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

Nuclear and degradative functions of the ESCRT-III pathway: implications for neurodegenerative disease

The ESCRT machinery plays a pivotal role in membrane-remodeling events across multiple cellular processes including nuclear envelope repair and reformation, nuclear pore complex surveillance, endolysosomal trafficking, and neuronal pruning. Alterations in ESCRT-III functionality have been associated with neurodegenerative diseases including Frontotemporal Dementia (FTD), Amyotrophic Lateral Sclerosis (ALS), and Alzheimer's Disease (AD). In addition, mutations in specific ESCRT-III proteins have been identified in FTD/ALS. Thus, understanding how disruptions in the fundamental functions of this pathway and its individual protein components in the human central nervous system (CNS) may offer valuable insights into mechanisms underlying neurodegenerative disease pathogenesis and identification of potential therapeutic targets. In this review, we discuss ESCRT components, dynamics, and functions, with a focus on the ESCRT-III pathway. In addition, we explore the implications of altered ESCRT-III function for neurodegeneration with a primary emphasis on nuclear surveillance and endolysosomal trafficking within the CNS.

Evolutionary origins of the lysosome-related organelle sorting machinery reveal ancient homology in post-endosome trafficking pathways

The major organelles of the endomembrane system were in place by the time of the last eukaryotic common ancestor (LECA) (~1.5 billion years ago). Their acquisitions were defining milestones during eukaryogenesis. Comparative cell biology and evolutionary analyses show multiple instances of homology in the protein machinery controlling distinct interorganelle trafficking routes. Resolving these homologous relationships allows us to explore processes underlying the emergence of additional, distinct cellular compartments, infer ancestral states predating LECA, and explore the process of eukaryogenesis itself. Here, we undertake a molecular evolutionary analysis (including providing a transcriptome of the jakobid flagellate Reclinomonas americana), exploring the origins of the machinery responsible for the biogenesis of lysosome-related organelles (LROs), the Biogenesis of LRO Complexes (BLOCs 1,2, and 3). This pathway has been studied only in animals and is not considered a feature of the basic eukaryotic cell plan. We show that this machinery is present across the eukaryotic tree of life and was likely in place prior to LECA, making it an underappreciated facet of eukaryotic cellular organisation. Moreover, we resolve multiple points of ancient homology between all three BLOCs and other post-endosomal retrograde trafficking machinery (BORC, CCZ1 and MON1 proteins, and an unexpected relationship with the "homotypic fusion and vacuole protein sorting" (HOPS) and "Class C core vacuole/endosomal tethering" (CORVET) complexes), offering a mechanistic and evolutionary unification of these trafficking pathways. Overall, this study provides a comprehensive account of the rise of the LROs biogenesis machinery from before the LECA to current eukaryotic diversity, integrating it into the larger mechanistic framework describing endomembrane evolution.

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.

The emerging view on the origin and early evolution of eukaryotic cells

The origin of the eukaryotic cell, with its compartmentalized nature and generally large size compared with bacterial and archaeal cells, represents a cornerstone event in the evolution of complex life on Earth. In a process referred to as eukaryogenesis, the eukaryotic cell is believed to have evolved between approximately 1.8 and 2.7 billion years ago from its archaeal ancestors, with a symbiosis with a bacterial (proto-mitochondrial) partner being a key event. In the tree of life, the branch separating the first from the last common ancestor of all eukaryotes is long and lacks evolutionary intermediates. As a result, the timing and driving forces of the emergence of complex eukaryotic features remain poorly understood. During the past decade, environmental and comparative genomic studies have revealed vital details about the identity and nature of the host cell and the proto-mitochondrial endosymbiont, enabling a critical reappraisal of hypotheses underlying the symbiotic origin of the eukaryotic cell. Here we outline our current understanding of the key players and events underlying the emergence of cellular complexity during the prokaryote-to-eukaryote transition and discuss potential avenues of future research that might provide new insights into the enigmatic origin of the eukaryotic cell.

