorganisms but also paves the way for exciting avenues of research into the intricacies of cellular membrane dynamics, offering valuable insights into the evolution of cellular complexity across diverse organisms and ecosystems.

Asgard archaea reveal the conserved principles of ESCRT-III membrane remodeling

ESCRT-III proteins assemble into composite polymers that undergo stepwise changes in composition and structure to deform membranes across the tree of life. Here, using a phylogenetic analysis, we demonstrate that the two endosomal sorting complex required for transport III (ESCRT-III) proteins present in eukaryote's closest Asgard archaeal relatives are evolutionarily related to the B- and A-type eukaryotic paralogs that initiate and execute membrane remodeling, respectively. We show that Asgard ESCRT-IIIB assembles into parallel arrays on planar membranes to initiate membrane deformation, from where it recruits ESCRT-IIIA to generate composite polymers. Last, we show that Asgard ESCRT-IIIA is able to remodel membranes into tubes as a likely prelude to scission. Together, these data reveal a set of conserved principles governing ESCRT-III-dependent membrane remodeling that first emerged in a two-component ESCRT-III system in archaea.

A relay race of ESCRT-III paralogs drives cell division in a hyperthermophilic archaeon

Cell division is a fundamental process ensuring the perpetuation of all cellular life forms. Archaea of the order Sulfolobales divide using a simpler version of the eukaryotic endosomal sorting complexes required for transport (ESCRT) machinery, composed of three ESCRT-III homologs (ESCRT-III, -III-1, and -III-2), AAA+ ATPase Vps4 and an archaea-specific component CdvA. Here, we clarify how these components act sequentially to drive the division of the hyperthermophilic archaeon Saccharolobus islandicus. Our data suggest that ESCRT-III plays an active role during the early stage of membrane constriction during cytokinesis, whereas ESCRT-III-1 and ESCRT-III-2 are indispensable for the "pre-late" and "late" stages of cytokinesis, respectively. In the escrt-III-1 deletion strain, the division is blocked when the mid-cell constriction reaches ~30% of the initial cell diameter ("pre-late" stage), yielding "chain-like" cellular aggregates. Depletion of ESCRT-III-2 leads to the accumulation of cells connected through narrow membrane bridges ("late" stage), consistent with the key role of this protein in the final membrane abscission. We used 3D-single molecule localization microscopy to image ESCRT-III rings of different diameters and show that the decrease in the ESCRT-III ring diameter and membrane constriction are inconsistent with a mechanism exclusively based on spiraling of the ESCRT-III filaments. By contrast, the cone-shaped assemblies of ESCRT-III-1 and ESCRT-III-2 are consistent with spiral formation, highlighting the distinct roles of the three ESCRT-III proteins during the cytokinesis. We propose the "relay race" model, whereby the cytokinesis is achieved through a sequential and concerted action of different ESCRT machinery components.

IMPORTANCE

Two major cytokinesis mechanisms, rooted in contractile FtsZ and endosomal sorting complexes required for transport (ESCRT) rings, respectively, have emerged in the course of evolution. Whereas bacteria rely on the FtsZ-based mechanism, different lineages of archaea use either of the two systems, and eukaryotes have inherited the ESCRT-based cell division machinery from their archaeal ancestors. The mechanism of ESCRT-based cell division in archaea remains poorly understood and mechanistic studies on different archaeal model systems are essential to unravel the natural history of the ESCRT machinery. Here we investigate the interplay between three major ESCRT-III homologs during the division of a hyperthermophilic archaeon Saccharolobus islandicus and propose the "relay race" model of cytokinesis.

The Archaeal Cell Cycle

Since first identified as a separate domain of life in the 1970s, it has become clear that archaea differ profoundly from both eukaryotes and bacteria. In this review, we look across the archaeal domain and discuss the diverse mechanisms by which archaea control cell cycle progression, DNA replication, and cell division. While the molecular and cellular processes archaea use to govern these critical cell biological processes often differ markedly from those described in bacteria and eukaryotes, there are also striking similarities that highlight both unique and common principles of cell cycle control across the different domains of life. Since much of the eukaryotic cell cycle machinery has its origins in archaea, exploration of the mechanisms of archaeal cell division also promises to illuminate the evolution of the eukaryotic cell cycle.

Origin of eukaryotic-like Vps23 shapes an ancient functional interplay between ESCRT and ubiquitin system in Asgard archaea

Functional interplay between the endosomal sorting complexes required for transport (ESCRT) and the ubiquitin system underlies the ubiquitin-dependent cargo-sorting pathway of the eukaryotic endomembrane system, yet its evolutionary origin remains unclear. Here, we show that a UEV-Vps23 protein family, which contains UEV and Vps23 domains, mediates an ancient ESCRT and ubiquitin system interplay in Asgard archaea. The UEV binds ubiquitin with high affinity, making the UEV-Vps23 a sensor for sorting ubiquitinated cargo. A steadiness box in the Vps23 domain undergoes ubiquitination through an Asgard E1, E2, and RING E3 cascade. The UEV-Vps23 switches between autoinhibited and active forms, regulating the ESCRT and ubiquitin system interplay. Furthermore, the shared sequence and structural homology among the UEV-Vps23, eukaryotic Vps23, and archaeal CdvA suggest a common evolutionary origin. Together, this work expands our understanding of the ancient ESCRT and ubiquitin system interplay that likely arose antedating divergent evolution between Asgard archaea and eukaryotes.

Archaeal virus entry and egress

Archaeal viruses display a high degree of structural and genomic diversity. Few details are known about the mechanisms by which these viruses enter and exit their host cells. Research on archaeal viruses has lately made significant progress due to advances in genetic tools and imaging techniques, such as cryo-electron tomography (cryo-ET). In recent years, a steady output of newly identified archaeal viral receptors and egress mechanisms has offered the first insight into how archaeal viruses interact with the archaeal cell envelope. As more details about archaeal viral entry and egress are unravelled, patterns are starting to emerge. This helps to better understand the interactions between viruses and the archaeal cell envelope and how these compare to infection strategies of viruses in other domains of life. Here, we provide an overview of recent developments in the field of archaeal viral entry and egress, shedding light onto the most elusive part of the virosphere.

