Tejas functions as a core component of nuage assembly and precursor processing in Drosophila piRNA biogenesis

Piwi-interacting RNAs (piRNAs), a class of 23- to 29-nt gonad-specific small RNAs, function to combat transposons in gonads. piRNAs are thought to be processed and amplified in membrane-less granules called nuage in germline cells. In Drosophila, two PIWI family proteins, several Tudor-domain containing (Tdrd) proteins and RNA helicases are assembled at perinuclear region of germline cells, forming nuage to process into piRNAs. Among those, Tejas (Tej), a fly homolog of mouse Tdrd5, has been known as a robust nuage component required for piRNA biogenesis in germline cells, yet its molecular functions remained elusive. To understand its molecular basis on nuage assembly and functions for piRNA biogenesis, we investigated subcellular localization of fluorescent-tagged nuage proteins including Tej and monitored the behavior of piRNA precursors. Tej functions as a core component for assembly of Vasa and Spindle-E to nuage granules through distinct motifs, respectively. The loss of Tej function resulted in malformation of nuage and accumulation of piRNA precursors en route in processing, perturbing further piRNA biogenesis in germline cells. Our study also revealed that the low complexity region of Tej regulates the mobility of nuage by phase separation. Collectively, we propose that Tej plays a pivotal role in processing of piRNA precursors by assembling RNA helicases, Vasa and Spindle-E, to nuage, by controlling the dynamics of nuage components.