JHIBRG:Abstract July 19 2007

Small RNAs associate with Argonaute proteins and serve as sequence-specific guides to regulate messenger RNA stability, protein synthesis, chromatin organization and genome structure. In animals, Argonaute proteins segregate into two subfamilies. 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 germlinespecific 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 was known.

In 2006, six groups converged upon the landmark finding that Piwi proteins in flies and mammals bind a class of small RNAs distinct from miRNAs and typical siRNAs. Two groups determined that Piwi and Aubergine bind repeat-associated siRNAs (rasiRNAs). The rasiRNAs were first characterized in small RNA cloning studies from flies at different stages of development, which revealed that the rasiRNAs were longer in length than canonical small RNAs (24–27 nucleotides, as opposed to 21–22), were enriched in the testes and early in development, and derived from retrotransposons and other repetitive elements. Working in mammals, four other groups revealed that Piwi proteins interact with novel RNAs derived from intergenic regions but were not as dominated with matches to repeat elements as the rasiRNAs from flies. Mammalian Piwi-interacting RNAs (piRNAs), like the rasiRNAs, were longer than miRNAs, and the pool of piRNA sequences was extremely complex. These piRNAs coimmunoprecipitated with two mouse Piwi subclade members, Miwi and Mili, whereas a conventional chromatographic purification of proteins associated with piRNAs from rat testes followed by protein sequencing identified piRC (piRNA complex), containing the rat Piwi protein, Riwi, and a RecQ1 helicase that has genetic links to known RNA silencing pathways.

On Aug 2, I’ll introduce the overview of small RNAs and focus on coming ages of Piwi proteins and piRNA.