The canonical single-stranded DNA-binding protein is not an essential replication factor but an RNA chaperon in Saccharolobus islandicus

Single-stranded DNA-binding proteins (SSBs) have been regarded as indispensable replication factors. Herein, we report that the genes encoding the canonical SSB (SisSSB) and the non-canonical SSB (SisDBP) in Saccharolobus islandicus REY15A are not essential for cell viability. Interestingly, at a lower temperature (55°C), the protein level of SisSSB increases and the growth of ΔSisssb and ΔSisssbΔSisdbp is retarded. SisSSB exhibits melting activity on dsRNA and DNA/RNA hybrid in vitro and is able to melt RNA hairpin in Escherichia coli. Furthermore, the core SisSSB domain is able to complement the absence of cold-shock proteins in E. coli. Importantly, these activities are conserved in the canonical SSBs from Crenarchaeota species that lack bacterial Csp homologs. Overall, our study has clarified the function of the archaeal canonical SSBs which do not function as a DNA-processing factor, but play a role in the processes requiring melting of dsRNA or DNA/RNA hybrid.

Roles of ESCRT-III polymers in cell division across the tree of life

Every cell becomes two through a carefully orchestrated process of division. Prior to division, contractile machinery must first be assembled at the cell midzone to ensure that the cut, when it is made, bisects the two separated copies of the genetic material. Second, this contractile machinery must be dynamically tethered to the limiting plasma membrane so as to bring the membrane with it as it constricts. Finally, the connecting membrane must be severed to generate two physically separate daughter cells. In several organisms across the tree of life, Endosomal Sorting Complex Required for Transport (ESCRT)-III family proteins aid cell division by forming composite polymers that function together with the Vps4 AAA-ATPase to constrict and cut the membrane tube connecting nascent daughter cells from the inside. In this review, we discuss unique features of ESCRT-III that enable it to play this role in division in many archaea and eukaryotes.

The patterned assembly and stepwise Vps4-mediated disassembly of composite ESCRT-III polymers drives archaeal cell division

ESCRT-III family proteins form composite polymers that deform and cut membrane tubes in the context of a wide range of cell biological processes across the tree of life. In reconstituted systems, sequential changes in the composition of ESCRT-III polymers induced by the AAA-adenosine triphosphatase Vps4 have been shown to remodel membranes. However, it is not known how composite ESCRT-III polymers are organized and remodeled in space and time in a cellular context. Taking advantage of the relative simplicity of the ESCRT-III-dependent division system in Sulfolobus acidocaldarius, one of the closest experimentally tractable prokaryotic relatives of eukaryotes, we use super-resolution microscopy, electron microscopy, and computational modeling to show how CdvB/CdvB1/CdvB2 proteins form a precisely patterned composite ESCRT-III division ring, which undergoes stepwise Vps4-dependent disassembly and contracts to cut cells into two. These observations lead us to suggest sequential changes in a patterned composite polymer as a general mechanism of ESCRT-III-dependent membrane remodeling.

A clade of RHH proteins ubiquitous in Sulfolobales and their viruses regulates cell cycle progression

Cell cycle regulation is crucial for all living organisms and is often targeted by viruses to facilitate their own propagation, yet cell cycle progression control is largely underexplored in archaea. In this work, we reveal a cell cycle regulator (aCcr1) carrying a ribbon-helix-helix (RHH) domain and ubiquitous in the Thermoproteota of the order Sulfolobales and their viruses. Overexpression of several aCcr1 members including gp21 of rudivirus SIRV2 and its host homolog SiL_0190 of Saccharolobus islandicus LAL14/1 results in impairment of cell division, evidenced by growth retardation, cell enlargement and an increase in cellular DNA content. Additionally, both gp21 and SiL_0190 can bind to the motif AGTATTA conserved in the promoter of several genes involved in cell division, DNA replication and cellular metabolism thereby repressing or inducing their transcription. Our results suggest that aCcr1 silences cell division and drives progression to the S-phase in Sulfolobales, a function exploited by viruses to facilitate viral propagation.

A novel RHH family transcription factor aCcr1 and its viral homologs dictate cell cycle progression in archaea

Cell cycle regulation is of paramount importance for all forms of life. Here, we report that a conserved and essential cell cycle-specific transcription factor (designated as aCcr1) and its viral homologs control cell division in Sulfolobales. We show that the transcription level of accr1 reaches peak during active cell division (D-phase) subsequent to the expression of CdvA, an archaea-specific cell division protein. Cells over-expressing the 58-aa-long RHH (ribbon-helix-helix) family cellular transcription factor as well as the homologs encoded by large spindle-shaped viruses Acidianus two-tailed virus (ATV) and Sulfolobus monocaudavirus 3 (SMV3) display significant growth retardation and cell division failure, manifesting as enlarged cells with multiple chromosomes. aCcr1 over-expression results in downregulation of 17 genes (>4-fold), including cdvA. A conserved motif, aCcr1-box, located between the TATA-binding box and the translation initiation site of 13 out of the 17 highly repressed genes, is critical for aCcr1 binding. The aCcr1-box is present in the promoters and 5' UTRs of cdvA genes across Sulfolobales, suggesting that aCcr1-mediated cdvA repression is an evolutionarily conserved mechanism by which archaeal cells dictate cytokinesis progression, whereas their viruses take advantage of this mechanism to manipulate the host cell cycle.

The archaeal Cdv cell division system

The Cdv system is the protein machinery that performs cell division and other membrane-deforming processes in a subset of archaea. Evolutionarily, the system is closely related to the eukaryotic ESCRT machinery, with which it shares many structural and functional similarities. Since its first description 15 years ago, the understanding of the Cdv system progressed rather slowly, but recent discoveries sparked renewed interest and insights. The emerging physical picture appears to be that CdvA acts as a membrane anchor, CdvB as a scaffold that localizes division to the mid-cell position, CdvB1 and CvdB2 as the actual constriction machinery, and CdvC as the ATPase that detaches Cdv proteins from the membrane. This paper provides a comprehensive overview of the research done on Cdv and explains how this relatively understudied machinery acts to perform its cell-division function. Understanding of the Cdv system helps to better grasp the biophysics and evolution of archaea, and furthermore provides new opportunities for the bottom-up building of a divisome for synthetic cells.

