Vaccinia-like cytoplasmic replication of the giant Mimivirus

Abstract

Poxviruses are considered to be unique among all DNA viruses, because their infection cycle is carried out exclusively in the host cytoplasm. Such an infection strategy is of interest, because it necessitates generation of elaborate factories in which viral replication and assembly are promoted. By using diverse imaging techniques, we show that the infection cycle of the largest virus currently identified, the Acanthamoeba polyphaga Mimivirus, similarly occurs exclusively in the host cytoplasm. We further show that newly synthesized mRNAs accumulate at discrete cytoplasmic sites that are distinct from the sites where viral replication occurs, and this is observed in vaccinia infection. By revealing substantial physiologic similarity between poxviruses and Mimivirus and thus, implying that an entirely cytoplasmic viral replication might be more common than generally considered, these findings underscore the ability of DNA viruses to generate large and elaborate replication factories.electron tomography | nucleocytoplasmic large DNA viruses | poxviruses | viral factories W ith the single exception of poxviruses, all currently known DNA viruses carry out replication and transcription either entirely or partially within host nuclei. The overwhelming bias to nucleus-centered viral transactions is rationalized by the fact that the nucleus provides the elaborate machinery required for these processes. Moreover, by concentrating essential factors into particular intranuclear sites as well as by enabling spatiotemporal regulation of viral replication and transcription, the nuclear environment considerably enhances the efficiency of these processes (1). Such underlying benefits render the entirely cytoplasmic infection cycle of poxviruses all of the more intriguing (2, 3). Nucleus-centered viral transactions present, however, remarkable hurdles associated with the prerequisite of transporting viral genomes from their entry sites at the cell periphery to and into the nucleus. Specifically, the cellular milieu is refractory to motion of DNA molecules because of the high viscosity of the cytosol and the dense molecular sieve generated by the cytoskeleton (4). Moreover, the nuclear envelope imposes a severe barrier for both entry and exit of long DNA molecules (1).The hurdles associated with genome trafficking are particularly high for nucleocytoplasmic large DNA viruses (NCLDV), which include the eukaryote-infecting families Poxviridae, Phycodnaviridae, Iridoviridae, and Asfarviridae, all characterized by DNA genomes longer than 100 kbp (5). After entry, genomes of phycodnaviruses (6), iridoviruses (7), and the African swine fever virus (8) are released into the cytoplasm at the host cell periphery and then, are shuttled toward and transported into the nucleus where initial replication cycles occur. Viral DNA is subsequently delivered to specific cytoplasmic factories in which all transactions required for viral assembly take place. The factors that attenuate the barriers imposed by the constrained DNA motion and thus, enabl...

Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae

“…However, at variance with Mimivirus where rounded cells remained adherent, Megavirus infection caused most of them to come off their support, making the cultures more difficult to monitor by regular microscopy. A. castellanii cells infected by Megavirus exhibited three distinctive ultrastructural features in succession, as described for Mimivirus (11,14,18). First, the "seed," corresponding to the Megavirus core nucleocapsid extracted from the external particle layers, appeared clearly separated from the cell cytoplasm by a well-delineated lipid membrane ( Fig.…”

“…This membrane likely derives from the most internal of the two membranes visible inside the virus particles, as expected from the infection process (opening of the stargate, followed by the fusion of the first virus membrane with the vacuole membrane). These seeds then progressively turn into early virion factories recognizable as electron-dense nucleic acid-rich regions (18) isolated from the surrounding cytoplasm by an exclusion zone constituted of a mesh of fibrils 16 nm in diameter ( Fig. 1 E and F).…”

Efficiency in Complexity: Composition and Dynamic Nature of Mimivirus Replication Factories

“…Previous studies revealed that, shortly after the release of the mimivirus genome into the host cytoplasm by the opening of a modified icosahedral capsid vertex, termed the "stargate" (34), initial replication centers are generated (26). Each replication center originates from one infecting virus, as was also demonstrated for the vaccinia virus (27).…”

“…With a diameter of 750 nm and harboring about 1,000 genes (25), the mimivirus, which belongs to the Mimiviridae family, represents one of the largest viruses known to date. Notably, the mimivirus infection cycle occurs entirely within the host cytoplasm (12,26), in contrast to other double-stranded DNA (dsDNA) viruses, with the exception of vaccinia virus (8,27) and pithovirus (23). Thus, host nuclei that generally provide optimal platforms for replication of dsDNA viruses (18) do not function as such with these viruses.…”

Sputnik and Mavirus: more than just satellite viruses

“…1). Giant viruses such as APMV and CroV replicate in large cytoplasmic virion factories, and encode an enzymatic complexity that is unrivalled among viruses and includes a presumably complete transcription apparatus, which is packaged in the virion and used to initiate viral gene transcription at the early stage of infection [9][10][11][12] . Although yet to be confirmed experimentally, Sputnik and Mavirus appear to depend on the transcription system of their associated giant viruses, as strongly suggested by the specific promoter and polyadenylation signals that are shared between these virophages and their giant viruses 4,13,14 .…”