A germline-specific class of small RNAs binds mammalian Piwi proteins

In RNA interference, small RNAs (siRNAs or miRNAs) act to regulate gene expression. They serve as specificity factors that direct the RISC (RNA-induced silencing) complex to the complementary mRNA targets. A major component of RISC is a protein of the Argonaute family. Two groups have now identified a new class of small RNAs that interact with one Argonaute subfamily, the Piwi class. These testis-specific small RNAs, called 'piRNAs', are slightly longer than the previously described small RNAs. The function of the piRNAs is not yet known, but they might be involved in sperm production. One of two studies that identifies a new class of small RNAs that interact with one Argonaute subfamily, the Piwi class. These testis-specific small RNAs, called 'piRNAs', are slightly longer than the previously described siRNAs and miRNAs. Small RNAs associate with Argonaute proteins and serve as sequence-specific guides to regulate messenger RNA stability, protein synthesis, chromatin organization and genome structure1,2,3. In animals, Argonaute proteins segregate into two subfamilies4. The Argonaute subfamily acts in RNA interference and in microRNA-mediated gene regulation using 21–22-nucleotide RNAs as guides. The Piwi subfamily is involved in germline-specific events such as germline stem cell maintenance and meiosis. However, neither the biochemical function of Piwi proteins nor the nature of their small RNA guides is known. Here we show that MIWI, a murine Piwi protein, binds a previously uncharacterized class of ∼29–30-nucleotide RNAs that are highly abundant in testes. We have therefore named these Piwi-interacting RNAs (piRNAs). piRNAs show distinctive localization patterns in the genome, being predominantly grouped into 20–90-kilobase clusters, wherein long stretches of small RNAs are derived from only one strand. Similar piRNAs are also found in human and rat, with major clusters occurring in syntenic locations. Although their function must still be resolved, the abundance of piRNAs in germline cells and the male sterility of Miwi mutants suggest a role in gametogenesis.