Are viruses alive? Until very recently, answering this question was often negative and viruses were not considered in discussions on the origin and definition of life. This situation is rapidly changing, following several discoveries that have modified our vision of viruses. It has been recognized that viruses have played (and still play) a major innovative role in the evolution of cellular organisms. New definitions of viruses have been proposed and their position in the universal tree of life is actively discussed. Viruses are no more confused with their virions, but can be viewed as complex living entities that transform the infected cell into a novel organism—the virus—producing virions. I suggest here to define life (an historical process) as the mode of existence of ribosome encoding organisms (cells) and capsid encoding organisms (viruses) and their ancestors. I propose to define an organism as an ensemble of integrated organs (molecular or cellular) producing individuals evolving through natural selection. The origin of life on our planet would correspond to the establishment of the first organism corresponding to this definition.
Archaea; Evolution; Mimivirus; Origin of life; Virus
Explaining the origin of viruses remains an important challenge for evolutionary biology. Previous explanatory frameworks described viruses as founders of cellular life, as parasitic reductive products of ancient cellular organisms or as escapees of modern genomes. Each of these frameworks endow viruses with distinct molecular, cellular, dynamic and emergent properties that carry broad and important implications for many disciplines, including biology, ecology and epidemiology. In a recent genome-wide structural phylogenomic analysis, we have shown that large-to-medium-sized viruses coevolved with cellular ancestors and have chosen the evolutionary reductive route. Here we interpret these results and provide a parsimonious hypothesis for the origin of viruses that is supported by molecular data and objective evolutionary bioinformatic approaches. Results suggest two important phases in the evolution of viruses: (1) origin from primordial cells and coexistence with cellular ancestors, and (2) prolonged pressure of genome reduction and relatively late adaptation to the parasitic lifestyle once virions and diversified cellular life took over the planet. Under this evolutionary model, new viral lineages can evolve from existing cellular parasites and enhance the diversity of the world’s virosphere.
giant viruses; parasitism; phylogenomics; protein domains; reductive evolution
Despite recent advances in our understanding of diverse aspects of virus evolution, particularly on the epidemiological scale, revealing the ultimate origins of viruses has proven to be a more intractable problem. Herein, I review some current ideas on the evolutionary origins of viruses and assess how well these theories accord with what we know about the evolution of contemporary viruses. I note the growing evidence for the theory that viruses arose before the last universal cellular ancestor (LUCA). This ancient origin theory is supported by the presence of capsid architectures that are conserved among diverse RNA and DNA viruses and by the strongly inverse relationship between genome size and mutation rate across all replication systems, such that pre-LUCA genomes were probably both small and highly error prone and hence RNA virus-like. I also highlight the advances that are needed to come to a better understanding of virus origins, most notably the ability to accurately infer deep evolutionary history from the phylogenetic analysis of conserved protein structures.
The discovery of giant viruses with genome and physical size comparable to cellular organisms, remnants of protein translation machinery and virus-specific parasites (virophages) have raised intriguing questions about their origin. Evidence advocates for their inclusion into global phylogenomic studies and their consideration as a distinct and ancient form of life.
Here we reconstruct phylogenies describing the evolution of proteomes and protein domain structures of cellular organisms and double-stranded DNA viruses with medium-to-very-large proteomes (giant viruses). Trees of proteomes define viruses as a ‘fourth supergroup’ along with superkingdoms Archaea, Bacteria, and Eukarya. Trees of domains indicate they have evolved via massive and primordial reductive evolutionary processes. The distribution of domain structures suggests giant viruses harbor a significant number of protein domains including those with no cellular representation. The genomic and structural diversity embedded in the viral proteomes is comparable to the cellular proteomes of organisms with parasitic lifestyles. Since viral domains are widespread among cellular species, we propose that viruses mediate gene transfer between cells and crucially enhance biodiversity.
Results call for a change in the way viruses are perceived. They likely represent a distinct form of life that either predated or coexisted with the last universal common ancestor (LUCA) and constitute a very crucial part of our planet’s biosphere.
