Crystal structure and ligand binding of the MID domain of a eukaryotic Argonaute protein

Argonaute (AGO) proteins are core components of RNA-induced silencing complexes and have essential roles in RNA-mediated gene silencing. They are characterized by a bilobal architecture, consisting of one lobe containing the amino-terminal and PAZ domains and another containing the MID and PIWI domains. Except for the PAZ domain, structural information on eukaryotic AGO domains is not yet available. In this study, we report the crystal structure of the MID domain of the eukaryotic AGO protein QDE-2 from Neurospora crassa. This domain adopts a Rossmann-like fold and recognizes the 5'-terminal nucleotide of a guide RNA in a manner similar to its prokaryotic counterparts. The 5'-nucleotide-binding site shares common residues with a second, adjacent ligand-binding site, suggesting a mechanism for the cooperative binding of ligands to the MID domain of eukaryotic AGOs.