Piwi-interacting RNA (piRNA) is a class of small RNA molecules that is expressed in mammalian testes and somatic cells and forms RNA-protein complexes with Piwi proteins. These piRNA complexes (piRCs) have been linked to transcriptional gene silencing of retrotransposons and other genetic elements in germ line cells, particularly those in spermatogenesis. Purification of these complexes has revealed that these oligonucleotides are approximately 29-30 nucleotides long. They are distinct in size from miRNA and are associated with distinct protein complexes.

It remains unclear how piRNAs are generated, but their biogenesis pathway is distinct from miRNA and siRNA.

RNA silencing

piRNA has a role in RNA silencing via the formation of an RNA-induced silencing complex (RISC). The Piwi proteins are part of a family of proteins called the argonautes, which are active in the testes of mammals and are required for germ-cell and stem-cell development in invertebrates. piRNAs are short stretches of RNAs with a typical length of 26-31 nucleotides, making them a distinct entity from microRNAs (miRNAs) and short interfering RNAs (siRNAs) which are 21-23 nucleotides long. Three Piwi subfamily proteins - MIWI, MIWI2 and MILI - have been found to be essential for spermatogenesis in mice.

References

• N.C. Lau et al., "Characterization of the piRNA Complex from Rat Testes," Science 313, 363 (2006)
• V.N. Kim, "Small RNAs Just Got Bigger: Piwi-Interacting RNAs (piRNAs) in Mammalian Testes," Genes Dev. 20, 1993 (2006)
• A. Girard et al., "A Germline-Specific Class of Small RNAs Binds Mammalian Piwi Proteins," Nature 442, 199 (2006)
• A. Aravin et al., "A Novel Class of Small RNAs Bind to MILI Protein in Mouse Testes," Nature 442, 203 (2006)
• S.T. Grivna et al., "A Novel Class of Small RNAs in Mouse Spermatogenic Cells," Genes Dev. 20, 1709 (2006)
• T. Watanabe et al., "Identification and characterization of two novel classes of small RNAs in the mouse germline," Genes Dev. 20, 1732 (2007)
• M.A. Carmell et al., "MIWI2 Is Essential for Spermatogenesis and Repression of Transposons in the Mouse Male Germline," Dev Cell. 12, 503 (2007)