TDRD1 phase separation drives intermitochondrial cement assembly to promote piRNA biogenesis and fertility.

The intermitochondrial cement (IMC) is a prominent germ granule that locates among clustered mitochondria in mammalian germ cells. Serving as a key platform for Piwi-interacting RNA (piRNA) biogenesis; however, how the IMC assembles among mitochondria remains elusive. Here, we identify that Tudor domain-containing 1 (TDRD1) triggers IMC assembly via phase separation. TDRD1 phase separation is driven by the cooperation of its tetramerized coiled-coil domain and dimethylarginine-binding Tudor domains but is independent of its intrinsically disordered region. TDRD1 is recruited to mitochondria by MILI and sequentially enhances mitochondrial clustering and triggers IMC assembly via phase separation to promote piRNA processing. TDRD1 phase separation deficiency in mice disrupts IMC assembly and piRNA biogenesis, leading to transposon de-repression and spermatogenic arrest. Moreover, TDRD1 phase separation is conserved in vertebrates but not in invertebrates. Collectively, our findings demonstrate a role of phase separation in germ granule formation and establish a link between membrane-bound organelles and membrane-less organelles. [Display omitted] • TDRD1 undergoes phase separation via oligomerization of coiled-coil domain • Tudor domains facilitate TDRD1 phase separation by recognizing sDMA • TDRD1 phase separation deficiency disrupts IMC assembly and piRNA biogenesis in mice • TDRD1 phase separation is conserved in vertebrates but not in invertebrates Gao et al. address the mechanism governing the assembly of mitochondrial-associated germ granules (IMCs) in mammalian germ cells. The IMC component TDRD1 undergoes phase separation to trigger IMC assembly, which is essential for piRNA biogenesis, transposon silencing, and male fertility in mice. [ABSTRACT FROM AUTHOR]