Sulfolobus islandicus Employs Orc1-2-Mediated DNA Damage Response in Defense against Infection by SSV2

All living organisms have evolved DNA damage response (DDR) strategies in coping with threats to the integrity of their genome. In response to DNA damage, Sulfolobus islandicus activates its DDR network in which Orc1-2, an ortholog of the archaeal Orc1/Cdc6 superfamily proteins, plays a central regulatory role. Here, we show that pretreatment with UV irradiation reduced virus genome replication in S. islandicus infected with the fusellovirus SSV2. Like treatment with UV or the DNA-damaging agent 4-nitroquinoline-1-oxide (NQO), infection with SSV2 facilitated the expression of orc1-2 and significantly raised the cellular level of Orc1-2. The inhibitory effect of UV irradiation on the virus DNA level was no longer apparent in the infected culture of an S. islandicus orc1-2 deletion mutant strain. On the other hand, the overexpression of orc1-2 decreased virus genomic DNA by ~10²-fold compared to that in the parent strain. Furthermore, as part of the Orc1-2-mediated DDR response genes for homologous recombination repair (HRR), cell aggregation and intercellular DNA transfer were upregulated, whereas genes for cell division were downregulated. However, the HRR pathway remained functional in host inhibition of SSV2 genome replication in the absence of UpsA, a subunit of pili essential for intercellular DNA transfer. In agreement with this finding, lack of the general transcriptional activator TFB3, which controls the expression of the ups genes, only moderately affected SSV2 genome replication. Our results demonstrate that infection of S. islandicus by SSV2 triggers the host DDR pathway that, in return, suppresses virus genome replication. IMPORTANCE Extremophiles thrive in harsh habitats and thus often face a daunting challenge to the integrity of their genome. How these organisms respond to virus infection when their genome is damaged remains unclear. We found that the thermophilic archaeon Sulfolobus islandicus became more inhibitory to genome replication of the virus SSV2 after preinfection UV irradiation than without the pretreatment. On the other hand, like treatment with UV or other DNA-damaging agents, infection of S. islandicus by SSV2 triggers the activation of Orc1-2-mediated DNA damage response, including the activation of homologous recombination repair, cell aggregation and DNA import, and the repression of cell division. The inhibitory effect of pretreatment with UV irradiation on SSV2 genome replication was no longer observed in an S. islandicus mutant lacking Orc1-2. Our results suggest that DNA damage response is employed by S. islandicus as a strategy to defend against virus infection.

The cell biology of archaea

The past decade has revealed the diversity and ubiquity of archaea in nature, with a growing number of studies highlighting their importance in ecology, biotechnology and even human health. Myriad lineages have been discovered, which expanded the phylogenetic breadth of archaea and revealed their central role in the evolutionary origins of eukaryotes. These discoveries, coupled with advances that enable the culturing and live imaging of archaeal cells under extreme environments, have underpinned a better understanding of their biology. In this Review we focus on the shape, internal organization and surface structures that are characteristic of archaeal cells as well as membrane remodelling, cell growth and division. We also highlight some of the technical challenges faced and discuss how new and improved technologies will help address many of the key unanswered questions.

Isolation of a virus causing a chronic infection in the archaeal model organism Haloferax volcanii reveals antiviral activities of a provirus

Viruses are important ecological, biogeochemical, and evolutionary drivers in every environment. Upon infection, they often cause the lysis of the host cell. However, some viruses exhibit alternative life cycles, such as chronic infections without cell lysis. The nature and the impact of chronic infections in prokaryotic host organisms remains largely unknown. Here, we characterize a novel haloarchaeal virus, Haloferax volcanii pleomorphic virus 1 (HFPV-1), which is currently the only virus infecting the model haloarchaeon Haloferax volcanii DS2, and demonstrate that HFPV-1 and H. volcanii are a great model system to study virus-host interactions in archaea. HFPV-1 is a pleomorphic virus that causes a chronic infection with continuous release of virus particles, but host and virus coexist without cell lysis or the appearance of resistant cells. Despite an only minor impact of the infection on host growth, we uncovered an extensive remodeling of the transcriptional program of the host (up to 1,049 differentially expressed genes). These changes are highlighted by a down-regulation of two endogenous provirus regions in the host genome, and we show that HFPV-1 infection is strongly influenced by a cross-talk between HFPV-1 and one of the proviruses mediated by a superinfection-like exclusion mechanism. Furthermore, HFPV-1 has a surprisingly wide host range among haloarchaea, and purified virus DNA can cause an infection after transformation into the host, making HFPV-1 a candidate for being developed into a genetic tool for a range of so far inaccessible haloarchaea.

Asgard archaea shed light on the evolutionary origins of the eukaryotic ubiquitin-ESCRT machinery

The ESCRT machinery, comprising of multiple proteins and subcomplexes, is crucial for membrane remodelling in eukaryotic cells, in processes that include ubiquitin-mediated multivesicular body formation, membrane repair, cytokinetic abscission, and virus exit from host cells. This ESCRT system appears to have simpler, ancient origins, since many archaeal species possess homologues of ESCRT-III and Vps4, the components that execute the final membrane scission reaction, where they have been shown to play roles in cytokinesis, extracellular vesicle formation and viral egress. Remarkably, metagenome assemblies of Asgard archaea, the closest known living relatives of eukaryotes, were recently shown to encode homologues of the entire cascade involved in ubiquitin-mediated membrane remodelling, including ubiquitin itself, components of the ESCRT-I and ESCRT-II subcomplexes, and ESCRT-III and Vps4. Here, we explore the phylogeny, structure, and biochemistry of Asgard homologues of the ESCRT machinery and the associated ubiquitylation system. We provide evidence for the ESCRT-I and ESCRT-II subcomplexes being involved in ubiquitin-directed recruitment of ESCRT-III, as it is in eukaryotes. Taken together, our analyses suggest a pre-eukaryotic origin for the ubiquitin-coupled ESCRT system and a likely path of ESCRT evolution via a series of gene duplication and diversification events.

The archaeal division protein CdvB1 assembles into polymers that are depolymerized by CdvC

The Cdv proteins constitute the cell division system of the Crenarchaea, a machinery closely related to the ESCRT system of eukaryotes. Using a combination of TEM imaging and biochemical assays, we here present an in vitro study of Metallosphaera sedula CdvB1, the Cdv protein that is believed to play a major role in the constricting ring that drives cell division in the Crenarchaea. We show that CdvB1 self-assembles into filaments that are depolymerized by the Vps4-homolog ATPase CdvC. Furthermore, we find that CdvB1 binds to negatively charged lipid membranes and can be detached from the membrane by the action of CdvC. Our findings provide novel insight into one of the main components of the archaeal cell division machinery.

