Conservation of the putative GTPase binding domain site 1 of aRF1. (A) Schematic presentation of eRF1 and two major groups of aRF1s (Euryarchaeota and Crenarchaeota). As a member of Crenarchaeota, A. pernix aRF1 lacks a large portion of domain C (“Δ”). Site 1, the predominant GTPase binding site found in the eRF1·eRF3-d23 crystal structure (located separately in sequences 1A and 1B) (10), and other relevant sites are indicated. The numbers of amino acid residues are indicated for representatives of each group in brackets. Homologous Pelota regions (domains B and C) are indicated in red. (B) Alignment of representative aRF1/eRF1/Pelota amino acid sequences around site 1 in A. The black dots above the sequences indicate the previously demonstrated critical contact residues for eRF3 binding (10). Distances in the eRF1·eRF3-d23 complex (4 Å, 6 Å) are also indicated in colored dots at the top. (C) Superposition of the domain C (site 1) structures of aRF1 and eRF1. The extra region of eRF1, which is truncated in the Crenarchaeota aRF1 group, is indicated as Δ. This is a rear side view of Fig. S3A.