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Updated: Oct 21, 2025
Sequence-specific and Selective Recognition of Double-stranded RNAs over Single-stranded RNAs by Chemically Modified Peptide Nucleic Acids
Published on: September 21, 2017
Structural basis for piRNA targeting
Todd A Anzelon1, Saikat Chowdhury1,2, Siobhan M Hughes1
1Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA, USA.
Abstract:
PIWI proteins use PIWI-interacting RNAs (piRNAs) to identify and silence transposable elements and thereby maintain genome integrity between metazoan generations1. The targeting of transposable elements by PIWI has been compared to mRNA target recognition by Argonaute proteins2,3, which use microRNA (miRNA) guides, but the extent to which piRNAs resemble miRNAs is not known. Here we present cryo-electron microscopy structures of a PIWI-piRNA complex from the sponge Ephydatia fluviatilis with and without target RNAs, and a biochemical analysis of target recognition. Mirroring Argonaute, PIWI identifies targets using the piRNA seed region. However, PIWI creates a much weaker seed so that stable target association requires further piRNA-target pairing, making piRNAs less promiscuous than miRNAs. Beyond the seed, the structure of PIWI facilitates piRNA-target pairing in a manner that is tolerant of mismatches, leading to long-lived PIWI-piRNA-target interactions that may accumulate on transposable-element transcripts. PIWI ensures targeting fidelity by physically blocking the propagation of piRNA-target interactions in the absence of faithful seed pairing, and by requiring an extended piRNA-target duplex to reach an endonucleolytically active conformation. PIWI proteins thereby minimize off-targeting cellular mRNAs while defending against evolving genomic threats.
Insights
PIWI proteins use PIWI-interacting RNAs (piRNAs) to silence transposable elements and maintain genome integrity. Unlike microRNAs (miRNAs), piRNAs form weaker initial interactions, ensuring precise targeting and genomic defense.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- PIWI proteins and PIWI-interacting RNAs (piRNAs) are crucial for silencing transposable elements and maintaining genome integrity across generations.
- The mechanism of PIWI-piRNA targeting is often compared to Argonaute proteins using microRNAs (miRNAs), but the similarities and differences remain unclear.
Purpose of the Study:
- To elucidate the structural basis of PIWI-piRNA target recognition.
- To compare the target recognition mechanisms of piRNAs and miRNAs.
Main Methods:
- Cryo-electron microscopy (cryo-EM) to determine the structure of a PIWI-piRNA complex.
- Biochemical analysis of PIWI-piRNA-target RNA interactions.
Main Results:
- PIWI proteins recognize targets via a piRNA seed region, similar to Argonaute proteins.
- PIWI-piRNA interactions exhibit a weaker seed but require extensive pairing, reducing promiscuity compared to miRNAs.
- PIWI structures facilitate mismatch-tolerant pairing and require extended duplexes for activity, ensuring targeting fidelity and minimizing off-target effects.
Conclusions:
- PIWI proteins employ a unique target recognition strategy involving a weak seed and extensive pairing, distinct from miRNA mechanisms.
- This mechanism allows PIWI to effectively defend against transposable elements while minimizing off-target binding to cellular mRNAs.
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