Mimiviruses: Giant viruses with novel and intriguing features (Review)

Abstract

The Mimivirus is a giant virus that infects amoebae and was long considered to be a bacterium due to its size. The viral particles are composed of a protein capsid of ~500 nm in diameter, which is enclosed in a polysaccharide layer in which ~120–140 nm long fibers are embedded, resulting in an overall diameter of 700 nm. The virus has a genome size of 1.2 Mb DNA, and surprisingly, replicates only in the cytoplasm of the infected cells without entering the nucleus, which is a unique characteristic among DNA viruses. Their existence is undeniable; however, as with any novel discovery, there is still uncertainty concerning their pathogenicity mechanisms in humans and the nature of the Mimivirus virophage resistance element system (MIMIVIRE), a term given to describe the immune network of the Mimivirus, which closely resembles the CRISPR-Cas system. The scope of the present review is to discuss the recent developments derived from structural and functional studies performed on the distinctive characteristics of the Mimivirus, and from studies concerning their putative clinical relevance in humans.

Widespread Horizontal Gene Transfer Among Animal Viruses

“…Megaviruses were originally discovered in protists (Kalafati et al, 2022). Namao sturgeon virus is the only established example of a megavirus transmitted among animals (Clouthier et al, 2020; Watson et al, 1998).…”

Virophages—Known and Unknown Facts

“…The organization of the Sputnik genome is typical of viral genomes, namely a tight arrangement but little overlap of the genes. The genome virophage contains 21 genes encoding proteins ranging in size from 88 to 779 amino acids, with only slight overlap [ 24 , 25 , 26 , 44 , 48 ]. Among these are genes encoding its trimeric MCP and mCP proteins, as well as proteins predicted to be involved in its replication [ 24 , 25 , 26 ].…”

“…For the Sputnik virophage, the dominant host among viruses of Lineages A, B, and C of the family Mimiviridae is the giant Mamavirus ACMV, which infects the amoebae A. castellanii [ 1 , 2 , 17 , 26 , 42 , 48 , 49 ]. However, with the same reaction kinetics, it can effectively infect the giant virus, Mimivirus APMV, of the same genus and family but living on the amoeba A. polyphaga [ 3 , 4 , 11 , 21 , 49 ].…”

“…Its four genes are similar to those of eukaryotes and bacteriophages, three to those of APMV giant viruses, and one to archaea viruses [ 11 , 25 , 26 ]. The virophage Sputnik 2 replicates in the viral particle factory of giant viruses of the Mimiviridae family of Lineages A, B, and C, causing their destruction, thus showing a protective effect against their amoeba hosts cell [ 2 , 3 , 4 , 11 , 17 , 26 , 43 , 48 ]. With the discovery of the Sputnik 2 virophage in the giant virus Lentillevirus , a provirophage integrated with it was also found [ 12 ], as well as a new class of mobile genetic elements; that is, small fragments of DNA in the form of independent mobile “pieces” found in both the virophage and giant virus genomes, which have been called transpovirions [ 12 , 17 , 25 , 26 , 49 ].…”

“…This mosaicism of Sputnik 3 virophage genes suggests the involvement of its genes in lateral transfers between different viruses that encode proteins of as yet unknown origin and function [ 25 , 26 ]. The virophage Sputnik 3, similar to the virophages Sputnik and Sputnik 2, replicates in the viral particle factory of giant viruses of the Mimivirdae family, mainly of Lineage C, although also of Lineages A and B, causing the formation of abnormal virions of these giant viruses and reducing their infectivity and lytic capacity against their hosts cells, which are amoebae [ 2 , 3 , 4 , 11 , 26 , 48 ]. Like Sputnik and Sputnik 2, this virophage belongs to the genus Sputnikovirus , the family Lavidaviridae .…”