The morphogenesis of poxviruses such as vaccinia virus (VACV) sees the virion shape mature from spherical to brick-shaped. Trimeric capsomers of the VACV D13 protein form a transitory, stabilizing lattice on the surface of the initial spherical immature virus particle. The crystal structure of D13 reveals that this major scaffolding protein comprises a double β barrel “jelly-roll” subunit arranged as pseudo-hexagonal trimers. These structural features are characteristic of the major capsid proteins of a lineage of large icosahedral double-stranded DNA viruses including human adenovirus and the bacteriophages PRD1 and PM2. Structure-based phylogenetic analysis confirms that VACV belongs to this lineage, suggesting that (analogously to higher organism embryogenesis) early poxvirus morphogenesis reflects their evolution from a lineage of viruses sharing a common icosahedral ancestor.
► Poxvirus D13 acts as a scaffold for the morphogenesis of spherical immature virions ► D13 has a double “jelly-roll” structure, like other large DNA virus capsid proteins ► Structure-based phylogenetics places D13 into an icosahedral viral lineage ► Poxvirus morphogenesis reflects Vaccinia virus evolution from an icosahedral ancestor
Darwin provided a great unifying theory for biology; its visual expression is the universal tree of life. The tree concept is challenged by the occurrence of horizontal gene transfer and—as summarized in this review—by the omission of viruses. Microbial ecologists have demonstrated that viruses are the most numerous biological entities on earth, outnumbering cells by a factor of 10. Viral genomics have revealed an unexpected size and distinctness of the viral DNA sequence space. Comparative genomics has shown elements of vertical evolution in some groups of viruses. Furthermore, structural biology has demonstrated links between viruses infecting the three domains of life pointing to a very ancient origin of viruses. However, presently viruses do not find a place on the universal tree of life, which is thus only a tree of cellular life. In view of the polythetic nature of current life definitions, viruses cannot be dismissed as non-living material. On earth we have therefore at least two large DNA sequence spaces, one represented by capsid-encoding viruses and another by ribosome-encoding cells. Despite their probable distinct evolutionary origin, both spheres were and are connected by intensive two-way gene transfers.
universal tree; viruses; phages
The genomes of most virus species have overlapping genes—two or more proteins coded for by the same nucleotide sequence. Several explanations have been proposed for the evolution of this phenomenon, and we test these by comparing the amount of gene overlap in all known virus species. We conclude that gene overlap is unlikely to have evolved as a way of compressing the genome in response to the harmful effect of mutation because RNA viruses, despite having generally higher mutation rates, have less gene overlap on average than DNA viruses of comparable genome length. However, we do find a negative relationship between overlap proportion and genome length among viruses with icosahedral capsids, but not among those with other capsid types that we consider easier to enlarge in size. Our interpretation is that a physical constraint on genome length by the capsid has led to gene overlap evolving as a mechanism for producing more proteins from the same genome length. We consider that these patterns cannot be explained by other factors, namely the possible roles of overlap in transcription regulation, generating more divergent proteins and the relationship between gene length and genome length.
genome compression; mutation; evolution; capsid; virus
Virus capsid assembly is a critical step in the viral life cycle. The underlying basis of capsid stability is key to understanding this process. Capsid subunits interact with weak individual contact energies to form a globally stable icosahedral lattice; this structure is ideal for encapsidating the viral genome and host partners and protecting its contents upon secretion, yet the unique properties of its assembly and intersubunit contacts allows for the capsid to dissociate upon entering a new host cell. The stability of the capsid can be analyzed by treating capsid assembly as an equilibrium polymerization reaction, modified from the traditional polymer model to account for the fact that a separate nucleus is formed for each individual capsid. From the concentrations of reactants and products in an equilibrated assembly reaction, it is possible to extract the thermodynamic parameters of assembly for a wide array of icosahedral viruses using well-characterized biochemical and biophysical methods. In this chapter we describe the basic analysis and provide examples of thermodynamic assembly data for several different icosahedral viruses. These data provide new insights into the assembly mechanisms of spherical virus capsids, as well as the biology of the viral life cycle.
