Pyrococcus abyssi virus 1 (PAV1) was the first virus particle infecting a hyperthermophilic Euryarchaeota (Pyrococcus abyssi strain GE23) that has been isolated and characterized. It is lemon shaped and is decorated with a short fibered tail. PAV1 morphologically resembles the fusiform members of the family Fuselloviridae or the genus Salterprovirus. The 18？kb dsDNA genome of PAV1 contains 25 predicted genes, most of them of unknown function. To help assigning functions to these proteins, we have initiated structural studies of the PAV1 proteome. We determined the crystal structure of a putative protein of 137 residues (PAV1-137) at a resolution of 2.2？？. The protein forms dimers both in solution and in the crystal. The fold of PAV1-137 is a four-α-helical bundle analogous to those found in some eukaryotic adhesion proteins such as focal adhesion kinase, suggesting that PAV1-137 is involved in protein-protein interactions. 1. Introduction The archaea domain is organized into two major phyla, the Crenarchaeota and the Euryarchaeota. The first phylum contains mainly the extremely thermophilic Sulfolobales, Desulfurococcales, and Thermoproteales. The vast majority of hyperthermophilic viruses were isolated from the Crenarchaeota infecting in particular the genera Sulfolobus, Thermoproteus, Acidianus, Pyrobaculum, Stygiolobus, and Aeropyrum [1, 2]. Their shapes are characterized by unusual morphologies very different from bacterialviruses and eukaryotic viruses. Genomic sequences were determined for some of these archaeal viruses and revealed a very high portion of ORFan genes [3]. Due to their exceptional morphological and genomic properties, they were assigned to eight novel viral families [1]. The Euryarchaeota phylum includes extreme halophiles, methanogens, and hyperthermophilic sulfur reducers (Thermococcales). Most of the viruses infecting this phylum are isolated from mesophilic hosts and are tailed viruses, whereas pleomorphic types are relatively rare [4]. The knowledge about archaeal viruses is still very limited and this is even more poignant for viruses that infect hyperthermophilic Euryarchaeota [5, 6]. To date, PAV1 and TPV1 (Thermococcus prieurii virus 1) are the only viruses isolated from cultivated marine hyperthermophilic euryarchaea. These spindle-shaped viruses are morphologically similar to the haloviruses of the genus Salterprovirus [7, 8] that infect extreme halophiles, and to crenarchaeal viruses assigned to the fusiform family Fuselloviridae [9], but they do not share any genomic properties. PAV1, isolated from Pyrococcus abyssi,
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