Taxonomy
The following genera and species are assigned to the family: * '' Alphalipothrixvirus'' * '' Betalipothrixvirus'' * '' Deltalipothrixvirus'' The family consists of three genera: ''Alphalipothrixvirus'', ''Betalipothrixvirus'', and ''Deltalipothrixvirus''. '' Captovirus'' used to be in this family as the genus Gammalipothrixvirus, but now it is the only genus in the family '' Ungulaviridae''.Häring M, Vestergaard G, Brügger K, Rachel R, Garrett RA, Prangishvili D (2005) Structure and genome organization of AFV2, a novel archaeal lipothrixvirus with unusual terminal and core structures. J Bacteriol 187(11): 3855–3858 They are classified into genera based on their genomic properties and on the diversity of their terminal appendages, which are involved in host cell recognition. The originally proposed genus ''Alphalipothrixvirus'' was renamed ''Alphatristromavirus'' and moved to family ''Tristromaviridae''. In 2020, the genus ''Alphalipothrixvirus'' was recreated for classification of Sulfolobus filamentous virus 1 and Sulfolobales Beppu filamentous virus 2. In the genus ''Gammalipothrixvirus'' claw-like structures are found at either end of the virion. Members of the ''Lipothrixviridae'' share structural and genomic characteristics with viruses from the '' Rudiviridae'' family, which contains non-enveloped rod-shaped viruses. Viruses from the two families have linear dsDNA genomes and share up to nine genes. In addition, the filamentous particles of rudiviruses and lipothrixviruses are built from structurally similar, homologous major capsid proteins. Due to these shared properties viruses from the two families are classified into an order '' Ligamenvirales''. Members of the ''Ligamenvirales'' are structurally related to viruses of the family ''Tristromaviridae'' which, similar to lipothrixviruses, are enveloped and encode two paralogous major capsid proteins with the same fold as those of ligamenviruses. Due to these structural similarities, order ''Ligamenvirales'' and family ''Tristromaviridae'' were proposed to be unified within a class 'Tokiviricetes' (toki means ‘thread’ in Georgian and ''viricetes'' is an official suffix for a virus class).Virology
The viruses are enveloped and filamentous. The capsid varies considerably in length – 410–1950 nanometers (nm) – and is 24–38 nm in diameter. The envelope has a monolayer structure and includes di-phytanyl tetraethers lipids. From either end of the viron are protrusions extending from the core through the envelope. The capsid itself is elongated and exhibits helical symmetry. The core itself is helical. There are two major capsid proteins (MCP1 and MCP2). MCP1 and MCP2 form a heterodimer, which wraps around the linear dsDNA genome transforming it into A-form. Interaction between the genome and the MCPs leads to condensation of the genome into the virion superhelix. Genomes are linear, up to 40 kb in length.Life cycle
Viral replication is cytoplasmic. Entry into the host cell is achieved by adsorption to the host cell. Acidianus filamentous virus 1 was found to bind to cellular pili-like appendages. DNA templated transcription is the method of transcription. Archaea serve as the natural host. Transmission routes are passive diffusion. Virion assembly and egress have been studied in the case of Sulfolobus islandicus filamentous virus (SIFV). The virions assemble inside the cell. Binding of the major capsid protein dimers to the linear dsDNA genome lead to the assembly of nucleocapsids, which are subsequently enveloped intracellularly through an unknown mechanism. All lipothrixviruses are likely to be lytic viruses. In the case of betalipothrixviruses and deltalipothrixviruses, virions are released through pyramidal portals, referred to as virus-associated pyramids (VAPs). The VAPs of SIFV have a hexagonal base (i.e., constructed from six triangular facets).References
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