SMG5-SMG7 authorize nonsense-mediated mRNA decay by enabling SMG6 endonucleolytic activity

Area of Science:

• Molecular Biology
• Gene Regulation
• RNA Biology

Background:

• Nonsense-mediated mRNA decay (NMD) is a crucial pathway regulating eukaryotic gene expression by degrading aberrant mRNAs.
• NMD activation involves UPF1 phosphorylation and subsequent mRNA clearance through distinct degradation branches.
• The interplay between these branches and their regulation remained incompletely understood.

Purpose of the Study:

• To investigate the functional relationship between the SMG5-SMG7 and SMG6-dependent NMD degradation branches.
• To elucidate the roles of SMG5 and SMG7 in NMD pathway execution.
• To refine the mechanistic model of NMD.

Main Methods:

• Transcriptome-wide analyses (RNA sequencing) in cells depleted of SMG5-SMG7.
• Functional assays to assess NMD pathway activity and protein interactions.
• Investigating the impact of SMG5 and SMG7 depletion on SMG6-mediated mRNA cleavage.

Main Results:

• The loss of the SMG5-SMG7 pathway leads to the inactivation of the SMG6-dependent branch, demonstrating functional interdependence.
• SMG5 can functionally replace SMG7 in NMD activation, highlighting its significant role.
• Either SMG5 or SMG7 is sufficient to enable SMG6-mediated endonucleolysis of NMD targets.
• Depletion of SMG5-SMG7 resulted in massive transcriptomic alterations due to exhaustive NMD inhibition.

Conclusions:

• NMD execution involves a coordinated mechanism where UPF1 phosphorylation and SMG5-SMG7 recruitment are necessary to activate SMG6-mediated mRNA decay.
• This suggests a 'two-factor authentication' model for NMD pathway activation.
• The study reveals an unexpected functional connection between the final steps of NMD, refining our understanding of mRNA surveillance.