Coupled 5' nucleotide recognition and processivity in Xrn1-mediated mRNA decay

Area of Science:

• Molecular Biology
• Structural Biology
• Gene Expression Regulation

Background:

• Messenger RNA (mRNA) decay regulates gene expression and cellular surveillance.
• The exoribonuclease Xrn1 enzyme degrades cytoplasmic RNA substrates.
• Xrn1's mechanism for 5'-to-3' degradation of RNA has remained unclear.

Purpose of the Study:

• To determine the structural basis of Xrn1's substrate recognition and degradation mechanism.
• To understand how Xrn1 targets 5'-monophosphorylated RNA substrates.

Main Methods:

• X-ray crystallography to obtain the structure of an Xrn1-substrate complex.
• Site-directed mutagenesis to investigate the role of specific residues.

Main Results:

• The crystal structure reveals substrate binding via aromatic stacking of the 5'-terminal trinucleotide and recognition of the 5' phosphate by a basic pocket.
• Mutations in key binding residues reduce Xrn1 processivity.
• The mechanism explains Xrn1's specificity for 5'-monophosphorylated substrates and its ability to unwind RNA duplexes.

Conclusions:

• The Xrn1-substrate complex structure provides insight into mRNA turnover fidelity.
• A model for translocation-coupled unwinding of structured RNA substrates by Xrn1 is proposed.