Archaeal extracellular vesicles are produced in an ESCRT-dependent manner and promote gene transfer and nutrient cycling in extreme environments

Membrane-bound extracellular vesicles (EVs), secreted by cells from all three domains of life, transport various molecules and act as agents of intercellular communication in diverse environments. Here we demonstrate that EVs produced by a hyperthermophilic and acidophilic archaeon Sulfolobus islandicus carry not only a diverse proteome, enriched in membrane proteins, but also chromosomal and plasmid DNA, and can transfer this DNA to recipient cells. Furthermore, we show that EVs can support the heterotrophic growth of Sulfolobus in minimal medium, implicating EVs in carbon and nitrogen fluxes in extreme environments. Finally, our results indicate that, similar to eukaryotes, production of EVs in S. islandicus depends on the archaeal ESCRT machinery. We find that all components of the ESCRT apparatus are encapsidated into EVs. Using synchronized S. islandicus cultures, we show that EV production is linked to cell division and appears to be triggered by increased expression of ESCRT proteins during this cell cycle phase. Using a CRISPR-based knockdown system, we show that archaeal ESCRT-III and AAA+ ATPase Vps4 are required for EV production, whereas archaea-specific component CdvA appears to be dispensable. In particular, the active EV production appears to coincide with the expression patterns of ESCRT-III-1 and ESCRT-III-2, rather than ESCRT-III, suggesting a prime role of these proteins in EV budding. Collectively, our results suggest that ESCRT-mediated EV biogenesis has deep evolutionary roots, likely predating the divergence of eukaryotes and archaea, and that EVs play an important role in horizontal gene transfer and nutrient cycling in extreme environments.

Egress of archaeal viruses

Viruses of Archaea, arguably the most mysterious part of the virosphere due to their unique morphotypes and genome contents, exploit diverse mechanisms for releasing virus progeny from the host cell. These include virus release as a result of the enzymatic degradation of the cell wall or budding through it, common for viruses of Bacteria and Eukarya, as well as a unique mechanism of virus egress through small polygonal perforations on the cell surface. The process of the formation of these perforations includes the development of pyramidal structures on the membrane of the infected cell, which gradually grow by the expansion of their faces and eventually open like flower petals. This mechanism of virion release is operating exclusively in cells of hyperthermophilic hosts from the phylum Crenarchaeota, which are encased solely by a layer of surface proteins, S-layer. The review focuses on recent developments in understanding structural and biochemical details of all three types of egress mechanisms of archaeal viruses. TAKE AWAYS: Many archaeal viruses exit the host via polygonal perforations on the cell membrane. The molecular mechanism of exit via specific apertures is unique for archaeal viruses. Some enveloped archaeal viruses exploit the budding mechanism for egress.

High-Temperature Live-Cell Imaging of Cytokinesis, Cell Motility, and Cell-Cell Interactions in the Thermoacidophilic Crenarchaeon Sulfolobus acidocaldarius

Significant technical challenges have limited the study of extremophile cell biology. Here we describe a system for imaging samples at 75°C using high numerical aperture, oil-immersion lenses. With this system we observed and quantified the dynamics of cell division in the model thermoacidophilic crenarchaeon Sulfolobus acidocaldarius with unprecedented resolution. In addition, we observed previously undescribed dynamic cell shape changes, cell motility, and cell-cell interactions, shedding significant new light on the high-temperature lifestyle of this organism.

Bacterial Vipp1 and PspA are members of the ancient ESCRT-III membrane-remodeling superfamily

Membrane remodeling and repair are essential for all cells. Proteins that perform these functions include Vipp1/IM30 in photosynthetic plastids, PspA in bacteria, and ESCRT-III in eukaryotes. Here, using a combination of evolutionary and structural analyses, we show that these protein families are homologous and share a common ancient evolutionary origin that likely predates the last universal common ancestor. This homology is evident in cryo-electron microscopy structures of Vipp1 rings from the cyanobacterium Nostoc punctiforme presented over a range of symmetries. Each ring is assembled from rungs that stack and progressively tilt to form dome-shaped curvature. Assembly is facilitated by hinges in the Vipp1 monomer, similar to those in ESCRT-III proteins, which allow the formation of flexible polymers. Rings have an inner lumen that is able to bind and deform membranes. Collectively, these data suggest conserved mechanistic principles that underlie Vipp1, PspA, and ESCRT-III-dependent membrane remodeling across all domains of life.

Virus-induced cell gigantism and asymmetric cell division in archaea

Archaeal viruses represent one of the most mysterious parts of the global virosphere, with many virus groups sharing no evolutionary relationship to viruses of bacteria or eukaryotes. How these viruses interact with their hosts remains largely unexplored. Here we show that nonlytic lemon-shaped virus STSV2 interferes with the cell cycle control of its host, hyperthermophilic and acidophilic archaeon Sulfolobus islandicus, arresting the cell cycle in the S phase. STSV2 infection leads to transcriptional repression of the cell division machinery, which is homologous to the eukaryotic endosomal sorting complexes required for transport (ESCRT) system. The infected cells grow up to 20-fold larger in size, have 8,000-fold larger volume compared to noninfected cells, and accumulate massive amounts of viral and cellular DNA. Whereas noninfected Sulfolobus cells divide symmetrically by binary fission, the STSV2-infected cells undergo asymmetric division, whereby giant cells release normal-sized cells by budding, resembling the division of budding yeast. Reinfection of the normal-sized cells produces a new generation of giant cells. If the CRISPR-Cas system is present, the giant cells acquire virus-derived spacers and terminate the virus spread, whereas in its absence, the cycle continues, suggesting that CRISPR-Cas is the primary defense system in Sulfolobus against STSV2. Collectively, our results show how an archaeal virus manipulates the cell cycle, transforming the cell into a giant virion-producing factory.

Comparative CRISPR type III-based knockdown of essential genes in hyperthermophilic Sulfolobales and the evasion of lethal gene silencing

CRISPR type III systems, which are abundantly found in archaea, recognize and degrade RNA in their specific response to invading nucleic acids. Therefore, these systems can be harnessed for gene knockdown technologies even in hyperthermophilic archaea to study essential genes. We show here the broader usability of this posttranscriptional silencing technology by expanding the application to further essential genes and systematically analysing and comparing silencing thresholds and escape mutants. Synthetic guide RNAs expressed from miniCRISPR cassettes were used to silence genes involved in cell division (cdvA), transcription (rpo8), and RNA metabolism (smAP2) of the two crenarchaeal model organisms Saccharolobus solfataricus and Sulfolobus acidocaldarius. Results were systematically analysed together with those obtained from earlier experiments of cell wall biogenesis (slaB) and translation (aif5A). Comparison of over 100 individual transformants revealed gene-specific silencing maxima ranging between 40 and 75%, which induced specific knockdown phenotypes leading to growth retardation. Exceedance of this threshold by strong miniCRISPR constructs was not tolerated and led to specific mutation of the silencing miniCRISPR array and phenotypical reversion of cultures. In two thirds of sequenced reverted cultures, the targeting spacers were found to be precisely excised from the miniCRISPR array, indicating a still hypothetical, but highly active recombination system acting on the dynamics of CRISPR spacer arrays. Our results indicate that CRISPR type III - based silencing is a broadly applicable tool to study in vivo functions of essential genes in Sulfolobales which underlies a specific mechanism to avoid malignant silencing overdose.

