Structure of the MIWI endoribonuclease bound to pachytene piRNAs from mouse testes

Area of Science:

• Molecular Biology
• Structural Biology
• Genetics

Background:

• PIWI-interacting RNAs (piRNAs) are crucial for maintaining genome stability by silencing transposable elements.
• They function by guiding PIWI endoribonucleases to complementary target transcripts, primarily transposon RNAs.
• Dysfunctional piRNA pathways are linked to infertility and genomic instability in animals.

Purpose of the Study:

• To determine the high-resolution structure of the MIWI-pachytene piRNA complex from mouse testes.
• To elucidate the molecular mechanisms of piRNA binding and PIWI protein activation.
• To understand how the PIWI complex distinguishes guide RNA from target RNA and accommodates mismatches.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structure of the MIWI-pachytene piRNA complex.
• Biochemical and structural analyses were performed on the isolated complex.

Main Results:

• The structure reveals specific interactions holding the piRNA, including recognition of the first uridine.
• The guide RNA adopts an A-form conformation for the initial six nucleotides, facilitating target pairing.
• The PIWI channel is wider than in insect homologs, allowing for piRNA-target mismatches.
• The PIWI endonuclease domain is in an inactive state, requiring re-orientation for catalysis.
• A conserved pocket in the PIWI domain suggests a binding site for GTSF1, potentially activating the enzyme.

Conclusions:

• The MIWI-piRNA complex structure provides insights into piRNA loading and recognition mechanisms.
• The structural features explain the tolerance for mismatches in piRNA-mediated silencing.
• The findings suggest a mechanism for PIWI enzyme activation involving conformational changes and interaction with co-factors like GTSF1.