Argonaute proteins form the functional core of the RNA-induced silencing complex (RISC) that mediates RNA silencing in eukaryotes. Argonaute proteins are loaded with small RNAs, which are used to guide RISC to complementary mRNA targets. To understand how Argonaute uses the sequence information encoded in guide RNAs, we determined high-resolution crystal structures of human Argonaute2 (Ago2) in complex with a guide siRNA and a variety of target RNAs, bearing different degrees of complementarity to the guide. These structures show that Ago2 binds to the 5' end in the guide RNA and splays out nucleotides 2 to 6, termed the �seed region�, in a helical conformation for base-pairing with mRNA targets. A narrow cavity adjacent to the guide 5' end specifically recognizes adenosine nucleotides in target RNAs. Target binding to the seed region promotes conformational changes in Ago2 that shift guide nucleotides 12 to 16 into a helical conformation, thereby generating a �supplementary seed region�. We suggest that these and additional observed conformational changes associated with target binding are involved in recruitment of further silencing factors to RISC.

Structural basis for target RNA recognition by human Argonaute-2

248th National Meeting (2014)

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