Figure 2.

Structure of Argonaute proteins. (a) X-ray crystal structure of the Argonaute protein from the archaeon Aquifex aeolicus. The amino-terminal domain (N, magenta) is linked by linker 1 (L1, green) to the PAZ domain (blue). Linker 2 (L2, yellow) connects the PAZ domain with the MID domain (magenta), which is followed by the PIWI domain (blue) at the carboxy-terminal end of the protein. The PIWI box (red) has been implicated in the interaction between Argonaute proteins and the nuclease Dicer in human cells [79]. (b) Schematic depiction of the human Ago2 protein. The domains are colored as in (a). In the PAZ domain, residues important for binding of small RNA 3' ends are indicated (R, arginine; F, phenylalanine; Y, tryptophan), and in the Mid domain, the residues required for 5' end binding to small RNAs and binding to the 7-methylguanine (m⁷G) cap of target mRNAs are shown (K, lysine; Q, glutamine) and the PIWI domain in red (catalytic residues are shown). (a) Reproduced with permission from [15].