SMG6 promotes endonucleolytic cleavage of nonsense mRNA in human cells

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• Nonsense-mediated mRNA decay (NMD) is a surveillance pathway that eliminates mRNAs with premature termination codons (PTCs).
• The precise mechanisms of NMD-mediated mRNA degradation are thought to differ across species, with endonucleolysis proposed for Drosophila and exonucleolysis for yeast and human cells.
• Understanding these species-specific differences is crucial for comprehending gene regulation and disease mechanisms.

Purpose of the Study:

• To investigate the degradation mechanism of human nonsense mRNAs.
• To identify the specific endonuclease responsible for initiating NMD in human cells.
• To revise the current mechanistic model of NMD in human cells.

Main Methods:

• Investigated nonsense mRNA degradation in human cells.
• Utilized site-directed mutagenesis to alter conserved residues in the nuclease domain of the metazoan-specific NMD factor SMG6 (specifically the C-terminal PIN motif).
• Performed in vivo and in vitro assays to assess the impact of mutations on endonucleolytic activity.

Main Results:

• Demonstrated that human nonsense mRNA degradation can be initiated by endonucleolytic cleavage near the PTC.
• Identified SMG6 as the endonuclease responsible for this PTC-proximal cleavage.
• Showed that mutations in SMG6's PIN motif abolish its endonucleolytic activity both in vivo and in vitro.

Conclusions:

• Human nonsense mRNA degradation can be initiated by endonucleolysis, challenging the prevailing exonucleolytic model.
• SMG6 acts as the endonuclease mediating this cleavage event in human NMD.
• Endonucleolytic cleavage represents a conserved mechanism in metazoan NMD, suggesting a unified model for this pathway across diverse species.