Mammalian PIWI-piRNA-target complexes reveal features for broad and efficient target silencing.

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• The PIWI-interacting RNA (piRNA) pathway provides adaptive defense against selfish genetic elements, ensuring genome integrity.
• Mammalian piRNA pathway adaptation for target silencing remains poorly understood despite rapid coevolution with transposons.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying piRNA targeting and silencing in mammals.
• To compare the functional and structural differences between mammalian and invertebrate piRNA-induced silencing complexes (piRISCs).

Main Methods:

• Cryo-electron microscopy (cryo-EM) studies of mouse MILI, human HILI, and sponge Ephydatia fluviatilis EfPiwi piRISCs.
• Functional analyses of target binding and cleavage efficiencies.
• Structural comparisons of piRISCs in the presence and absence of target RNA.

Main Results:

• Mouse MILI and human HILI piRISCs exhibit enhanced target binding and cleavage compared to EfPiwi piRISC.
• Mammalian piRISCs possess a wider nucleic-acid-binding channel and a more accessible piRNA seed.
• A conserved lysine residue in mammalian piRISCs relaxes the seed gate, enabling tolerance of seed-target mismatches and broadening target recognition.

Conclusions:

• The study provides a molecular basis for mammalian piRNA targeting, highlighting structural adaptations for efficient silencing.
• Mammalian piRNA machinery can achieve broad target silencing with a limited piRNA repertoire.
• These findings offer insights into the coevolutionary arms race between host defense and selfish genetic elements.