Structure and assembly of archaeal viruses

Viruses of archaea represent one of the most enigmatic parts of the virosphere. Most of the characterized archaeal viruses infect extremophilic hosts and display remarkable diversity of virion morphotypes, many of which have never been observed among bacteriophages or viruses of eukaryotes. However, recent environmental studies have shown that archaeal viruses are widespread also in moderate ecosystems, where they play an important ecological role by influencing the turnover of microbial communities, with a global impact on the carbon and nitrogen cycles. In this review, we summarize recent advances in understanding the molecular details of virion organization and assembly of archaeal viruses. We start by briefly introducing the 20 officially recognized families of archaeal viruses and then outline the similarities and differences of archaeal virus assembly with the morphogenesis pathways used by bacterial and eukaryotic viruses, and discuss the evolutionary implications of these observations. Generally, the assembly of the icosahedral archaeal viruses closely follows the mechanisms employed by evolutionarily related bacterial and eukaryotic viruses with the HK97 fold and double jelly-roll major capsid proteins, emphasizing the overall conservation of these pathways over billions of years of evolution. By contrast, archaea-specific viruses employ unique virion assembly mechanisms. We also highlight some of the molecular adaptations underlying the stability of archaeal viruses in extreme environments. Despite considerable progress during the past few years, the archaeal virosphere continues to represent one of the least studied parts of the global virome, with many molecular features awaiting to be discovered and characterized.

The proteasome controls ESCRT-III-mediated cell division in an archaeon

Sulfolobus acidocaldarius is the closest experimentally tractable archaeal relative of eukaryotes and, despite lacking obvious cyclin-dependent kinase and cyclin homologs, has an ordered eukaryote-like cell cycle with distinct phases of DNA replication and division. Here, in exploring the mechanism of cell division in S. acidocaldarius, we identify a role for the archaeal proteasome in regulating the transition from the end of one cell cycle to the beginning of the next. Further, we identify the archaeal ESCRT-III homolog, CdvB, as a key target of the proteasome and show that its degradation triggers division by allowing constriction of the CdvB1:CdvB2 ESCRT-III division ring. These findings offer a minimal mechanism for ESCRT-III-mediated membrane remodeling and point to a conserved role for the proteasome in eukaryotic and archaeal cell cycle control.

Live Imaging of a Hyperthermophilic Archaeon Reveals Distinct Roles for Two ESCRT-III Homologs in Ensuring a Robust and Symmetric Division

Live-cell imaging has revolutionized our understanding of dynamic cellular processes in bacteria and eukaryotes. Although similar techniques have been applied to the study of halophilic archaea [1-5], our ability to explore the cell biology of thermophilic archaea has been limited by the technical challenges of imaging at high temperatures. Sulfolobus are the most intensively studied members of TACK archaea and have well-established molecular genetics [6-9]. Additionally, studies using Sulfolobus were among the first to reveal striking similarities between the cell biology of eukaryotes and archaea [10-15]. However, to date, it has not been possible to image Sulfolobus cells as they grow and divide. Here, we report the construction of the Sulfoscope, a heated chamber on an inverted fluorescent microscope that enables live-cell imaging of thermophiles. By using thermostable fluorescent probes together with this system, we were able to image Sulfolobus acidocaldarius cells live to reveal tight coupling between changes in DNA condensation, segregation, and cell division. Furthermore, by imaging deletion mutants, we observed functional differences between the two ESCRT-III proteins implicated in cytokinesis, CdvB1 and CdvB2. The deletion of cdvB1 compromised cell division, causing occasional division failures, whereas the ΔcdvB2 exhibited a profound loss of division symmetry, generating daughter cells that vary widely in size and eventually generating ghost cells. These data indicate that DNA separation and cytokinesis are coordinated in Sulfolobus, as is the case in eukaryotes, and that two contractile ESCRT-III polymers perform distinct roles to ensure that Sulfolobus cells undergo a robust and symmetrical division.

Coevolution of Eukaryote-like Vps4 and ESCRT-III Subunits in the Asgard Archaea

The emergence of the endomembrane system is a key step in the evolution of cellular complexity during eukaryogenesis. The endosomal sorting complex required for transport (ESCRT) machinery is essential and required for the endomembrane system functions in eukaryotic cells. Recently, genes encoding eukaryote-like ESCRT protein components have been identified in the genomes of Asgard archaea, a newly proposed archaeal superphylum that is thought to include the closest extant prokaryotic relatives of eukaryotes. However, structural and functional features of Asgard ESCRT remain uncharacterized. Here, we show that Vps4, Vps2/24/46, and Vps20/32/60, the core functional components of the Asgard ESCRT, coevolved eukaryote-like structural and functional features. Phylogenetic analysis shows that Asgard Vps4, Vps2/24/46, and Vps20/32/60 are closely related to their eukaryotic counterparts. Molecular dynamics simulation and biochemical assays indicate that Asgard Vps4 contains a eukaryote-like microtubule-interacting and transport (MIT) domain that binds the distinct type 1 MIT-interacting motif and type 2 MIT-interacting motif in Vps2/24/46 and Vps20/32/60, respectively. The Asgard Vps4 partly, but much more efficiently than homologs from other archaea, complements the vps4 null mutant of Saccharomyces cerevisiae, further supporting the functional similarity between the membrane remodeling machineries of Asgard archaea and eukaryotes. Thus, this work provides evidence that the ESCRT complexes from Asgard archaea and eukaryotes are evolutionarily related and functionally similar. Thus, despite the apparent absence of endomembranes in Asgard archaea, the eukaryotic ESCRT seems to have been directly inherited from an Asgard ancestor, to become a key component of the emerging endomembrane system.IMPORTANCE The discovery of Asgard archaea has changed the existing ideas on the origins of eukaryotes. Researchers propose that eukaryotic cells evolved from Asgard archaea. This hypothesis partly stems from the presence of multiple eukaryotic signature proteins in Asgard archaea, including homologs of ESCRT proteins that are essential components of the endomembrane system in eukaryotes. However, structural and functional features of Asgard ESCRT remain unknown. Our study provides evidence that Asgard ESCRT is functionally comparable to the eukaryotic counterparts, suggesting that despite the apparent absence of endomembranes in archaea, eukaryotic ESCRT was inherited from an Asgard archaeal ancestor, alongside the emergence of endomembrane system during eukaryogenesis.

