Figure 7. The model of dynamics of Armi and Piwi in OSCs.

Armi localizes to Yb bodies Piwi independently, but Piwi localization to Yb bodies depends on Armi. At Yb bodies, Piwi is loaded with piRNA precursor (through their 5′ ends) to become pre‐piRISC. Armi joins the complex by binding the downstream region in the precursor and also Piwi. The Armi–Piwi–pre‐piRISC complex then leaves Yb bodies to head to mitochondria for Zuc cleavage. The Armi–Piwi–pre‐piRISC complex is docked onto mitochondria through the Gasz–Daed complex, under which Zuc cleaves the precursor and releases Piwi–piRISC to the cytosol, while Armi returns to Yb bodies for another round of reaction (not shown). Piwi–piRISC is translocated to the nucleus for transposon repression. Nascent Piwi may bind 5′ end of piRNA precursor produced by Zuc cleavage, while the RNA is still bound with Armi. Another Zuc cleavage gives rise to phased piRNAs. The Piwi mutant MIDmt does not bind piRNA precursor at Yb bodies, abrogating the whole processing. Another Piwi mutant PAZmt does bind piRNA precursor at Yb bodies, but after Zuc reaction, the 3′ end of piRNA is not bound by the PAZ domain because of the mutation, and so functional piRISC is not sufficiently produced in the cells. The Armi mutant N756A fails to bind piRNA precursor pre‐bound with Piwi at Yb bodies, leading to the failure of Piwi–pre‐piRISC departure from Yb bodies. Another Armi mutant ΔN34 is able to bind Piwi–pre‐piRISC at Yb bodies but fails to leave the bodies for an unknown reason. The whole processing of Piwi–piRISC formation is well regulated by the interdependence between Armi and Piwi and also the RNA‐binding activity of the two proteins. Functions of SoYb, Vret, Shu, and Mino (not shown) in the pathway remain to be determined.