Crystal structures of Argonaute proteins. (a) P. furiosus Argonaute (Pf-Ago, PDB code 1U04 ) adopts crescent-shaped (1+3) architecture with the N-terminal domain (blue), the middle domain (green), and the PIWI domain (purple) as the base and the PAZ domain (red) at the top. The linker region is yellow. (b) Aquifex aeolicus Argonaute (Aa-Ago, PDB code 1YVU ) adopts bi-lobal architecture (2+2) with the PAZ and N-terminal domains as one lobe and the PIWI and middle domain as another lobe. The domains of Aa-Ago have the same color scheme as that of Pf-Ago. This color scheme is used in all subsequent figures. This and all subsequent structural figures were created using VMD .