Preserving Genome Integrity: Unveiling the Roles of ESCRT Machinery

The endosomal sorting complex required for transport (ESCRT) machinery is composed of an articulated architecture of proteins that assemble at multiple cellular sites. The ESCRT machinery is involved in pathways that are pivotal for the physiology of the cell, including vesicle transport, cell division, and membrane repair. The subunits of the ESCRT I complex are mainly responsible for anchoring the machinery to the action site. The ESCRT II subunits function to bridge and recruit the ESCRT III subunits. The latter are responsible for finalizing operations that, independently of the action site, involve the repair and fusion of membrane edges. In this review, we report on the data related to the activity of the ESCRT machinery at two sites: the nuclear membrane and the midbody and the bridge linking cells in the final stages of cytokinesis. In these contexts, the machinery plays a significant role for the protection of genome integrity by contributing to the control of the abscission checkpoint and to nuclear envelope reorganization and correlated resilience. Consistently, several studies show how the dysfunction of the ESCRT machinery causes genome damage and is a codriver of pathologies, such as laminopathies and cancer.

Three-dimensional architecture of ESCRT-III flat spirals on the membrane

The endosomal sorting complexes required for transport (ESCRTs) are responsible for membrane remodeling in many cellular processes, such as multivesicular body biogenesis, viral budding, and cytokinetic abscission. ESCRT-III, the most abundant ESCRT subunit, assembles into flat spirals as the primed state, essential to initiate membrane invagination. However, the three-dimensional architecture of ESCRT-III flat spirals remained vague for decades due to highly curved filaments with a small diameter and a single preferred orientation on the membrane. Here, we unveiled that yeast Snf7, a component of ESCRT-III, forms flat spirals on the lipid monolayers using cryogenic electron microscopy. We developed a geometry-constrained Euler angle-assigned reconstruction strategy and obtained moderate-resolution structures of Snf7 flat spirals with varying curvatures. Our analyses showed that Snf7 subunits recline on the membrane with N-terminal motifs α0 as anchors, adopt an open state with fused α2/3 helices, and bend α2/3 gradually from the outer to inner parts of flat spirals. In all, we provide the orientation and conformations of ESCRT-III flat spirals on the membrane and unveil the underlying assembly mechanism, which will serve as the initial step in understanding how ESCRTs drive membrane abscission.

The discovery of archaea: from observed anomaly to consequential restructuring of the phylogenetic tree

Observational and experimental discoveries of new factual entities such as objects, systems, or processes, are major contributors to some advances in the life sciences. Yet, whereas discovery of theories was extensively deliberated by philosophers of science, very little philosophical attention was paid to the discovery of factual entities. This paper examines historical and philosophical aspects of the experimental discovery by Carl Woese of archaea, prokaryotes that comprise one of the three principal domains of the phylogenetic tree. Borrowing Kuhn's terminology, this discovery of a major biological entity was made during a 'normal science' project of building molecular taxonomy for prokaryotes. Unexpectedly, however, an observed anomaly instigated the discovery of archaea. Substantiation of the existence of the new archaeal entity and consequent reconstruction of the phylogenetic tree prompted replacement of a long-held model of a prokarya and eukarya bipartite tree of life by a new model of a tripartite tree comprising of bacteria, archaea, and eukarya. This paper explores the history and philosophical implications of the progression of Woese's project from normal science to anomaly-instigated model-changing discovery. It is also shown that the consequential discoveries of RNA splicing and of ribozymes were similarly prompted by unexpected irregularities during normal science activities. It is thus submitted that some discoveries of factual biological entities are triggered by unforeseen observational or experimental anomalies.

