piRNA-induced silencing in C. elegans

In the nematode Caenorhabditis elegans, the 21U RNA, or piRNA, pathway is required to establish proper gene regulation and an immortal germline. To achieve this, PRG-1-bound 21U RNAs trigger a downstream silencing mechanism, mediated by RNA-dependent RNA polymerase (RdRP)-synthetized 22G RNAs. This silencing can become PRG-1 independent, and heritable over many generations. This state is named RNAe. It is unknown how RNAe is established and maintained. We show that maternally provided 21U RNAs can be sufficient to trigger RNAe in embryos. Additionally, we identify the novel, IDR-containing protein, PID-2, as being required for an immortal germline, and essential to establish and maintain RNAe. Effects on small RNA populations suggest that PID-2 affects RdRP activity. PID-2 interacts with two novel, partially redundant, Tudor-domain proteins, PID-4 and PID-5, whose joint loss closely phenocopies pid-2 mutants. Additionally, PID-5 has domain related to the protease APP-1, and binds APP-1 itself, implicating N-terminal proteolysis in RNAe. Finally, we show that PID-2, -4 and -5 localize to perinuclear foci and affect Z-granules, which have been linked to RdRP activity.