The Function of Piwi is Independent of Its Slicer Activity

In diverse organisms, the highly-conserved Piwi proteins bind to a complex class of small non-coding RNAs called piRNAs, and are essential for germline stem cell maintenance, transposon silencing, fertility, and offspring viability1. By homology with other Argonautes, Piwi proteins have been proposed and later reported to possess endonuclease activity in vitro, effected by its conserved aspartate catalytic triad2,3. Indeed, mutation of these residues in one of the three mouse PIWI proteins, MILI, affects the production of Line 1 piRNAs and spermatogenesis4. In Drosophila, the conserved slicer function has been proposed to be crucial for all Piwi proteins in piRNA amplification and transposon silencing5. Here, we report in vivo evidence that, in contrast to common belief, the catalytic triad of Piwi is not required for the function of PIwi  in the germline for stem cell maintenance, fertility, transposon silencing, or  piRNA biogenesis. In addition, it is not required for piRNA biogenesis in the soma where Piwi is the sole Piwi-subfamily protein involved in the process. These observations, together with the recent findings on MILI and MIWI2 triad mutant, indicate that even though the slicer function may be required for a subset of piRNA-mediated process, it is overall inconsequential for the function of Piwi proteinsÂ