Diversity, origin, and evolution of the ESCRT systems

Endosomal sorting complexes required for transport (ESCRT) play key roles in protein sorting between membrane-bounded compartments of eukaryotic cells. Homologs of many ESCRT components are identifiable in various groups of archaea, especially in Asgardarchaeota, the archaeal phylum that is currently considered to include the closest relatives of eukaryotes, but not in bacteria. We performed a comprehensive search for ESCRT protein homologs in archaea and reconstructed ESCRT evolution using the phylogenetic tree of Vps4 ATPase (ESCRT IV) as a scaffold and using sensitive protein sequence analysis and comparison of structural models to identify previously unknown ESCRT proteins. Several distinct groups of ESCRT systems in archaea outside of Asgard were identified, including proteins structurally similar to ESCRT-I and ESCRT-II, and several other domains involved in protein sorting in eukaryotes, suggesting an early origin of these components. Additionally, distant homologs of CdvA proteins were identified in Thermoproteales which are likely components of the uncharacterized cell division system in these archaea. We propose an evolutionary scenario for the origin of eukaryotic and Asgard ESCRT complexes from ancestral building blocks, namely, the Vps4 ATPase, ESCRT-III components, wH (winged helix-turn-helix fold) and possibly also coiled-coil, and Vps28-like domains. The last archaeal common ancestor likely encompassed a complex ESCRT system that was involved in protein sorting. Subsequent evolution involved either simplification, as in the TACK superphylum, where ESCRT was co-opted for cell division, or complexification as in Asgardarchaeota. In Asgardarchaeota, the connection between ESCRT and the ubiquitin system that was previously considered a eukaryotic signature was already established.IMPORTANCEAll eukaryotic cells possess complex intracellular membrane organization. Endosomal sorting complexes required for transport (ESCRT) play a central role in membrane remodeling which is essential for cellular functionality in eukaryotes. Recently, it has been shown that Asgard archaea, the archaeal phylum that includes the closest known relatives of eukaryotes, encode homologs of many components of the ESCRT systems. We employed protein sequence and structure comparisons to reconstruct the evolution of ESCRT systems in archaea and identified several previously unknown homologs of ESCRT subunits, some of which can be predicted to participate in cell division. The results of this reconstruction indicate that the last archaeal common ancestor already encoded a complex ESCRT system that was involved in protein sorting. In Asgard archaea, ESCRT systems evolved toward greater complexity, and in particular, the connection between ESCRT and the ubiquitin system that was previously considered a eukaryotic signature was established.

Micro Trojan horses: Engineering extracellular vesicles crossing biological barriers for drug delivery

The biological barriers of the body, such as the blood-brain, placental, intestinal, skin, and air-blood, protect against invading viruses and bacteria while providing necessary physical support. However, these barriers also hinder the delivery of drugs to target tissues, reducing their therapeutic efficacy. Extracellular vesicles (EVs), nanostructures with a diameter ranging from 30 nm to 10 μm secreted by cells, offer a potential solution to this challenge. These natural vesicles can effectively pass through various biological barriers, facilitating intercellular communication. As a result, artificially engineered EVs that mimic or are superior to the natural ones have emerged as a promising drug delivery vehicle, capable of delivering drugs to almost any body part to treat various diseases. This review first provides an overview of the formation and cross-species uptake of natural EVs from different organisms, including animals, plants, and bacteria. Later, it explores the current clinical applications, perspectives, and challenges associated with using engineered EVs as a drug delivery platform. Finally, it aims to inspire further research to help bioengineered EVs effectively cross biological barriers to treat diseases.

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.