capsid; self-assembly; protein-protein interactions; thermodynamics; hepatitis B virus; cowpea chlorotic mottle virus
Where there is life, there are viruses. The impact of viruses on evolution, global nutrient cycling, and disease has driven research on their cellular and molecular biology. Knowledge exists for a wide range of viruses, however, a major exception are viruses with archaeal hosts. Archaeal virus-host systems are of great interest because they have similarities to both eukaryotic and bacterial systems and often live in extreme environments. Here we report the first proteomics-based experiments on archaeal host response to viral infection. Sulfolobus Turreted Icosahedral Virus (STIV) infection of Sulfolobus solfataricus P2 was studied using 1D and 2D differential gel electrophoresis (DIGE) to measure abundance and redox changes. Cysteine reactivity was measured using novel fluorescent zwitterionic chemical probes that, together with abundance changes, suggest that virus and host are both vying for control of redox status in the cells. Proteins from nearly 50% of the predicted viral open reading frames were found along with a new STIV protein with a homolog in STIV2. This study provides insight to features of viral replication novel to the archaea, makes strong connections to well described mechanisms used by eukaryotic viruses such as ESCRT-III mediated transport, and emphasizes the complementary nature of different omics approaches.
Archaea; virus infection; Sulfolobus solfataricus strain P2; STIV; Sulfolobus turreted icosahedral virus; Proteomics; virus–host interaction; LC/MS/MS; liquid chromatography mass spectrometry; differential gene expression; membrane protein; VAPs; Virus-associated pyramids; 2-D Fluorescence Difference Gel Electrophoresis; Thiol-reactive maleimide probe
Abrescia and colleagues demonstrate how the bacteriophage PRD1, a model membrane-containing virus, generates a self-polymerizing protein-lipid nanotube to deliver its viral genome to a host cell.
In internal membrane-containing viruses, a lipid vesicle enclosed by the icosahedral capsid protects the genome. It has been postulated that this internal membrane is the genome delivery device of the virus. Viruses built with this architectural principle infect hosts in all three domains of cellular life. Here, using a combination of electron microscopy techniques, we investigate bacteriophage PRD1, the best understood model for such viruses, to unveil the mechanism behind the genome translocation across the cell envelope. To deliver its double-stranded DNA, the icosahedral protein-rich virus membrane transforms into a tubular structure protruding from one of the 12 vertices of the capsid. We suggest that this viral nanotube exits from the same vertex used for DNA packaging, which is biochemically distinct from the other 11. The tube crosses the capsid through an aperture corresponding to the loss of the peripentonal P3 major capsid protein trimers, penton protein P31 and membrane protein P16. The remodeling of the internal viral membrane is nucleated by changes in osmolarity and loss of capsid-membrane interactions as consequence of the de-capping of the vertices. This engages the polymerization of the tail tube, which is structured by membrane-associated proteins. We have observed that the proteo-lipidic tube in vivo can pierce the gram-negative bacterial cell envelope allowing the viral genome to be shuttled to the host cell. The internal diameter of the tube allows one double-stranded DNA chain to be translocated. We conclude that the assembly principles of the viral tunneling nanotube take advantage of proteo-lipid interactions that confer to the tail tube elastic, mechanical and functional properties employed also in other protein-membrane systems.
Viral survival and propagation depend on the ability of the viruses to transfer their genetic material to a host cell. Viral genome delivery has been described for viruses that directly enclose their genome in a capsid or nucleocapsid, but not for internal membrane-containing viruses in which the genome is protected by a lipid vesicle enclosed by the icosahedral capsid. The latter infect organisms across the three domains of life. We use a range of electron microscopy techniques to reveal how one such virus, the bacteriophage PRD1, which uses gram negative bacteria as its host, delivers its double-stranded DNA to the bacteria across the cell envelope. The PRD1 bacteriophage is special in that it doesn't carry a rigid tail; rather it creates a tube tail when needed at the time of infection to pass its DNA through to the host. We now show that this tube formation is accomplished via concerted restructuring of the icosahedral capsid and remodeling of the internal icosahedral protein-rich virus membrane. We also find that this tail tube is studded with membrane-associated proteins and its internal diameter allows one double-stranded DNA chain to be injected. Finally, we capture PRD1 in 3-D with the proteo-lipidic tail piercing the gram-negative bacterial cell and shuttling its viral genome in vivo. These results provide insights into a new mechanism of viral genome delivery.
