Artificial tethering of Argonaute proteins for studying their role in translational repression of target mRNAs.

Small RNAs such as microRNAs (miRNAs) and small-interfering RNAs (siRNAs) associate with members of the RNA-binding Argonaute family proteins. Together they participate in transcriptional and posttranscriptional gene silencing mechanisms. The fate of the target mRNA is determined, in part, by the degree of complementarity with the small RNA. To examine the exact role of the Argonaute protein in the silencing complex, human Argonautes were artificially recruited to reporter mRNAs in a small RNA-independent manner by the BoxB-N-peptide tethering system. Tethering of Argonaute proteins to a reporter mRNA leads to the inhibition of translation, mimicking the repression seen with miRNAs. Similar tethering experiments were performed with fly and fission yeast Argonaute proteins and other components of the small RNP (ribonucleoprotein) complex, uncovering their specific roles in the silencing complexes containing them.

Recognition of 2'-O-methylated 3'-end of piRNA by the PAZ domain of a Piwi protein.

Piwi proteins are germline-specific Argonautes that associate with small RNAs called Piwi-interacting RNAs (piRNAs), and together with these RNAs are implicated in transposon silencing. The PAZ domain of Argonaute proteins recognizes the 3'-end of the RNA, which in the case of piRNAs is invariably modified with a 2'-O-methyl group. Here, we present the solution structure of the PAZ domain from the mouse Piwi protein, MIWI, in complex with an 8-mer piRNA mimic. The methyl group is positioned in a hydrophobic cavity made of conserved amino acids from strand ß7 and helix α3, where it is contacted by the side chain of methionine-382. Our structure is similar to that of Ago-PAZ, but subtle differences illustrate how the PAZ domain has evolved to accommodate distinct 3' ends from a variety of RNA substrates.