Fig. 2

Biogenesis and functions of the piRNA/PIWI complex. piRNA and PIWI proteins form a ribonucleoprotein complex that is primarily responsible for the maintenance of genome integrity through transposable element (TE) silencing in the germline at both the transcriptional and post-transcriptional level. a The ribonucleoprotein complex is active in piRNA biogenesis, where it cleaves target RNAs at the position 10 and 11 of the guide strand, generating the 5′ end of the cleavage product that will be loaded to a second PIWI protein, and gives rise to a secondary piRNA, after nucleolytic processing of the 3′ end. Primary piRNAs have uridine (U) bias at their 5′ nucleic acid, while secondary piRNAs, which shows 10 nucleotide complementarity with primary piRNAs at their 5′ ends, exhibits a bias for adenosine (A) at the tenth nucleotide. b In the nucleus, the complex is active in epigenetic silencing, through the establishment of a repressive chromatin state as a result of the recruitment of Heterochromatin protein 1 (HP1a) and histone methyltransferases (HMT); and epigenetic activation, through euchromatic histone modifications that allows binding of proteins such as JmjC domain-containing histone demethylation protein 1 (Epe1), chromodomain protein1 and 2 (Chp1, Chp2) and Chromatin-associated protein Swi6. In the cytoplasm, it is active in mRNA degradation through association with the carbon catabolite repressed 4 - negative on TATA-less (CCR4–NOT) deadenylation complex, together with Smaug (Smg)