Viruses are cellular parasites. The linkage between viral and host functions makes the study of a viral life cycle an important key to cellular functions. A deeper understanding of many aspects of viral life cycles has emerged from coordinated molecular and structural studies carried out with a wide range of viral pathogens. Structural studies of viruses by means of cryo-electron microscopy and three-dimensional image reconstruction methods have grown explosively in the last decade. Here we review the use of cryo-electron microscopy for the determination of the structures of a number of icosahedral viruses. These studies span more than 20 virus families. Representative examples illustrate the use of moderate- to low-resolution (7- to 35-Å) structural analyses to illuminate functional aspects of viral life cycles including host recognition, viral attachment, entry, genome release, viral transcription, translation, proassembly, maturation, release, and transmission, as well as mechanisms of host defense. The success of cryo-electron microscopy in combination with three-dimensional image reconstruction for icosahedral viruses provides a firm foundation for future explorations of more-complex viral pathogens, including the vast number that are nonspherical or nonsymmetrical.
Herpesviruses, a family of animal viruses with large (125 – 250 kbp) linear DNA genomes, are highly diversified in terms of host range; nevertheless, their virions conform to a common architecture. The genome is confined at high density within a thick-walled icosahedral capsid with the uncommon (among viruses, generally) but unvarying triangulation number T=16. The envelope is a membrane in which some 11 different viral glycoproteins are implanted. Between the capsid and the envelope is a capacious compartment called the tegument that accommodates ~ 20 – 40 different viral proteins (depending on which virus) destined for delivery into a host cell. A strong body of evidence supports the hypothesis that herpesvirus capsids and those of tailed bacteriophages stem from a distant common ancestor, whereas their radically different infection apparatuses - envelope on one hand and tail on the other - reflect subsequent co-evolution with divergent hosts. Here we review the molecular components of herpesvirus capsids and the mechanisms that regulate their assembly, with particular reference to the archetypal alphaherpesvirus, herpes simplex virus type 1; assess their duality with the capsids of tailed bacteriophages; and discuss the mechanism whereby, once DNA packaging has been completed, herpesvirus nucleocapsids exit from the nucleus to embark on later stages of the replication cycle.
herpesvirus; herpes simplex virus type 1; tailed bacteriophage; tegument; nuclear exit; cryo-electron microscopy
Recent investigations of archaeal viruses have revealed novel features of their structures and life cycles when compared to eukaryotic and bacterial viruses, yet there are structure-based unifying themes suggesting common ancestral relationships among dsDNA viruses in the three kingdoms of life. Sulfolobus solfataricus and the infecting virus Sulfolobus turreted icosahedral virus (STIV) is one of the well-established model systems to study archaeal virus replication and viral-host interactions. Reliable laboratory conditions to propagate STIV and available genetic tools allowed structural characterization of the virus and viral components that lead to the proposal of common capsid ancestry with PRD1 (bacteriophage), Adenovirus (eukaryotic virus) and PBCV (chlorellavirus). Micro-array and proteomics approaches systematically analyzed viral replication and the corresponding host responses. Cellular cryo-electron tomography and thin-section EM studies uncovered the assembly and maturation pathway of STIV and revealed dramatic cellular ultra-structure changes upon infection. The viral induced pyramid-like protrusions on cell surfaces represent a novel viral release mechanism and previously uncharacterized functions in viral replication.
