The evolutionarily conserved Argonaute/PIWI (AGO/PIWI; a.k.a. PAZ-PIWI Domain, or PPD) family of proteins is crucial for the biogenesis and function of small non-coding RNAs (ncRNAs). This family can be divided into AGO and PIWI subfamilies. The AGO proteins are ubiquitously present in diverse tissues. They bind to small interfering RNAs (siRNAs) and microRNAs (miRNAs). In contrast, the PIWI proteins are predominantly present in the germline, and associate with a novel class of small RNAs known as PIWI-interacting RNAs (piRNAs). Tens of thousands of piRNA species, typically 24-32 nucleotide long, have been found in mammals, zebrafish, and Drosophila. Most piRNAs appear to be generated from a small number of long single-stranded RNA precursors that are often encoded by repetitive intergenic sequences in the genome. PIWI proteins play crucial roles during germline development and gametogenesis of many metazoan species, from germline determination and germline stem cell maintenance to meiosis, spermiogenesis, and transposon silencing. These diverse functions may involve piRNAs, and may be achieved via novel mechanisms of epigenetic and post-transcriptional regulation.
Keywords: epigenetic regulation, germ cell, stem cell, transposon silencing, translational regulation