Multidomain ribosomal protein trees and the planctobacterial origin of neomura (eukaryotes, archaebacteria)

Palaeontologically, eubacteria are > 3× older than neomura (eukaryotes, archaebacteria). Cell biology contrasts ancestral eubacterial murein peptidoglycan walls and derived neomuran N-linked glycoprotein coats/walls. Misinterpreting long stems connecting clade neomura to eubacteria on ribosomal sequence trees (plus misinterpreted protein paralogue trees) obscured this historical pattern. Universal multiprotein ribosomal protein (RP) trees, more accurate than rRNA trees, are taxonomically undersampled. To reduce contradictions with genically richer eukaryote trees and improve eubacterial phylogeny, we constructed site-heterogeneous and maximum-likelihood universal three-domain, two-domain, and single-domain trees for 143 eukaryotes (branching now congruent with 187-protein trees), 60 archaebacteria, and 151 taxonomically representative eubacteria, using 51 and 26 RPs. Site-heterogeneous trees greatly improve eubacterial phylogeny and higher classification, e.g. showing gracilicute monophyly, that many 'rDNA-phyla' belong in Proteobacteria, and reveal robust new phyla Synthermota and Aquithermota. Monoderm Posibacteria and Mollicutes (two separate wall losses) are both polyphyletic: multiple outer membrane losses in Endobacteria occurred separately from Actinobacteria; neither phylum is related to Chloroflexi, the most divergent prokaryotes, which originated photosynthesis (new model proposed). RP trees support an eozoan root for eukaryotes and are consistent with archaebacteria being their sisters and rooted between Filarchaeota (=Proteoarchaeota, including 'Asgardia') and Euryarchaeota sensu-lato (including ultrasimplified 'DPANN' whose long branches often distort trees). Two-domain trees group eukaryotes within Planctobacteria, and archaebacteria with Planctobacteria/Sphingobacteria. Integrated molecular/palaeontological evidence favours negibacterial ancestors for neomura and all life. Unique presence of key pre-neomuran characters favours Planctobacteria only as ancestral to neomura, which apparently arose by coevolutionary repercussions (explained here in detail, including RP replacement) of simultaneous outer membrane and murein loss. Planctobacterial C-1 methanotrophic enzymes are likely ancestral to archaebacterial methanogenesis and β-propeller-α-solenoid proteins to eukaryotic vesicle coats, nuclear-pore-complexes, and intraciliary transport. Planctobacterial chaperone-independent 4/5-protofilament microtubules and MamK actin-ancestors prepared for eukaryote intracellular motility, mitosis, cytokinesis, and phagocytosis. We refute numerous wrong ideas about the universal tree.

Multidomain ribosomal protein trees and the planctobacterial origin of neomura (eukaryotes, archaebacteria)

Palaeontologically, eubacteria are > 3× older than neomura (eukaryotes, archaebacteria). Cell biology contrasts ancestral eubacterial murein peptidoglycan walls and derived neomuran N-linked glycoprotein coats/walls. Misinterpreting long stems connecting clade neomura to eubacteria on ribosomal sequence trees (plus misinterpreted protein paralogue trees) obscured this historical pattern. Universal multiprotein ribosomal protein (RP) trees, more accurate than rRNA trees, are taxonomically undersampled. To reduce contradictions with genically richer eukaryote trees and improve eubacterial phylogeny, we constructed site-heterogeneous and maximum-likelihood universal three-domain, two-domain, and single-domain trees for 143 eukaryotes (branching now congruent with 187-protein trees), 60 archaebacteria, and 151 taxonomically representative eubacteria, using 51 and 26 RPs. Site-heterogeneous trees greatly improve eubacterial phylogeny and higher classification, e.g. showing gracilicute monophyly, that many 'rDNA-phyla' belong in Proteobacteria, and reveal robust new phyla Synthermota and Aquithermota. Monoderm Posibacteria and Mollicutes (two separate wall losses) are both polyphyletic: multiple outer membrane losses in Endobacteria occurred separately from Actinobacteria; neither phylum is related to Chloroflexi, the most divergent prokaryotes, which originated photosynthesis (new model proposed). RP trees support an eozoan root for eukaryotes and are consistent with archaebacteria being their sisters and rooted between Filarchaeota (=Proteoarchaeota, including 'Asgardia') and Euryarchaeota sensu-lato (including ultrasimplified 'DPANN' whose long branches often distort trees). Two-domain trees group eukaryotes within Planctobacteria, and archaebacteria with Planctobacteria/Sphingobacteria. Integrated molecular/palaeontological evidence favours negibacterial ancestors for neomura and all life. Unique presence of key pre-neomuran characters favours Planctobacteria only as ancestral to neomura, which apparently arose by coevolutionary repercussions (explained here in detail, including RP replacement) of simultaneous outer membrane and murein loss. Planctobacterial C-1 methanotrophic enzymes are likely ancestral to archaebacterial methanogenesis and β-propeller-α-solenoid proteins to eukaryotic vesicle coats, nuclear-pore-complexes, and intraciliary transport. Planctobacterial chaperone-independent 4/5-protofilament microtubules and MamK actin-ancestors prepared for eukaryote intracellular motility, mitosis, cytokinesis, and phagocytosis. We refute numerous wrong ideas about the universal tree.

ESCRT Machinery Mediates Cytokinetic Abscission in the Unicellular Red Alga Cyanidioschyzon merolae