Sulfolobus turreted icosahedral virus (STIV); Archaea; virus
We described a new computer program for calculation of RNA secondary structure. Calculation of 20 viral RNAs with this program showed that genomes of the icosahedral capsid viruses had higher folding probabilities than those of the helical capsid viruses. As this explains virus assembly quite well, the information of capsid structure must be imprinted not only in the capsid protein structures but also in the base sequence of the whole genome. We compared folding probability of the original sequence with that of the random sequence in which base composition was the same as the original. All the actual genomes of RNA viruses were more folded than the corresponding random sequences, even though most transcripts of chromosomal genes tended to be less folded. The data can be related to encapsidation of viral genomes. It was thus suggested that there exists a relation between actual sequences and random sequences with the same base ratios, and that the base ratio itself has some evolutional meaning.
Studies on viral capsid architectures and coat protein folds have revealed the evolutionary lineages of viruses branching to all three domains of life. A widespread group of icosahedral tailless viruses, the PRD1-adenovirus lineage, was the first to be established. A double β-barrel fold for a single major capsid protein is characteristic of these viruses. Similar viruses carrying genes coding for two major capsid proteins with a more complex structure, such as Thermus phage P23-77 and haloarchaeal virus SH1, have been isolated. Here, we studied the host range, life cycle, biochemical composition, and genomic sequence of a new isolate, Haloarcula hispanica icosahedral virus 2 (HHIV-2), which resembles SH1 despite being isolated from a different location. Comparative analysis of these viruses revealed that their overall architectures are very similar except that the genes for the receptor recognition vertex complexes are unrelated even though these viruses infect the same hosts.
Human papillomaviruses (HPVs) are small dsDNA tumor viruses, which are the etiologic agents of most cervical cancers and are associated with a growing percentage of oropharyngeal cancers. The HPV capsid is non-enveloped, having a T=7 icosahedral symmetry formed via the interaction among 72 pentamers of the major capsid protein, L1. The minor capsid protein L2 associates with L1 pentamers, although it is not known if each L1 pentamer contains a single L2 protein. The HPV life cycle strictly adheres to the host cell differentiation program, and as such, native HPV virions are only produced in vivo or in organotypic “raft” culture. Research producing synthetic papillomavirus particles—such as virus-like particles (VLPs), papillomavirus-based gene transfer vectors, known as pseudovirions (PsV), and papillomavirus genome-containing quasivirions (QV)—has bypassed the need for stratifying and differentiating host tissue in viral assembly and has allowed for the rapid analysis of HPV infectivity pathways, transmission, immunogenicity, and viral structure.
papillomavirus; assembly; structure; cancer; life cycle
Viruses are replication competent genomes which are relatively gene-poor. Even the largest viruses (i.e. Herpesviruses) encode only slightly >200 open reading frames (ORFs). However, because viruses replicate obligatorily inside cells, and considering that evolution may be driven by a principle of economy of scale, it is reasonable to surmise that many viruses have evolved the ability to co-opt cell-encoded proteins to provide needed surrogate functions. An in silico survey of viral sequence databases reveals that most positive-strand and double-stranded RNA viruses have ORFs for RNA helicases. On the other hand, the genomes of retroviruses are devoid of virally-encoded helicase. Here, we review in brief the notion that the human immunodeficiency virus (HIV-1) has adopted the ability to use one or more cellular RNA helicases for its replicative life cycle.
Many viruses infecting animals and plants share common cores of homologous genes involved in the key processes of viral replication. In contrast, genes that mediate virus – host interactions including in many cases capsid protein genes are markedly different. There are three distinct scenarios for the origin of related viruses of plants and animals: i) evolution from a common ancestral virus predating the divergence of plants and animals; ii) horizontal transfer of viruses, for example, through insect vectors; iii) parallel origin from related genetic elements. We present evidence that each of these scenarios contributed, to a varying extent, to the evolution of different groups of viruses.
