Relaxed targeting rules help PIWI proteins silence transposons.

Area of Science:

• Molecular Biology
• Genetics
• RNA Biology

Background:

• Eukaryotes utilize small RNAs (siRNAs, miRNAs) to guide AGO proteins for gene regulation and genome defense.
• Animal-specific PIWI proteins use PIWI-interacting RNAs (piRNAs) to silence transposon transcripts.
• Transposons may evade silencing via mutations reducing piRNA complementarity.

Purpose of the Study:

• To investigate the target binding and cleavage capabilities of PIWI proteins compared to AGO proteins.
• To determine the tolerance of PIWI proteins to mismatches in piRNA-target interactions.
• To understand the evolutionary advantage of the PIWI-piRNA pathway in genome defense.

Main Methods:

• Examination of target binding and cleavage by mouse and sponge PIWI proteins.
• Analysis of PIWI protein activity with partially paired target transcripts.
• Comparison of PIWI protein mismatch tolerance with known AGO protein requirements.

Main Results:

• PIWI proteins efficiently cleave transcripts with partial pairing to piRNA guides.
• PIWI slicing tolerates mismatches at any target nucleotide, including around the scissile phosphate.
• Canonical seed pairing is not essential for PIWI binding or cleavage, unlike AGO proteins.

Conclusions:

• PIWI proteins exhibit superior capability over AGO proteins in targeting evolving transposons.
• The PIWI pathway's tolerance to mismatches provides robust defense against transposons.
• This mechanism explains the retention of the PIWI-piRNA pathway in animal evolution for genome defense.