Rapid Deadenylation Triggered by a Nonsense Codon Precedes Decay of the RNA Body in a Mammalian Cytoplasmic Nonsense-Mediated Decay Pathway

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Abstract
Nonsense-mediated mRNA decay (NMD) is an RNA surveillance pathway that detects and destroys aberrant mRNAs containing nonsense or premature termination codons (PTCs) in a translation-dependent manner in eukaryotes. In yeast, the NMD pathway bypasses the deadenylation step and directly targets PTC-containing messages for decapping, followed by 5′-to-3′ exonuclease digestion of the RNA body. In mammals, most PTC-containing mRNAs are subject to active nucleus-associated NMD. Here, using two distinct transcription-pulsing approaches to monitor mRNA deadenylation and decay kinetics, we demonstrate the existence of an active cytoplasmic NMD pathway in mammalian cells. In this pathway, a nonsense codon triggers accelerated deadenylation that precedes decay of the PTC-containing mRNA body. Transcript is stabilized when accelerated deadenylation is impeded by blocking translation initiation; by ectopically expressing two RNA-binding proteins, UNR and NSAP1; or by ectopically expressing a UPF1 dominant-negative mutant. These results are consistent with the notion that the nonsense codon can function in the cytoplasm by promoting rapid removal of the poly(A) tail as a necessary first step in the decay process.