It is proposed that the pre-cellular stage of biological evolution unraveled within networks of inorganic compartments that harbored a diverse mix of virus-like genetic elements. This stage of evolution might comprise the Last Universal Cellular Ancestor (LUCA) that more appropriately could be denoted Last Universal Cellular Ancestral State (LUCAS). This scenario for the origin of cellular life recapitulates the early ideas of J. B. S. Haldane sketched in his classic 1928 essay. However, unlike in Haldane’s day, there is now considerable support for this scenario from three major lines of comparative-genomic evidence: i) lack of homology between the core components of the DNA replication systems of the two primary lines of descent of cellular life forms, archaea and bacteria, ii) distinct membrane chemistries and lack of homology between the enzymes of lipid biosynthesis in archaea and bacteria, iii) spread of several viral hallmark genes, which encode proteins with key functions in viral replication and morphogenesis, among numerous and extremely diverse groups of viruses, in contrast to their absence in cellular life forms, iv) the extant archaeal and bacterial chromosomes appear to be shaped by accretion of diverse, smaller replicons, suggesting a continuity between the hypothetical, primordial virus stage of life’s evolution and the dynamic prokaryotic world that existed ever since. Under the viral model of pre-cellular evolution, the key components of cells including the replication apparatus, membranes, and molecular complexes involved in membrane transport and translocation originated as components of virus-like entities. The two surviving types of cellular life forms, archaea and bacteria, might have emerged from the LUCAS independently, along with, probably, numerous forms now extinct.
comparative genomics; evolution of cells; evolution of viruses; origin of membranes; viral hallmark genes
Icosahedral nontailed double-stranded DNA (dsDNA) viruses are present in all three domains of life, leading to speculation about a common viral ancestor that predates the divergence of Eukarya, Bacteria, and Archaea. This suggestion is supported by the shared general architecture of this group of viruses and the common fold of their major capsid protein. However, limited information on the diversity and replication of archaeal viruses, in general, has hampered further analysis. Sulfolobus turreted icosahedral virus (STIV), isolated from a hot spring in Yellowstone National Park, was the first icosahedral virus with an archaeal host to be described. Here we present a detailed characterization of the components forming this unusual virus. Using a proteomics-based approach, we identified nine viral and two host proteins from purified STIV particles. Interestingly, one of the viral proteins originates from a reading frame lacking a consensus start site. The major capsid protein (B345) was found to be glycosylated, implying a strong similarity to proteins from other dsDNA viruses. Sequence analysis and structural predication of virion-associated viral proteins suggest that they may have roles in DNA packaging, penton formation, and protein-protein interaction. The presence of an internal lipid layer containing acidic tetraether lipids has also been confirmed. The previously presented structural models in conjunction with the protein, lipid, and carbohydrate information reported here reveal that STIV is strikingly similar to viruses associated with the Bacteria and Eukarya domains of life, further strengthening the hypothesis for a common ancestor of this group of dsDNA viruses from all domains of life.
Although no physical fossils of viruses have been found, retroviruses are known to leave their molecular fossils in the genomes of their hosts, the so-called endogenous retroviral elements. These have provided us with important information about retroviruses in the past and their co-evolution with their hosts. On the other hand, because non-retroviral viruses were considered not to leave such fossils, even the existence of prehistoric non-retroviral viruses has been enigmatic. Recently, we discovered that elements derived from ancient bornaviruses, non-segmented, negative strand RNA viruses, are found in the genomes of several mammalian species, including humans. In addition, at approximately the same time, several endogenous elements of RNA viruses, DNA viruses and reverse-transcribing DNA viruses have been independently reported, which revealed that non-retroviral viruses have played significant roles in the evolution of their hosts and provided novel insights into virology and cell biology. Here we review non-retroviral virus-like elements in vertebrate genomes, non-retroviral integration and the knowledge obtained from these endogenous non-retroviral virus-like elements.
viral fossil; endogenous non-retroviral virus-like elements; non-retroviral endogenization; non-retroviral integration; exaptation
Some cellular editing functions can restrict the replication of some viruses but contribute to completion of the life cycle of others. A recent study has identified an isoform of the adenosine deaminase acting on RNA type 1 (ADAR 1) as required for embryogenesis, and as a restriction factor for a number of important RNA virus pathogens . The dual implication of key cellular functions in the innate immunity against viruses, or, paradoxically, as mediators of virus replication is interpreted in the light of the concept of virus-host coevolution and tinkering proposed for general evolution by François Jacob decades ago.
