Revised Mimivirus major capsid protein sequence reveals intron-containing gene structure and extra domain

Background: Acanthamoebae polyphaga Mimivirus (APM) is the largest known dsDNA virus. The viral particle has a nearly icosahedral structure with an internal capsid shell surrounded with a dense layer of fibrils. A Capsid protein sequence, D13L, was deduced from the APM L425 coding gene and was shown to be the most abundant protein found within the viral particle. However this protein remained poorly characterised until now. A revised protein sequence deposited in a database suggested an additional N-terminal stretch of 142 amino acids missing from the original deduced sequence. This result led us to investigate the L425 gene structure and the biochemical properties of the complete APM major Capsid protein. Results: This study describes the full length 3430 bp Capsid coding gene and characterises the 593 amino acids long corresponding Capsid protein 1. The recombinant full length protein allowed the production of a specific monoclonal antibody able to detect the Capsid protein 1 within the viral particle. This protein appeared to be post-translationnally modified by glycosylation and phosphorylation. We proposed a secondary structure prediction of APM Capsid protein 1 compared to the Capsid protein structure of Paramecium Bursaria Chlorella Virus 1, another member of the Nucleo-Cytoplasmic Large DNA virus family. Conclusion: The characterisation of the full length L425 Capsid coding gene of Acanthamoebae polyphaga Mimivirus provides new insights into the structure of the main Capsid protein. The production of a full length recombinant protein will be useful for further structural studies

Evolutionary Dynamics of Giant Viruses and their Virophages

Giant viruses contain large genomes, encode many proteins atypical for viruses, replicate in large viral factories, and tend to infect protists. The giant virus replication factories can in turn be infected by so called virophages, which are smaller viruses that negatively impact giant virus replication. An example are Mimiviruses that infect the protist Acanthamoeba and that are themselves infected by the virophage Sputnik. This paper examines the evolutionary dynamics of this system, using mathematical models. While the models suggest that the virophage population will evolve to increasing degrees of giant virus inhibition, it further suggests that this renders the virophage population prone to extinction due to dynamic instabilities over wide parameter ranges. Implications and conditions required to avoid extinction are discussed. Another interesting result is that virophage presence can fundamentally alter the evolutionary course of the giant virus. While the giant virus is predicted to evolve towards increasing its basic reproductive ratio in the absence of the virophage, the opposite is true its presence. Therefore, virophages can not only benefit the host population directly by inhibiting the giant viruses, but also indirectly by causing giant viruses to evolve towards weaker phenotypes. Experimental tests for this model are suggested

Ultrastructural Characterization of the Giant Volcano-like Virus Factory of Acanthamoeba polyphaga Mimivirus

Acanthamoeba polyphaga Mimivirus is a giant double-stranded DNA virus defining a new genus, the Mimiviridae, among the Nucleo-Cytoplasmic Large DNA Viruses (NCLDV). We used utrastructural studies to shed light on the different steps of the Mimivirus replication cycle: entry via phagocytosis, release of viral DNA into the cell cytoplasm through fusion of viral and vacuolar membranes, and finally viral morphogenesis in an extraordinary giant cytoplasmic virus factory (VF). Fluorescent staining of the AT-rich Mimivirus DNA showed that it enters the host nucleus prior to the generation of a cytoplasmic independent replication centre that forms the core of the VF. Assembly and filling of viral capsids were observed within the replication centre, before release into the cell cytoplasm where progeny virions accumulated. 3D reconstruction from fluorescent and differential contrast interference images revealed the VF emerging from the cell surface as a volcano-like structure. Its size dramatically grew during the 24 h infectious lytic cycle. Our results showed that Mimivirus replication is an extremely efficient process that results from a rapid takeover of cellular machinery, and takes place in a unique and autonomous giant assembly centre, leading to the release of a large number of complex virions through amoebal lysis

Analysis of Virion Structural Components Reveals Vestiges of the Ancestral Ichnovirus Genome

Many thousands of endoparasitic wasp species are known to inject polydnavirus (PDV) particles into their caterpillar host during oviposition, causing immune and developmental dysfunctions that benefit the wasp larva. PDVs associated with braconid and ichneumonid wasps, bracoviruses and ichnoviruses respectively, both deliver multiple circular dsDNA molecules to the caterpillar. These molecules contain virulence genes but lack core genes typically involved in particle production. This is not completely unexpected given that no PDV replication takes place in the caterpillar. Particle production is confined to the wasp ovary where viral DNAs are generated from proviral copies maintained within the wasp genome. We recently showed that the genes involved in bracovirus particle production reside within the wasp genome and are related to nudiviruses. In the present work we characterized genes involved in ichnovirus particle production by analyzing the components of purified Hyposoter didymator Ichnovirus particles by LC-MS/MS and studying their organization in the wasp genome. Their products are conserved among ichnovirus-associated wasps and constitute a specific set of proteins in the virosphere. Strikingly, these genes are clustered in specialized regions of the wasp genome which are amplified along with proviral DNA during virus particle replication, but are not packaged in the particles. Clearly our results show that ichnoviruses and bracoviruses particles originated from different viral entities, thus providing an example of convergent evolution where two groups of wasps have independently domesticated viruses to deliver genes into their hosts

Defining Life: The Virus Viewpoint

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

A scoping review and thematic analysis of social and behavioural research among HIV-serodiscordant couples in high-income settings.

CAPRISA, 2015.Abstract available in pdf