# Relaxed targeting rules help PIWI proteins silence transposons

## TL;DR

PIWI proteins can silence transposons more effectively than AGO proteins by tolerating RNA mismatches, helping defend the genome without needing new RNA guides.

## Contribution

PIWI proteins tolerate mismatches in RNA targets, making them more effective at silencing transposons compared to AGO proteins.

## Key findings

- PIWI proteins can cleave transcripts with partial complementarity to their piRNA guides.
- PIWI slicing does not require canonical seed pairing, unlike AGO proteins.
- PIWI proteins avoid silencing host RNAs while efficiently targeting transposons.

## Abstract

In eukaryotes, small RNA guides, such as small interfering RNAs and microRNAs, direct AGO-clade Argonaute proteins to regulate gene expression and defend the genome against external threats. Only animals make a second clade of Argonaute proteins: PIWI proteins. PIWI proteins use PIWI-interacting RNAs (piRNAs) to repress complementary transposon transcripts1,2. In theory, transposons could evade silencing through target site mutations that reduce piRNA complementarity. Here we report that, unlike AGO proteins, PIWI proteins efficiently cleave transcripts that are only partially paired to their piRNA guides. Examination of target binding and cleavage by mouse and sponge PIWI proteins revealed that PIWI slicing tolerates mismatches to any target nucleotide, including those flanking the scissile phosphate. Even canonical seed pairing is dispensable for PIWI binding or cleavage, unlike plant and animal AGOs, which require uninterrupted target pairing from the seed to the nucleotides past the scissile bond3,4. PIWI proteins are therefore better equipped than AGO proteins to target newly acquired or rapidly diverging endogenous transposons without recourse to new small RNA guides. Conversely, the minimum requirements for PIWI slicing are sufficient to avoid inadvertent silencing of host RNAs. Our results demonstrate the biological advantage of PIWI over AGO proteins in defending the genome against transposons and suggest an explanation for why the piRNA pathway was retained in animal evolution.

Of the two types of Argonaute proteins produced by animals, AGO and PIWI, PIWI proteins can bind RNAs with less complementarity, enabling efficient silencing of transposons without the need to produce new RNA guides.