Sorting of Drosophila small silencing RNAs.

In Drosophila, small interfering RNAs (siRNAs), which direct RNA interference through the Argonaute protein Ago2, are produced by a biogenesis pathway distinct from microRNAs (miRNAs), which regulate endogenous mRNA expression as guides for Ago1. Here, we report that siRNAs and miRNAs are sorted into Ago1 and Ago2 by pathways independent from the processes that produce these two classes of small RNAs. Such small-RNA sorting reflects the structure of the double-stranded assembly intermediates--the miRNA/miRNA( *) and siRNA duplexes--from which Argonaute proteins are loaded. We find that the Dcr-2/R2D2 heterodimer acts as a gatekeeper for the assembly of Ago2 complexes, promoting the incorporation of siRNAs and disfavoring miRNAs as loading substrates for Drosophila Ago2. A separate mechanism acts in parallel to favor miRNA/miRNA( *) duplexes and exclude siRNAs from assembly into Ago1 complexes. Thus, in flies small-RNA duplexes are actively sorted into Argonaute-containing complexes according to their intrinsic structures.