Although Argonaute (Ago) proteins are key players in RNA-mediated post-transcriptional gene silencing in eukaryotes, recent studies on prokaryotic Agos suggest a protective role against invading foreign DNA. Here we present crystal structures of the archaeal Methanocaldococcus jannaschii (Mj) Ago in both the apo and guide DNA-bound forms, providing molecular details of nucleic acid recognition by a prokaryotic Ago1.
Comparison of the MjAgo apo-enzyme with the DNA-bound complex reveals that the interaction with guide DNA leads to pronounced structural rearrangements within the protein, including a structural transformation of a hinge region containing a switch helix, which has been shown to be critical for dynamic target search processes in human Ago2. Analyses of MjAgo mutants identified several key residues crucial for ternary complex formation and DNA cleavage. Interestingly, we observe that the identity of the 3'- and 5'- nucleotide, as well as the switch helix, appear to impact MjAgo cleavage activity. In summary, we provide insights into the molecular mechanisms that drive DNA-guided DNA silencing by an archaeal Ago1.
1Willkomm, S*., Oellig, C. A*., et al. Structural and mechanistic insights into an archaeal DNA-guided Argonaute protein. Nature Microbiology 2017, 2, 17035 (*joint first author)