Virophages—Known and Unknown Facts

Abstract

The paper presents virophages, which, like their host, giant viruses, are “new” infectious agents whose role in nature, including mammalian health, is important. Virophages, along with their protozoan and algal hosts, are found in fresh inland waters and oceanic and marine waters, including thermal waters and deep-sea vents, as well as in soil, plants, and in humans and animals (ruminants). Representing “superparasitism”, almost all of the 39 described virophages (except Zamilon) interact negatively with giant viruses by affecting their replication and morphogenesis and their “adaptive immunity”. This causes them to become regulators and, at the same time, defenders of the host of giant viruses protozoa and algae, which are organisms that determine the homeostasis of the aquatic environment. They are classified in the family Lavidaviridae with two genus (Sputnikovirus, Mavirus). However, in 2023, a proposal was presented that they should form the class Maveriviricetes, with four orders and seven families. Their specific structure, including their microsatellite (SSR-Simple Sequence Repeats) and the CVV (cell—virus—virophage, or transpovirion) system described with them, as well as their function, makes them, together with the biological features of giant viruses, form the basis for discussing the existence of a fourth domain in addition to Bacteria, Archaea, and Eukaryota. The paper also presents the hypothetical possibility of using them as a vector for vaccine antigens.

Virophages, Satellite Viruses, Virophage Replication and Its Effects and Virophage Defence Mechanisms for Giant Virus Hosts and Giant Virus Defence Systems against Virophages

“…The virophage capsids 'form' outside the viral particle factory of the giant viruses, and the formed virophages reside in the cytoplasm of the eukaryotic host of the giant viruses until their lysis, which usually occurs approximately 16-24 h after their 'infection' [8,17]. The replication cycle of virophages, as well as their structure, has been best described in the virophage Sputnik, which, 'enters' the host amoeba cell together with the giant virus Mamavirus ACMV by attaching to the fibres of its capsid [8,19,39,40,63,64]. These ACMV giant virus filaments covering its capsid, referred to as 'fungus-like formations', are similar in structure to peptidoglycan in bacteria and condition the tight coupling of the particles of these viruses [8,39,40].…”

“…Viruses, although they do not carry out metabolic processes themselves, replicate in Eukaryota, Bacteria and Archaea and are the most abundant entities in all natural environments of the biosphere [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Giant viruses were included in 2003, and their ‘parasites’, the virophages, were included in 2008 and, due to their biological characteristics, have become a potential evolutionary driving force in microbiology, including and ecology [ 2 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Giant “host” viruses of virophages are classified into NCLDVs (Nucleocytoplasmic Large DNA Viruses) characterised by large virions (greater than 200 nm), and their genetic material is linear dsDNA, although, in some of them, it can also occur in a circular form [ 3 , 4 , 10 , 20 ].…”

“…It has also been recorded that underneath the virophage, Sputnik’s capsids have a 4 nm thick double lipid layer [ 8 ]. Virophages parasitising giant viruses replicate in the viral particle factory of these viruses by taking over their transcription and replication mechanism, with the participation of their enzymes encoded in their genetic material [ 8 , 19 , 25 , 29 , 32 ]. Because virophages use giant viruses for replication, which can be described as helper viruses, they resemble satellite viruses, which also use helper viruses [ 1 , 8 , 10 , 19 , 24 , 25 , 33 , 34 , 35 ].…”

Virophages Found in Viromes from Lake Baikal

“…[17], was recently identified. To date, only 22 virophages have been formally characterized, i.e., for which a giant host virus and/or a eukaryotic host has been described; the rest have been identified in metagenomic assemblies [18].…”

“…We provide brief and basic information on virophages here because recent review articles fully reflect the current state of research on virophages [18,22,[29][30][31]. To date, there is no detailed information on the presence of virophages in the ancient oligotrophic Lake Baikal, just as there are no data on virophages in other ancient and large lakes of the Earth.…”