In many eukaryotes, cytokinesis proceeds in two successive steps: first, ingression of the cleavage furrow and second, abscission of the intercellular bridge. In animal cells, the actomyosin contractile ring is involved in the first step, while the endosomal sorting complex required for transport (ESCRT), which participates in various membrane fusion/fission events, mediates the second step. Intriguingly, in archaea, ESCRT is involved in cytokinesis, raising the hypothesis that the function of ESCRT in eukaryotic cytokinesis descended from the archaeal ancestor. In eukaryotes other than in animals, the roles of ESCRT in cytokinesis are poorly understood. To explore the primordial core mechanisms for eukaryotic cytokinesis, we investigated ESCRT functions in the unicellular red alga Cyanidioschyzon merolae that diverged early in eukaryotic evolution. C. merolae provides an excellent experimental system. The cell has a simple organelle composition. The genome (16.5 Mb, 5335 genes) has been completely sequenced, transformation methods are established, and the cell cycle is synchronized by a light and dark cycle. Similar to animal and fungal cells, C. merolae cells divide by furrowing at the division site followed by abscission of the intercellular bridge. However, they lack an actomyosin contractile ring. The proteins that comprise ESCRT-I-IV, the four subcomplexes of ESCRT, are partially conserved in C. merolae. Immunofluorescence of native or tagged proteins localized the homologs of the five ESCRT-III components [charged multivesicular body protein (CHMP) 1, 2, and 4-6], apoptosis-linked gene-2-interacting protein X (ALIX), the ESCRT-III adapter, and the main ESCRT-IV player vacuolar protein sorting (VPS) 4, to the intercellular bridge. In addition, ALIX was enriched around the cleavage furrow early in cytokinesis. When the ESCRT function was perturbed by expressing dominant-negative VPS4, cells with an elongated intercellular bridge accumulated-a phenotype resulting from abscission failure. Our results show that ESCRT mediates cytokinetic abscission in C. merolae. The fact that ESCRT plays a role in cytokinesis in archaea, animals, and early diverged alga C. merolae supports the hypothesis that the function of ESCRT in cytokinesis descended from archaea to a common ancestor of eukaryotes.

CRISPR-mediated gene silencing reveals involvement of the archaeal S-layer in cell division and virus infection

The S-layer is a proteinaceous surface lattice found in the cell envelope of bacteria and archaea. In most archaea, a glycosylated S-layer constitutes the sole cell wall and there is evidence that it contributes to cell shape maintenance and stress resilience. Here we use a gene-knockdown technology based on an endogenous CRISPR type III complex to gradually silence slaB, which encodes the S-layer membrane anchor in the hyperthermophilic archaeon Sulfolobus solfataricus. Silenced cells exhibit a reduced or peeled-off S-layer lattice, cell shape alterations and decreased surface glycosylation. These cells barely propagate but increase in diameter and DNA content, indicating impaired cell division; their phenotypes can be rescued through genetic complementation. Furthermore, S-layer depleted cells are less susceptible to infection with the virus SSV1. Our study highlights the usefulness of the CRISPR type III system for gene silencing in archaea, and supports that an intact S-layer is important for cell division and virus susceptibility.

The S-layer is a proteinaceous envelope often found in bacterial and archaeal cells. Here, the authors use CRISPR-based technology to silence slaB, encoding the S-layer membrane anchor, to show that an intact S-layer is important for cell division and virus susceptibility in the archaeon Sulfolobus solfataricus.

Cell Structure Changes in the Hyperthermophilic Crenarchaeon Sulfolobus islandicus Lacking the S-Layer

The S-layer is considered to be the sole component of the cell wall in Sulfolobales, a taxonomic group within the Crenarchaeota whose cellular features have been suggested to have a close relationship to the last archaea-eukaryote common ancestor. In this study, we genetically dissect how the two previously characterized S-layer genes as well as a newly identified S-layer-associated protein-encoding gene contribute to the S-layer architecture in Sulfolobus. We provide genetic evidence for the first time showing that the slaA gene is a key cell morphology determinant and may play a role in Sulfolobus cell division or/and cell fusion.

Rediscovery of the ancient evolutionary relationship between archaea and eukaryotes has revitalized interest in archaeal cell biology. Key to the understanding of archaeal cells is the surface layer (S-layer), which is commonly found in Archaea but whose in vivo function is unknown. Here, we investigate the architecture and cellular roles of the S-layer in the hyperthermophilic crenarchaeon Sulfolobus islandicus. Electron micrographs of mutant cells lacking slaA or both slaA and slaB confirm the absence of the outermost layer (SlaA), whereas cells with intact or partially or completely detached SlaA are observed for the ΔslaB mutant. We experimentally identify a novel S-layer-associated protein, M164_1049, which does not functionally replace its homolog SlaB but likely assists SlaB to stabilize SlaA. Mutants deficient in the SlaA outer layer form large cell aggregates, and individual cell size varies, increasing significantly up to six times the diameter of wild-type cells. We show that the ΔslaA mutant cells exhibit more sensitivity to hyperosmotic stress but are not reduced to wild-type cell size. The ΔslaA mutant contains aberrant chromosome copy numbers not seen in wild-type cells, in which the cell cycle is tightly regulated. Together, these data suggest that the lack of SlaA results in either cell fusion or irregularities in cell division. Our studies show the key physiological and cellular functions of the S-layer in this archaeal cell.

An Orc1/Cdc6 ortholog functions as a key regulator in the DNA damage response in Archaea

While bacteria and eukaryotes show distinct mechanisms of DNA damage response (DDR) regulation, investigation of ultraviolet (UV)-responsive expression in a few archaea did not yield any conclusive evidence for an archaeal DDR regulatory network. Nevertheless, expression of Orc1-2, an ortholog of the archaeal origin recognition complex 1/cell division control protein 6 (Orc1/Cdc6) superfamily proteins was strongly activated in Sulfolobus solfataricus and Sulfolobus acidocaldarius upon UV irradiation. Here, a series of experiments were conducted to investigate the possible functions of Orc1-2 in DNA damage repair in Sulfolobus islandicus. Study of DDR in Δorc1-2 revealed that Orc1-2 deficiency abolishes DNA damage-induced differential expression of a large number of genes and the mutant showed hypersensitivity to DNA damage treatment. Reporter gene and DNase I footprinting assays demonstrated that Orc1-2 interacts with a conserved hexanucleotide motif present in several DDR gene promoters and regulates their expression. Manipulation of orc1-2 expression by promoter substitution in this archaeon revealed that a high level of orc1-2 expression is essential but not sufficient to trigger DDR. Together, these results have placed Orc1-2 in the heart of the archaeal DDR regulation, and the resulting Orc1-2-centered regulatory circuit represents the first DDR network identified in Archaea, the third domain of life.