Diversity, Origin and Evolution of the ESCRT Systems

Endosomal Sorting Complexes Required for Transport (ESCRT) play key roles in protein sorting between membrane-bounded compartments of eukaryotic cells. Homologs of many ESCRT components are identifiable in various groups of archaea, especially in Asgardarchaeota, the archaeal phylum that is currently considered to include the closest relatives of eukaryotes, but not in bacteria. We performed a comprehensive search for ESCRT protein homologs in archaea and reconstructed ESCRT evolution using the phylogenetic tree of Vps4 ATPase (ESCRT IV) as a scaffold, using sensitive protein sequence analysis and comparison of structural models to identify previously unknown ESCRT proteins. Several distinct groups of ESCRT systems in archaea outside of Asgard were identified, including proteins structurally similar to ESCRT-I and ESCRT-II, and several other domains involved in protein sorting in eukaryotes, suggesting an early origin of these components. Additionally, distant homologs of CdvA proteins were identified in Thermoproteales which are likely components of the uncharacterized cell division system in these archaea. We propose an evolutionary scenario for the origin of eukaryotic and Asgard ESCRT complexes from ancestral building blocks, namely, the Vps4 ATPase, ESCRT-III components, wH (winged helix-turn-helix fold) and possibly also coiled-coil, and Vps28-like domains. The Last Archaeal Common Ancestor likely encompassed a complex ESCRT system that was involved in protein sorting. Subsequent evolution involved either simplification, as in the TACK superphylum, where ESCRT was co-opted for cell division, or complexification as in Asgardarchaeota. In Asgardarchaeota, the connection between ESCRT and the ubiquitin system that was previously considered a eukaryotic signature was already established.

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.

On the origin of the nucleus: a hypothesis

SUMMARYIn this hypothesis article, we explore the origin of the eukaryotic nucleus. In doing so, we first look afresh at the nature of this defining feature of the eukaryotic cell and its core functions-emphasizing the utility of seeing the eukaryotic nucleoplasm and cytoplasm as distinct regions of a common compartment. We then discuss recent progress in understanding the evolution of the eukaryotic cell from archaeal and bacterial ancestors, focusing on phylogenetic and experimental data which have revealed that many eukaryotic machines with nuclear activities have archaeal counterparts. In addition, we review the literature describing the cell biology of representatives of the TACK and Asgardarchaeaota - the closest known living archaeal relatives of eukaryotes. Finally, bringing these strands together, we propose a model for the archaeal origin of the nucleus that explains much of the current data, including predictions that can be used to put the model to the test.

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.

Probing archaeal cell biology: exploring the use of dyes in the imaging of Sulfolobus cells

Archaea are key players in many critical ecological processes. In comparison to eukaryotes and bacteria, however, our understanding of both the cell biology and diversity of archaea remains limited. While archaea inhabit a wide range of environmental conditions, many species are extremophiles, surviving in extreme temperature, salt or pH conditions, making their cell biology hard to study. Recently, our understanding of archaeal cell biology has been advanced significantly by the advent of live cell imaging in extremis as well as the development of genetic tools to exogenously express fluorescent proteins in some mesophilic archaeal model systems, e.g., Haloferax volcanii. However, for most archaeal species, especially thermophilic species or emerging model systems without well characterized genetic tools, live cell imaging remains dependent on fluorescent chemical probes to label and track the dynamics of living cells. While a wide range of fluorescent stains and markers that label different components of the cell are available commercially, their use has usually been optimized for use in a small number of eukaryotic cell systems. Here we report the successes and failures of the application of membrane, DNA, S-layer and cytoplasm markers in live cell imaging of archaea, as well as the optimization of fixation and immunolabelling approaches. We have applied these markers to the thermoacidophilic archaeon Sulfolobus acidocaldarius, but expect some to work in other archaeal species. Furthermore, those procedures that failed in S. acidocaldarius may still prove useful for imaging archaea that grow at a more neutral pH and/or at a less extreme temperature.

Inference and reconstruction of the heimdallarchaeial ancestry of eukaryotes

In the ongoing debates about eukaryogenesis-the series of evolutionary events leading to the emergence of the eukaryotic cell from prokaryotic ancestors-members of the Asgard archaea play a key part as the closest archaeal relatives of eukaryotes1. However, the nature and phylogenetic identity of the last common ancestor of Asgard archaea and eukaryotes remain unresolved2-4. Here we analyse distinct phylogenetic marker datasets of an expanded genomic sampling of Asgard archaea and evaluate competing evolutionary scenarios using state-of-the-art phylogenomic approaches. We find that eukaryotes are placed, with high confidence, as a well-nested clade within Asgard archaea and as a sister lineage to Hodarchaeales, a newly proposed order within Heimdallarchaeia. Using sophisticated gene tree and species tree reconciliation approaches, we show that analogous to the evolution of eukaryotic genomes, genome evolution in Asgard archaea involved significantly more gene duplication and fewer gene loss events compared with other archaea. Finally, we infer that the last common ancestor of Asgard archaea was probably a thermophilic chemolithotroph and that the lineage from which eukaryotes evolved adapted to mesophilic conditions and acquired the genetic potential to support a heterotrophic lifestyle. Our work provides key insights into the prokaryote-to-eukaryote transition and a platform for better understanding the emergence of cellular complexity in eukaryotic cells.

