Recent, primarily structural observations indicate that related viruses, harboring no sequence similarity, infect hosts of different domains of life. One such clade of viruses, defined by common capsid architecture and coat protein fold, is the so-called PRD1-adenovirus lineage. Here we report the structure of the marine lipid-containing bacteriophage PM2 determined by crystallographic analyses of the entire approximately 45 MDa virion and of the outer coat proteins P1 and P2, revealing PM2 to be a primeval member of the PRD1-adenovirus lineage with an icosahedral shell and canonical double beta barrel major coat protein. The view of the lipid bilayer, richly decorated with membrane proteins, constitutes a rare visualization of an in vivo membrane. The viral membrane proteins P3 and P6 are organized into a lattice, suggesting a possible assembly pathway to produce the mature virus.

A novel halovirus, SH1, with a spherical morphology is described. Isolated from a hypersaline lake, SH1 is divalent, producing clear plaques on Haloarcula hispanica and a natural Halorubrum isolate. Single-step growth curves gave a latent period of 5-6 h and a burst size of around 200 PFU/cell. The host can differentiate to form tight clusters of thick cell-walled forms, and these were shown to be resistant to infection. Purified virions had no visible tail, were about 70 nm in diameter, and displayed a fragile outer capsid layer, possibly with an underlying membrane component. The structural proteins of the virion were analyzed by SDS-PAGE and several were found to be cross-linked, forming protein complexes. The genome was linear, dsDNA, of approximately 30 kb in length. This morphology and linear genome are features not observed in any other euryarchaeal viruses, but have properties similar to the bacterial virus PRD1.

The Archaea, and the viruses that infect them, are the least well understood of all of the three domains of life. They often grow in extreme conditions such as hypersaline lakes and sulfuric hot springs. Only rare glimpses have been gained into the structures of archaeal viruses. Here, we report the subnanometer resolution structure of a recently isolated, hypersalinic, membrane-containing, euryarchaeal virus, SH1, in which different viral proteins can be localized. The results indicate that SH1 has a complex capsid formed from single ␤-barrels, an important missing link in hypotheses on viral capsid protein evolution. Unusual, symmetry-mismatched spikes seem to play a role in host adsorption. They are connected to highly organized membrane proteins providing a platform for capsid assembly and potential machinery for host infection.archaeal virus ͉ electron cryomicroscopy ͉ infection ͉ symmetry mismatch ͉ capsid protein evolution B ecause of the high mutation rates in viruses, amino acid sequence-analysis methods generally only reveal the relationships between closely related viruses. Instead, viral evolutionary links can be studied by using conserved structural information; for example, a tentative viral lineage has been constructed by analysis of the structures of the major capsid proteins (MCPs) of adenoviruses (infect mammals and birds), Paramecium bursaria Chlorella virus type 1 (infects unicellular algae) (1), and bacteriophages PRD1 (infects Gram-negative bacteria) and Bam35 (infects Gram-positive bacteria). The MCPs of all these viruses are trimers of double ␤-barrels in which the axis of the barrel is oriented normal to the capsid shell (2-4). Could this tentative viral lineage be expanded further? The pool of structural information on viruses that infect eukaryotic and bacterial hosts is relatively large and constantly growing, but our knowledge about viruses that infect Archaea is still very limited. The only three-dimensional structure available at 27-Å resolution is that of the Sulfolobus turreted icosahedral virus (STIV) (5), which infects Sulfolobus solfataricus in the archaeal kingdom Crenarchaeota. As the name suggests, the STIV capsid is icosahedral and has elaborate turret-like structures at the fivefold vertices (5). Its glycosylated MCP (6) also has a double ␤-barrel fold (7), suggesting a common ancestry with the PRD1-adenovirus lineage.SH1 is a virus that infects the halophilic organism Haloarcula hispanica in the archaeal kingdom of the Euryarchaeota (8). Its linear dsDNA genome of 30,898 bp has 56 identified ORFs (9). It has been shown to have several lipid moieties and 11 structural protein species. The proteins have been classified into capsid proteins and lipid core proteins (found with the membrane vesicle enclosing the nucleic acid) by biochemical analysis of virion dissociation products (10). The most abundant proteins are the capsid-associated proteins VP3, VP4, and VP7 and the lipid core protein VP12. The viral membrane is composed of neutral lipids and three major archaeal phosp...