Asterix/Gtsf1 links tRNAs and piRNA silencing of retrotransposons

Abstract

The piRNA pathway safeguards genomic integrity by silencing transposable elements in the germline. While Piwi is the central piRNA factor, others including Asterix/Gtsf1 have also been demonstrated to be critical for effective silencing. Here, using eCLIP with a custom informatic pipeline, we show that Asterix/Gtsf1 specifically binds tRNAs in cellular contexts. We determined the structure of mouse Gtsf1 by NMR spectroscopy and identified the RNA binding interface on the protein's first zinc finger, which was corroborated by biochemical analysis as well as cryo-EM structures of Gtsf1 in complex with co-purifying tRNA. We further show that LTR retrotransposons are preferentially de-repressed in Asterix mutants. Given the role of tRNAs as LTR retrotransposon primers, our work implicates Asterix/Gtsf1 as exploiting tRNA dependence to identify transposon transcripts and promote piRNA silencing.

Taming transposable elements in vertebrates: from epigenetic silencing to domestication

“…tRNAs are required to prime the reverse transcription of long terminal repeats (LTR) retrotransposons [207]. Interestingly, tRNAs are bound by Gtsf1 (Table 1), a conserved interactor of Piwi proteins required for retrotransposon silencing [209][210][211]. Such Gtsf1-tRNA interaction was proposed to be a strategy exploited by the piRNA pathway to target and silence retrotransposons in a sequence-specific manner [209].…”

“…Interestingly, tRNAs are bound by Gtsf1 (Table 1), a conserved interactor of Piwi proteins required for retrotransposon silencing [209][210][211]. Such Gtsf1-tRNA interaction was proposed to be a strategy exploited by the piRNA pathway to target and silence retrotransposons in a sequence-specific manner [209]. These findings suggest that at least two sRNA classes, tRFs and piRNAs, make use of the cellular pool of tRNAs to silence TEs.…”

The conserved zinc-finger protein GTSF1 helps PIWI proteins achieve their full catalytic potential

“…Only three known eukaryotic proteins contain CHHC zinc-fingers: the spliceosomal RNA-binding protein U11-48K, the TRM13 tRNA methyltransferase, and GTSF1 and its paralogs 92 . In vitro, the first zinc-finger of GTSF1 directly binds RNA; RNA binding requires four basic surface residues (R26, R29, K36 and K39) 93 ). Potentiation of target cleavage required RNA-binding by GTSF1: purified mutant GTSF1 R26A,R29A,K36A,K39A (Extended Data Fig.…”

The two Gtsf paralogs in silkworms orthogonally activate their partner PIWI proteins for target cleavage

“…Indeed, AlphaFold-Multimer predicts that both Gtsf proteins locate around the catalytic PIWI domain (Fig EV1F). Moreover, the first zinc finger motif of mouse GTSF1, which has RNA binding activity, is required to promote PIWI slicing (Ipsaro et al , 2021; Arif et al , 2022). Therefore, it is likely that Gtsf binds to the PIWI domain and the target RNA simultaneously to stabilize the catalytic competent conformation of PIWI proteins.…”

“…Indeed, AlphaFold-Multimer predicts that both Gtsf proteins locate around the catalytic PIWI domain (Fig EV1F). Moreover, the first zinc finger motif of mouse GTSF1, which has RNA binding activity, is required to promote PIWI slicing (Arif et al, 2022;Ipsaro et al, 2021).…”