Acidianus filamentous virus 1 coat proteins display a helical fold spanning the filamentous archaeal viruses lineage.

Acidianus filamentous virus 1 (AFV1), a member of the Lipothrixviridae family, infects the hyperthermophilic, acidophilic crenarchaeaon Acidianus hospitalis. The virion, covered with a lipidic outer shell, is 9,100-A long and contains a 20.8-kb linear dsDNA genome. We have identified the two major coat proteins of the virion (MCPs; 132 and 140 amino acids). They bind DNA and form filaments when incubated with linear dsDNA. A C-terminal domain is identified in their crystal structure with a four-helix-bundle fold. In the topological model of the virion filament core, the genomic dsDNA superhelix wraps around the AFV1-132 basic protein, and the AFV1-140 basic N terminus binds genomic DNA, while its lipophilic C-terminal domain is imbedded in the lipidic outer shell. The four-helix bundle fold of the MCPs from AFV1 is identical to that of the coat protein (CP) of Sulfolobus islandicus rod-shaped virus (SIRV), a member of the Rudiviridae family. Despite low sequence identity between these proteins, their high degree of structural similarity suggests that they could have derived from a common ancestor and could thus define an yet undescribed viral lineage.