Mollivirus

Mollivirus was originally detected by light microscopy as rounded particles that multiplied in Acanthamoeba castellanii ameba culture, which interacted with the same permafrost specimen from Siberia in which Pithovirus was detected. The particles had a diameter of about 600 nm and were covered with a “fuzzy” tegument. Inside the particle there is at least one internal lipid membrane . Virions are crowned with 2–4 rings separated by gaps of 20 nm; these rings correspond to different layers of “fibers” of various lengths surrounding the particle. Tegument consists of two layers of different density. The outer layer is 10–12 nm thick and consists of parallel bands separated by gaps of 30–40 nm, which are visible only under an electron microscope on the tangential plane of the tegument. The intermediate layer 12-14 nm thick is represented by a network of fibrils resembling those that make up the central layer of the pandoravirus tegument. Unlike the Pandora virus particles and Pithovirus , the hole at the top of the Mollivirus particle is represented by a funnel with a diameter of 160–200 nm. After entering the vacuoles of the host cell, the virions apparently lose their spherical appearance. Like other non- icosahedral virions, there are no electron dense regions inside the Mollivirus virus particle in which viral DNA can be located ^[5] .

During an infection caused by Mollivirus , amoebic cells retain their trophozoite form throughout the entire life cycle of the virus; however, 4-5 hours after infection, there is a decrease in the number of vacuoles in the cells . Like all giant viruses that affect Acanthamoeba , the Mollivirus replication cycle begins with phagocytosis of amoeba virus particles. They can fall into separate small vacuoles or assemble into a large vacuole. Although the discovery of the viral particle has not yet been observed, the fusion of the inner lipid membrane of the virus with the vacuole membrane and the penetration of the contents of the viral particle into the cytoplasm are seen very well. As with infections caused by pandoraviruses, 3-4 hours after infection with Mollivirus, the cell nucleus undergoes a strong reorganization, manifested in the appearance of numerous invaginations of the nuclear membrane . Soon, the newly formed virions appear on the periphery of the newly formed virin factory, which includes a modified cell nucleus , devoid of a visible nucleolus . Other cellular organelles are not included in the virion factory. The assembly process of Mollivirus virions resembles that of pandoravirus viruses: the shell and the inner part of the virion are assembled simultaneously, with the exception of individual fibers that are individually included in the maturing virion. In the virion factory, fragments of lipid membranes can accumulate, which can participate in the assembly of the inner membrane. After 6-8 hours after infection, virions are observed at different stages of maturation in the same virion factory. After 6 hours, new virions begin to appear in the medium, while their release from the cells is not accompanied by culture lysis . Sometimes there are mature virions enclosed in a vacuole, so most of the virions are excreted out by exocytosis ^[5] .

The Mollivirus genome is represented by a linear DNA molecule with a length of 651 thousand base pairs (kilobases, kb) and a GC-composition of 60%, at each end of the molecule there are tandem repeats with a length of 10 kb. It encodes 523 predicted proteins , for 65% of which there are no known homologues (ORFans). Thus, Mollivirus is a separate family of giant viruses. The rest of its genome can be divided into 3 groups: 18% of genes that have homologues in other viruses (for 89% of them, the closest relatives belong to the proteins of pandoraviruses), 14% of genes that are homologous to eukaryotic genes (67% of them are closest to genes A . castellanii ), and 3% of genes related to bacterial genes. For 80% of Mollivirus proteins, the function is undefined. The weak phylogenetic relationship of Mollivirus sibericum to pandoraviruses is confirmed by the fact that of the 136 proteins that make up the Mollivirus particles, only 28 are homologous to the proteins of the virions of pandoraviruses, and the relative amount of these proteins is very different. For example, the first of the proteins common to Mollivirus and pandoraviruses is ninth in the number of molecules in the Mollivirus virions and 39th in the Pandoravirus salinus virions. Interestingly, one of the most abundant proteins of the Mollivirus virion is homologous to the main capsid protein of large icosahedral DNA-containing viruses. Despite the lack of symmetry, this protein may be involved in the assembly of the viral particle. It is curious that the closest relative of this main capsid protein is A. castellanii protein, which, apparently, was borrowed by amoeba from the icosahedral virus that infects single-celled algae . Therefore, the corresponding Mollivirus gene could get into its genome as a result of two successful acts of horizontal gene transfer : from virus to host and from host to Mollivirus ^[5] .

The characteristics of the genome and life cycle make it possible to assign Mollivirus to a group of large nuclear-cytoplasmic DNA-containing viruses . The lack of transcriptional machinery in the Mollivirus proteome , the aggressive rearrangement of the cell nucleus during infection, and the presence of spliceosomal introns in 3% of Mollivirus genes indicate the need for Mollivirus to transfer its genome to the nucleus at least to initiate a replication cycle. An analysis of the Mollivirus proteome unexpectedly revealed the presence of several ribosomal proteins in it, which is unique to DNA-containing viruses. The functions of these proteins in the life cycle of the virus have not yet been determined ^[5] .