Martin Jinek & Jennifer A. Doudna Nature 457, 405-412(22 January 2009)

a, Eukaryotic Argonaute proteins have four domains: the N-terminal, PAZ, MID and PIWI domains. In some cases, notably for the structures of Archaeoglobus fulgidus Piwi protein, the MID domain and PIWI domain have been referred to as domain A and domain B, respectively. b, A crystal structure of Pyrococcus furiosus Argonaute (Ago) soaked with Mn2+ (PDB identity 1Z25) is shown. The protein adopts a bilobate architecture, with the N-terminal and PAZ domains forming one lobe and the MID and PIWI domains forming the other. The metal ion in the active site is shown as a purple sphere. The amino-acid residues involved in metal-ion binding in the slicer catalytic site are shown in stick format.