The mouse homolog of HEN1 is a potential methylase for Piwi-interacting RNAs

Piwi-interacting RNAs (piRNAs) are a novel class of small regulatory RNAs that are expressed specifically and abundantly in germ cells. Mammalian piRNAs are 26-31 nucleotides in length and bind to Piwi proteins, but their function and biogenesis remain elusive. We previously showed that mammalian piRNAs are 2'-O-methylated at their 3' termini. The biosynthetic mechanism and function of this modification is unknown. Here, we report that the mouse homolog (mHEN1) of HEN1, a plant microRNA (miRNA) 2'-O-methyltransferase, is expressed specifically in testis and methylates 3' termini of piRNAs in vitro. These findings provide insight into the biogenesis of piRNAs.