SMG1:SMG8:SMG9-complex integrity supports efficient execution of nonsense-mediated mRNA decay

Area of Science:

• Molecular Biology
• Cell Biology
• Genetics

Background:

• Nonsense-mediated mRNA decay (NMD) is a crucial post-transcriptional regulatory pathway.
• NMD degrades aberrant mRNAs with premature termination codons, preventing cellular damage.
• SMG1 kinase phosphorylates UPF1 to activate NMD, with SMG8 and SMG9 implicated as regulators.

Purpose of the Study:

• To systematically investigate the roles of SMG8 and SMG9 in human NMD.
• To determine the in vivo significance of the SMG8 kinase inhibitory domain (KID).
• To assess the impact of SMG8 and SMG9 loss on NMD efficiency and cellular response to SMG1 inhibition.

Main Methods:

• Genetic manipulation (gene deletion) in human cell lines.
• Pharmacological inhibition of the SMG1 kinase.
• Analysis of UPF1 phosphorylation levels.
• Transcriptome-wide assessment of NMD target mRNA stability.

Main Results:

• The SMG8 KID is dispensable for NMD in vivo.
• Complete loss of SMG8 or SMG9 caused only modest NMD impairment and moderate UPF1 hyperphosphorylation.
• SMG8- or SMG9-deficient cells showed hypersensitivity to partial SMG1 inhibition, leading to synergistic NMD target stabilization.

Conclusions:

• SMG8 and SMG9 are not essential for NMD but act as nonessential modulators.
• These proteins safeguard NMD pathway efficiency and its tolerance to perturbations.
• SMG8 and SMG9 play a general regulatory role in NMD across different cellular contexts.