Abstract
Viruses infecting hyperthermophilic archaea represent one of the most enigmatic parts of the global virome, with viruses from different families showing no genomic relatedness to each other or to viruses of bacteria and eukaryotes. Tristromaviruses, which build enveloped filamentous virions and infect hyperthermophilic neutrophiles of the order Thermoproteales, represent one such enigmatic virus families. They do not share genes with viruses from other families and have been believed to represent an evolutionarily independent virus lineage. A cryo-electron microscopic reconstruction of the tristromavirus Pyrobaculum filamentous virus 2 at 3.4 Å resolution shows that the virion is constructed from two paralogous major capsid proteins (MCP) which transform the linear dsDNA genome of the virus into A-form by tightly wrapping around it. Unexpectedly, the two MCP are homologous to the capsid proteins of other filamentous archaeal viruses, uncovering a deep evolutionary relationship within the archaeal virosphere.
Highlights
Systematic comparison of sequences and structures of the majorcapsid proteins (MCP) from across the virosphere has shown that viruses from $80 per cent of known virus families can be categorized into seventeen architectural classes represented by unique major (nucleo)capsid proteins (MCP) folds (Krupovic and Koonin 2017)
Clavavirus APBV1 infects Aeropyrum pernix and encodes one of the simplest known MCPs, folded as two a-helices connected by a b-hairpin
The refined MCP1 and MCP2 single model were used to generate a filamentous model using the determined helical symmetry, and this filament model plus ADNA were refined against the full cryo-electron micrograph map using PHENIX
Summary
Systematic comparison of sequences and structures of the major (nucleo)capsid proteins (MCP) from across the virosphere has shown that viruses from $80 per cent of known virus families can be categorized into seventeen architectural classes represented by unique MCP folds (Krupovic and Koonin 2017). Whereas virions of rudiviruses are fairly rigid and non-enveloped, those of lipothrixviruses are more flexible and covered with a lipid envelope containing other viral proteins. Despite these differences, rudiviruses and lipothrixviruses share extensive gene content and are unified into an order Ligamenvirales (Prangishvili and Krupovic 2012). To study the virion organization of tristromaviruses, we have determined the structure of the Pyrobaculum filamentous virus 2 (PFV2) at 3.4 Aresolution
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