Virus Life Cycle
LIFE CYCLE: AMOEBA
Amoebas are considered a genus of Protozoa and obtain their food (algae, bacteria, other protozoans, and tiny particles of dead plant or animal matter) via phagocytosis. This is important to know because the Mimivirus infection is triggered by phagocytosis (Ghigo, 2008 & Monti, 2007). First off, the Mimivirus has a strong resemblance towards Gram-positive bacteria due to its large size and mimic abilities of Gram positive cell wall. Because of those characteristics, the amoeba, with no distinctive abilities, thought that the large virus was a bacterium and swallowed the virus via the mechanism of phagocytosis. Once the viral particle is absorbed by phagocytosis, a vacuole surrounds the viral particle and fuses to become a phagosome (figure 2). The absorbed viral particles in the phagosome will then merge with the lysosome. The degradation from the lysosomal activity in the amoeba will weaken the outer fiber layer, which will produce an opening of the five icosahedral faces on the vertex of the capsid (Zauberman, 2006 and Xiao, 2009). This allows the virus to release its DNA to the host cell nucleus. From there, the Mimivirus replication will break into three different zones known as the viral factory (VF). The three VF zones are: (1) inner replication center, (2) intermediate assembly zone, and (3) peripheral zone (Monti, 2007). The replication center will produce viral proteins and the assembly zone will assemble these proteins into capsids. The peripheral zone will add the fibers to the capsid. Empty or filled DNA capsids will then accumulate near the central core, which will result in the formation of a large massive membrane, to which the host cell could not withstand the large massive membrane and bursts (process known to be amoebal lysis). The viral nucleocapsid DNA is released into the cell cytoplasm (Monti, 2007). Future studies of the Mimivirus may be linked with finding out its attachment phase and possibly a more in depth detail on its transcription factors.
Figure 2. Schematic representation of APMV replication cycle (Monti, 2007). (a1) Phagocytosis of viral particle. (a2) Fusion of phagosome and lysosome, (b3-b4) Virus releases DNA to host nucleus (c5) Mimivirus DNA came out the host nucleus to form the virus factory (VF) replication center. (d6) Replication center produce viral proteins, assembly zone assembles capsids, and peripheral zone adds the fibers, (e7) Empty or DNA filled capsids accumulate near the central core resulting in the formation of a large massive membrane. (f8) Host cells could not withstand the large massive membrane and bursts (amoebal lysis), viral nucleocapsid DNA is released into the cell cytoplasm.
LIFE CYCLE: HUMAN
The life cycle of the Mimivirus in humans is similar to amoebas, in that, entry is mediated via phagocytosis. Resources did not state how the virus attaches to, replicates, assemble, or released in the host. However, scientist had proposed that the Mimivirus could cause further penetration of the cytoplasm by using enzymatic activities necessary for the opening of the five icosahedral faces at the vertex of the capsid (Zauberman, 2006 and Xiao, 2009). Sources were not clear on the replication cycle in humans. The question still remains whether or not to consider Mimivirus as a potential pneumonia agent. Some studies had found relations correlating Mimivirus to pneumonia patients, but were unsure if the virus was the cause of the disease. See medical relevance for more details.
LIFE CYCLE: SPUTNIK VIRUS
Recent studies discovered a satellite virus called “Sputnik virophage.” Satellite viruses are viruses that are dependent on the co-infection of a helper virus in the host cell for its replication. The Sputnik is an icosahedral small virus (50 nm) with ~18 kb circular dsDNA genome that uses other virus machinery for replication. Investigations of this small virus suggest that Sputnik cannot reproduce in host amoeba cells without the infection of Mimivirus. Sputnik reproduction occurs within the replication machinery of the Mimivirus VF.
Figure 3 shows Sputnik starts its replication cycle using Mimivirus as a helper virus to penetrate the host cell for entry. Once inside the host, the Sputnik first travels to the viral factory created by the Mimivirus, and then invade the Mimivirus VF to replicate its own genome. The resulting viral mRNA would be translated in the host cytoplasm. Sputnik proteins will then assemble into viral particles at the periphery of the Mimivirus factories and eventually be released in the cell cytoplasm via lysis. The effects this will have on Mimivirus are that few of its own viruses will be produced much lesser and that they will become deformed and less infectious. There were also signs of extra layers from the capsid of Mimivirus. Details of the replication were not provided.
Figure 3. Replication of traditional satellite virus, Sputnik (Legendre, 2011). Following entry within the host cytoplasm, Mimivirus and Sputnik goes separate ways. Mimivirus generates factors where the viral replication takes place. Sputnik takes advantage of the Mimivirus VF zone for the replication of its genome to generate its transcripts. At the assembly stage, Sputnik requires the help of Mimivirus.