The symbiotic origin of the eukaryotic cell

Eukaryogenesis represented a major evolutionary transition that led to the emergence of complex cells from simpler ancestors. For several decades, the most accepted scenario involved the evolution of an independent lineage of proto-eukaryotes endowed with an endomembrane system, including a nuclear compartment, a developed cytoskeleton and phagocytosis, which engulfed the alphaproteobacterial ancestor of mitochondria. However, the recent discovery by metagenomic and cultural approaches of Asgard archaea, which harbour many genes in common with eukaryotes and are their closest relatives in phylogenomic trees, rather supports scenarios based on the symbiosis of one Asgard-like archaeon and one or more bacteria at the origin of the eukaryotic cell. Here, we review the recent discoveries that led to this conceptual shift, briefly evoking current models of eukaryogenesis and the challenges ahead to discriminate between them and to establish a detailed, plausible scenario that accounts for the evolution of eukaryotic traits from those of their prokaryotic ancestors.

Widespread PRC barrel proteins play critical roles in archaeal cell division

Cell division is fundamental to all cellular life. Most of the archaea employ one of two alternative division machineries, one centered around the prokaryotic tubulin homolog FtsZ and the other around the endosomal sorting complex required for transport (ESCRT). However, neither of these mechanisms has been thoroughly characterized in archaea. Here, we show that three of the four PRC (Photosynthetic Reaction Center) barrel domain proteins of Haloferax volcanii (renamed Cell division proteins B1/2/3 (CdpB1/2/3)), play important roles in division. CdpB1 interacts directly with the FtsZ membrane anchor SepF and is essential for division, whereas deletion of cdpB2 and cdpB3 causes a major and a minor division defect, respectively. Orthologs of CdpB proteins are also involved in cell division in other haloarchaea. Phylogenetic analysis shows that PRC barrel proteins are widely distributed among archaea, including the highly conserved CdvA protein of the crenarchaeal ESCRT-based division system. Thus, diverse PRC barrel proteins appear to be central to cell division in most if not all archaea. Further study of these proteins is expected to elucidate the division mechanisms in archaea and their evolution.

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.

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.

Actin cytoskeleton and complex cell architecture in an Asgard archaeon

Asgard archaea are considered to be the closest known relatives of eukaryotes. Their genomes contain hundreds of eukaryotic signature proteins (ESPs), which inspired hypotheses on the evolution of the eukaryotic cell1-3. A role of ESPs in the formation of an elaborate cytoskeleton and complex cellular structures has been postulated4-6, but never visualized. Here we describe a highly enriched culture of 'Candidatus Lokiarchaeum ossiferum', a member of the Asgard phylum, which thrives anaerobically at 20 °C on organic carbon sources. It divides every 7-14 days, reaches cell densities of up to 5 × 107 cells per ml and has a significantly larger genome compared with the single previously cultivated Asgard strain7. ESPs represent 5% of its protein-coding genes, including four actin homologues. We imaged the enrichment culture using cryo-electron tomography, identifying 'Ca. L. ossiferum' cells on the basis of characteristic expansion segments of their ribosomes. Cells exhibited coccoid cell bodies and a network of branched protrusions with frequent constrictions. The cell envelope consists of a single membrane and complex surface structures. A long-range cytoskeleton extends throughout the cell bodies, protrusions and constrictions. The twisted double-stranded architecture of the filaments is consistent with F-actin. Immunostaining indicates that the filaments comprise Lokiactin-one of the most highly conserved ESPs in Asgard archaea. We propose that a complex actin-based cytoskeleton predated the emergence of the first eukaryotes and was a crucial feature in the evolution of the Asgard phylum by scaffolding elaborate cellular structures.

