Mechanisms governing poly(A)-tail-length specificity of the human PAN2-PAN3 deadenylase complex

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Eukaryotic mRNA lifespan is regulated by poly(A) tail shortening and poly(A)-binding protein removal.
• The human PAN2-PAN3 complex initiates deadenylation by shortening long poly(A) tails bound to PABPC1.
• PAN2-PAN3 and PABPC1 are conserved across species, but human adaptation to longer poly(A) tails is unclear.

Purpose of the Study:

• To investigate how the human PAN2-PAN3 complex recognizes and processes longer poly(A) tails found in mammalian mRNAs.
• To elucidate the structural basis for the enhanced activity of the human deadenylase on longer substrates.

Main Methods:

• Developed a method to synthesize long homo-polymeric poly(A) RNA (up to 240 nt).
• Reconstituted and analyzed human deadenylation activity in vitro using synthetic poly(A) RNA and PABPC1.
• Utilized single-particle cryo-electron microscopy (cryo-EM) to determine the structure of PAN2-PAN3 bound to poly(A)-PABPC1 ribonucleoproteins.

Main Results:

• Human PAN2-PAN3 exhibits enhanced deadenylation activity on long poly(A)-PABPC1 substrates compared to shorter ones.
• Cryo-EM analysis revealed a significantly longer substrate-binding path in the human PAN2-PAN3 complex relative to its fungal counterparts.
• Demonstrated that the human complex can efficiently process poly(A) tails mimicking mammalian mRNA lengths.

Conclusions:

• The structural adaptation of a longer substrate-binding path in human PAN2-PAN3 explains its ability to act on extended poly(A) tails.
• These findings provide a molecular rationale for the co-evolution of deadenylase function and poly(A) tail length regulation in mammals.
• The study offers insights into the mechanisms controlling mRNA stability and gene expression in eukaryotes.