This is a Paz domain found in Argonaute proteins from Aquifex aeolicus bacteria. The PAZ core fold in Aquifex aeolicus bacteria Ago (Aa-Ago) proteins is closely related to the human hAgo1 PAZ domain [1]. Structural and functional studies of Aa-Ago indicate that conformational rearrangement of the PAZ domain may be critical for the catalytic cycle of Argonaute and the RNA-induced silencing complex [3].

1: Crystal structure of A. aeolicus argonaute, a site-specific DNA-guided endoribonuclease, provides insights into RISC-mediated mRNA cleavage. Yuan YR, Pei Y, Ma JB, Kuryavyi V, Zhadina M, Meister G, Chen HY, Dauter Z, Tuschl T, Patel DJ; Mol Cell. 2005;19:405-419. PMID:16061186

2: A potential protein-RNA recognition event along the RISC-loading pathway from the structure of A. aeolicus Argonaute with externally bound siRNA. Yuan YR, Pei Y, Chen HY, Tuschl T, Patel DJ; Structure. 2006;14:1557-1565. PMID:17027504

3: Structure of Aquifex aeolicus argonaute highlights conformational flexibility of the PAZ domain as a potential regulator of RNA-induced silencing complex function. Rashid UJ, Paterok D, Koglin A, Gohlke H, Piehler J, Chen JC; J Biol Chem. 2007;282:13824-13832. PMID:17130125