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PIWI-interacting RNAs, or piRNAs, are the most abundant short non-coding RNAs. More than 20,000 genes have been found in humans that code for piRNAs while only 2000 genes have been found for miRNAs. piRNAs can act at the transcriptional and post-transcriptional levels and have a vital role in silencing transposable elements present in germ cells. They are also involved in epigenetic silencing and activation. Previously, they were thought to function only in germ cells but new evidence suggests...
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RNA interference (RNAi) is a cellular mechanism that inhibits gene expression by suppressing its transcription or activating the RNA degradation process. The mechanism was discovered by Andrew Fire and Craig Mello in 1998 in plants. Today, it is observed in almost all eukaryotes, including protozoa, flies, nematodes, insects, parasites, and mammals. This precise cellular mechanism of gene silencing has been developed into a technique that provides an efficient way to identify and determine the...
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Updated: Sep 8, 2025

In Situ Detection of Ribonucleoprotein Complex Assembly in the C. elegans Germline using Proximity Ligation Assay
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A conserved PIWI silencing complex detects piRNA-target engagement.

Dipayan De1, Sucharita Sarkar1, Luca F R Gebert1

  • 1Department of Integrative Structural and Computational Biology, Scripps Research, La Jolla, CA, USA.

Molecular Cell
|September 5, 2025
PubMed
Summary
This summary is machine-generated.

PIWI proteins use piRNAs to defend against selfish genetic elements. This study reveals a conserved PIWI-associated complex that recognizes and cleaves transposon RNAs, providing a crucial defense mechanism across animals.

Keywords:
GTSF1MaelstromPiwiSiwipiRNAsilencingtransposible elementtransposon

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Area of Science:

  • Molecular Biology
  • Genetics
  • Structural Biology

Background:

  • PIWI proteins and PIWI-interacting RNAs (piRNAs) are essential for silencing selfish genetic elements in animal germ cells.
  • The precise mechanism by which piRNA-guided transposon recognition activates PIWI proteins for defense remains incompletely understood.

Purpose of the Study:

  • To elucidate the molecular mechanism of transposon recognition and silencing by PIWI proteins.
  • To identify the protein complex involved in piRNA-mediated defense against transposons.

Main Methods:

  • Biochemical assays to characterize protein-protein interactions and enzymatic activity.
  • Cryo-electron microscopy (cryo-EM) to determine the structural basis of the complex.
  • Structural predictions to assess evolutionary conservation.

Main Results:

  • Identification of a novel transposon recognition complex comprising Siwi, GTSF1, and Maelstrom in silkworms.
  • Demonstration that extended piRNA-target pairing induces a specific conformation of Siwi, enabling recruitment of GTSF1 and Maelstrom.
  • Structural analysis revealed that Maelstrom and GTSF1 cooperatively activate Siwi's endonuclease activity, leading to target RNA cleavage and recruitment of Spindle-E.

Conclusions:

  • The study defines a conserved PIWI-associated complex (PIWI*) as a key effector in piRNA-mediated transposon defense.
  • This mechanism of PIWI* assembly and activation is conserved across metazoans, from sponges to humans.
  • The findings provide a structural and mechanistic framework for understanding how animals defend their genomes against mobile genetic elements.