RNA virus; host-virus interaction; coevolution
After recognizing and binding to its host cell, poliovirus (like other nonenveloped viruses) faces the challenge of translocating its genome across a cellular membrane and into the cytoplasm. To avoid entanglement with the capsid, the RNA must exit via a single site on the virion surface. However, the mechanism by which a single site is selected (from among 60 equivalents) is unknown; and until now, even its location on the virion surface has been controversial. To help to elucidate the mechanism of infection, we have used single-particle cryo-electron microscopy and tomography to reconstruct conformationally altered intermediates that are formed by the poliovirion at various stages of the poliovirus infection process. Recently, we reported icosahedrally symmetric structures for two forms of the end-state 80S empty capsid particle. Surprisingly, RNA was frequently visible near the capsid; and in a subset of the virions, RNA was seen on both the inside and outside of the capsid, caught in the act of exiting. To visualize RNA exiting, we have now determined asymmetric reconstructions from that subset, using both single-particle cryo-electron microscopy and cryo-electron tomographic methods, producing independent reconstructions at ∼50-Å resolution. Contrary to predictions in the literature, the footprint of RNA on the capsid surface is located close to a viral 2-fold axis, covering a slot-shaped area of reduced density that is present in both of the symmetrized 80S reconstructions and which extends by about 20 Å away from the 2-fold axis toward each neighboring 5-fold axis.
It is uncertain whether nonenveloped karyophilic virus particles may actively traffic from the nucleus outward. The unordered amino-terminal domain of the VP2 major structural protein (2Nt) of the icosahedral parvovirus minute virus of mice (MVM) is internal in empty capsids, but it is exposed outside of the shell through the fivefold axis of symmetry in virions with an encapsidated single-stranded DNA genome, as well as in empty capsids subjected to a heat-induced structural transition. In productive infections of transformed and normal fibroblasts, mature MVM virions were found to efficiently exit from the nucleus prior to cell lysis, in contrast to the extended nuclear accumulation of empty capsids. Newly formed mutant viruses lacking the three phosphorylated serine residues of 2Nt were hampered in their exit from the human transformed NB324K nucleus, in correspondence with the capacity of 2Nt to drive microinjected phosphorylated heated capsids out of the nucleus. However, in normal mouse A9 fibroblasts, in which the MVM capsid was phosphorylated at similar sites but with a much lower rate, the nuclear exit of virions and microinjected capsids harboring exposed 2Nt required the infection process and was highly sensitive to inhibition of the exportin CRM1 in the absence of a demonstrable interaction. Thus, the MVM virion exits the nucleus by accessing nonconventional export pathways relying on cell physiology that can be intensified by infection but in which the exposure of 2Nt remains essential for transport. The flexible 2Nt nuclear transport signal may illustrate a common structural solution used by nonenveloped spherical viruses to propagate in undamaged host tissues.
The origin and evolution of viruses is currently a heavily discussed issue. One element in this discussion is the innate viral "self" concept, which suggests that viral structures and functions can be divided into two categories. The first category consists of genetic determinants that are inherited from a viral ancestor and encode the viral "self". The second group consists of another set of structures and functions, the "nonself", which is interchangeable between different viruses and can be obtained via lateral gene transfer. Comparing the structures and sequences of the "self" elements, we have proposed that viruses can be grouped into lineages regardless of which domain of life (bacteria, archaea, eukarya) they infect. It has also been suggested that viruses are ancient and possibly predate modern cells.
Here we identified thirteen putative prophages (viral genomes integrated into bacterial chromosome) closely related to the virulent icosahedral tailless lipid-containing bacteriophage PM2. Using the comparative genomics approach, we present evidence to support the viral "self" hypothesis and divide genes of the bacteriophage PM2 and related prophages into "self" and "nonself" categories.
We show here that the previously proposed most conserved viral "self" determinants, the major coat protein and the packaging ATPase, were the only proteins that could be recognized in all detected corticoviral elements. We also argue here that the genes needed for viral genome replication, as well as for host cell lysis, belong to the "nonself" category of genes.
Furthermore, we suggest that abundance of PM2-like viruses in the aquatic environment as well as their importance in the ecology of aquatic microorganisms might have been underestimated.