The structural landscape of Microprocessor mediated pri-let-7 miRNA processing.

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• MicroRNA (miRNA) biogenesis is crucial for gene regulation.
• The Microprocessor complex (MP), comprising Drosha and DGCR8, initiates miRNA biogenesis by cleaving pri-miRNA hairpins.
• Sequence motifs within pri-miRNAs influence MP recognition, cleavage fidelity, and efficiency.

Purpose of the Study:

• To elucidate the structural basis of pri-miRNA recognition and cleavage by the human Microprocessor complex.
• To investigate the role of specific sequence motifs, including the fUN motif, in MP function.
• To understand how accessory factors like SRSF3 contribute to MP-mediated pri-miRNA processing.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine high-resolution structures of MP-pri-miRNA complexes.
• Biochemical assays to assess MP recognition, cleavage kinetics, and fidelity.
• Structural analysis of MP complexes with various let-7 family pri-miRNAs and the SRSF3 protein.

Main Results:

• MP exhibits structural plasticity, accommodating diverse pri-miRNA structures.
• Key features of the 5' UG sequence motif, termed the "fUN" motif, were identified.
• Cleavage of class-II pri-let-7s with bulged nucleotides generates precursors with a 1-nt 3' overhang.
• The MP-SRSF3-pri-let-7f1 structure reveals SRSF3's role in enhancing MP fidelity via interaction with the CNNC-motif and Drosha's PAZ-like domain.

Conclusions:

• The study provides mechanistic insights into the flexible recognition of pri-miRNAs by MP.
• Structural plasticity and specific sequence motifs contribute to accurate and regulated pri-miRNA processing.
• SRSF3 acts as a fidelity factor, enhancing the accuracy of miRNA biogenesis.