Asgard ESCRT-III and VPS4 reveal conserved chromatin binding properties of the ESCRT machinery

The archaeal Asgard superphylum currently stands as the most promising prokaryotic candidate, from which eukaryotic cells emerged. This unique superphylum encodes for eukaryotic signature proteins (ESP) that could shed light on the origin of eukaryotes, but the properties and function of these proteins is largely unresolved. Here, we set to understand the function of an Asgard archaeal protein family, namely the ESCRT machinery, that is conserved across all domains of life and executes basic cellular eukaryotic functions, including membrane constriction during cell division. We find that ESCRT proteins encoded in Loki archaea, express in mammalian and yeast cells, and that the Loki ESCRT-III protein, CHMP4-7, resides in the eukaryotic nucleus in both organisms. Moreover, Loki ESCRT-III proteins associated with chromatin, recruited their AAA-ATPase VPS4 counterpart to organize in discrete foci in the mammalian nucleus, and directly bind DNA. The human ESCRT-III protein, CHMP1B, exhibited similar nuclear properties and recruited both human and Asgard VPS4s to nuclear foci, indicating interspecies interactions. Mutation analysis revealed a role for the N terminal region of ESCRT-III in mediating these phenotypes in both human and Asgard ESCRTs. These findings suggest that ESCRT proteins hold chromatin binding properties that were highly preserved through the billion years of evolution separating Asgard archaea and humans. The conserved chromatin binding properties of the ESCRT membrane remodeling machinery, reported here, may have important implications for the origin of eukaryogenesis.

Is an archaeon the ancestor of eukaryotes?

The origin of complex cellular life is a key puzzle in evolutionary research, which has broad implications for various neighbouring scientific disciplines. Naturally, views on this topic vary widely depending on the world view and context from which this topic is approached. In the following, I will share my perspective about our current scientific knowledge on the origin of eukaryotic cells, that is, eukaryogenesis, from a biological point of view focusing on the question as to whether an archaeon was the ancestor of eukaryotes.

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.

The ESCRT Machinery: Remodeling, Repairing, and Sealing Membranes

The ESCRT machinery is an evolutionarily conserved membrane remodeling complex that is used by the cell to perform reverse membrane scission in essential processes like protein degradation, cell division, and release of enveloped retroviruses. ESCRT-III, together with the AAA ATPase VPS4, harbors the main remodeling and scission function of the ESCRT machinery, whereas early-acting ESCRTs mainly contribute to protein sorting and ESCRT-III recruitment through association with upstream targeting factors. Here, we review recent advances in our understanding of the molecular mechanisms that underlie membrane constriction and scission by ESCRT-III and describe the involvement of this machinery in the sealing and repairing of damaged cellular membranes, a key function to preserve cellular viability and organellar function.

Diving into the Evolutionary History of HSC70-Linked Selective Autophagy Pathways: Endosomal Microautophagy and Chaperone-Mediated Autophagy

Autophagy is a pleiotropic and evolutionarily conserved process in eukaryotes that encompasses different types of mechanisms by which cells deliver cytoplasmic constituents to the lysosome for degradation. Interestingly, in mammals, two different and specialized autophagic pathways, (i) the chaperone-mediated autophagy (CMA) and (ii) the endosomal microautophagy (eMI), both rely on the use of the same cytosolic chaperone HSPA8 (also known as HSC70) for targeting specific substrates to the lysosome. However, this is not true for all organisms, and differences exist between species with respect to the coexistence of these two autophagic routes. In this paper, we present an in-depth analysis of the evolutionary history of the main components of CMA and eMI and discuss how the observed discrepancies between species may contribute to improving our knowledge of these two functions and their interplays.