Functional Insights Into Protein Acetylation in the Hyperthermophilic Archaeon Sulfolobus islandicus

Proteins undergo acetylation at the Nε-amino group of lysine residues and the Nα-amino group of the N terminus in Archaea as in Bacteria and Eukarya. However, the extent, pattern and roles of the modifications in Archaea remain poorly understood. Here we report the proteomic analyses of a wild-type Sulfolobus islandicus strain and its mutant derivative strains lacking either a homolog of the protein acetyltransferase Pat (ΔSisPat) or a homolog of the Nt-acetyltransferase Ard1 (ΔSisArd1). A total of 1708 Nε-acetylated lysine residues in 684 proteins (26% of the total proteins), and 158 Nt-acetylated proteins (44% of the identified proteins) were found in S. islandicus ΔSisArd1 grew more slowly than the parental strain, whereas ΔSisPat showed no significant growth defects. Only 24 out of the 1503 quantifiable Nε-acetylated lysine residues were differentially acetylated, and all but one of the 24 residues were less acetylated by >1.3 fold in ΔSisPat than in the parental strain, indicating the narrow substrate specificity of the enzyme. Six acyl-CoA synthetases were the preferred substrates of SisPat in vivo, suggesting that Nε-acetylation by the acetyltransferase is involved in maintaining metabolic balance in the cell. Acetylation of acyl-CoA synthetases by SisPat occurred at a sequence motif conserved among all three domains of life. On the other hand, 92% of the acetylated N termini identified were acetylated by SisArd1 in the cell. The enzyme exhibited broad substrate specificity and could modify nearly all types of the target N termini of human NatA-NatF. The deletion of the SisArd1 gene altered the cellular levels of 18% of the quantifiable proteins (1518) by >1.5 fold. Consistent with the growth phenotype of ΔSisArd1, the cellular levels of proteins involved in cell division and cell cycle control, DNA replication, and purine synthesis were significantly lowered in the mutant than those in the parental strain.

Archaeal cell biology: diverse functions of tubulin-like cytoskeletal proteins at the cell envelope

The tubulin superfamily of cytoskeletal proteins is widespread in all three domains of life — Archaea, Bacteria and Eukarya. Tubulins build the microtubules of the eukaryotic cytoskeleton, whereas members of the homologous FtsZ family construct the division ring in prokaryotes and some eukaryotic organelles. Their functions are relatively poorly understood in archaea, yet these microbes contain a remarkable diversity of tubulin superfamily proteins, including FtsZ for division, a newly described major family called CetZ that is involved in archaeal cell shape control, and several other divergent families of unclear function that are implicated in a variety of cell envelope-remodelling contexts. Archaeal model organisms, particularly halophilic archaea such as Haloferax volcanii, have sufficiently developed genetic tools and we show why their large, flattened cells that are capable of controlled differentiation are also well suited to cell biological investigations by live-cell high-resolution light and electron microscopy. As most archaea only have a glycoprotein lattice S-layer, rather than a peptidoglycan cell wall like bacteria, the activity of the tubulin-like cytoskeletal proteins at the cell envelope is expected to vary significantly, and may involve direct membrane remodelling or directed synthesis or insertion of the S-layer protein subunits. Further studies of archaeal cell biology will provide fresh insight into the evolution of cells and the principles in common to their fundamental activities across the full spectrum of cellular life.

The essential genome of the crenarchaeal model Sulfolobus islandicus

Sulfolobus islandicus is a model microorganism in the TACK superphylum of the Archaea, a key lineage in the evolutionary history of cells. Here we report a genome-wide identification of the repertoire of genes essential to S. islandicus growth in culture. We confirm previous targeted gene knockouts, uncover the non-essentiality of functions assumed to be essential to the Sulfolobus cell, including the proteinaceous S-layer, and highlight essential genes whose functions are yet to be determined. Phyletic distributions illustrate the potential transitions that may have occurred during the evolution of this archaeal microorganism, and highlight sets of genes that may have been associated with each transition. We use this comparative context as a lens to focus future research on archaea-specific uncharacterized essential genes that may provide valuable insights into the evolutionary history of cells.

Sulfolobus islandicus is a model organism within the TACK superphylum of the Archaea. Here, the authors perform a genome-wide analysis of essential genes in this organism, show that the proteinaceous S-layer is not essential, and explore potential stages of evolution of the essential gene repertoire in Archaea.

Dividing the Archaeal Way: The Ancient Cdv Cell-Division Machinery

Cell division in most prokaryotes is mediated by the well-studied fts genes, with FtsZ as the principal player. In many archaeal species, however, division is orchestrated differently. The Crenarchaeota phylum of archaea features the action of the three proteins, CdvABC. This Cdv system is a unique and less-well-studied division mechanism that merits closer inspection. In vivo, the three Cdv proteins form a composite band that contracts concomitantly with the septum formation. Of the three Cdv proteins, CdvA is the first to be recruited to the division site, while CdvB and CdvC are thought to participate in the active part of the Cdv division machinery. Interestingly, CdvB shares homology with a family of proteins from the eukaryotic ESCRT-III complex, and CdvC is a homolog of the eukaryotic Vps4 complex. These two eukaryotic complexes are key factors in the endosomal sorting complex required for transport (ESCRT) pathway, which is responsible for various budding processes in eukaryotic cells and which participates in the final stages of division in Metazoa. There, ESCRT-III forms a contractile machinery that actively cuts the membrane, whereas Vps4, which is an ATPase, is necessary for the turnover of the ESCRT membrane-abscission polymers. In contrast to CdvB and CdvC, CdvA is unique to the archaeal Crenarchaeota and Thaumarchaeota phyla. The Crenarchaeota division mechanism has often been suggested to represent a simplified version of the ESCRT division machinery thus providing a model system to study the evolution and mechanism of cell division in higher organisms. However, there are still many open questions regarding this parallelism and the division mechanism of Crenarchaeota. Here, we review the existing data on the role of the Cdv proteins in the division process of Crenarchaeota as well as concisely review the ESCRT system in eukaryotes. We survey the similarities and differences between the division and abscission mechanisms in the two cases. We suggest that the Cdv system functions differently in archaea than ESCRT does in eukaryotes, and that, unlike the eukaryotic case, the Cdv system's main function may be related to surplus membrane invagination and cell-wall synthesis.

The enigmatic archaeal virosphere

One of the most prominent features of archaea is the extraordinary diversity of their DNA viruses. Many archaeal viruses differ substantially in morphology from bacterial and eukaryotic viruses and represent unique virus families. The distinct nature of archaeal viruses also extends to the gene composition and architectures of their genomes and the properties of the proteins that they encode. Environmental research has revealed prominent roles of archaeal viruses in influencing microbial communities in ocean ecosystems, and recent metagenomic studies have uncovered new groups of archaeal viruses that infect extremophiles and mesophiles in diverse habitats. In this Review, we summarize recent advances in our understanding of the genomic and morphological diversity of archaeal viruses and the molecular biology of their life cycles and virus-host interactions, including interactions with archaeal CRISPR-Cas systems. We also examine the potential origins and evolution of archaeal viruses and discuss their place in the global virosphere.