Dissecting the functions of SMG5, SMG7, and PNRC2 in nonsense-mediated mRNA decay of human cells

Jan 20, 2018RNA (New York, N.Y.)

Roles of SMG5, SMG7, and PNRC2 in the process that removes faulty mRNA in human cells

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Abstract

Nonsense-mediated mRNA decay (NMD) affects 3%-10% of all mRNAs.

NMD involves the degradation of mRNAs with premature translation-termination codons (PTCs).
The degradation processes include both exonucleolytic and endonucleolytic pathways in mammalian cells.
An interaction between SMG5 and SMG7 was confirmed as necessary for NMD.
No evidence was found for a physical or functional interaction between SMG5 and PNRC2.
UPF1 interacts with PNRC2, which triggers 5'-3' exonucleolytic decay in specific assays.
PNRC2 is likely an important mRNA decapping factor but does not seem required for NMD.

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The term "nonsense-mediated mRNA decay" (NMD) originally described the degradation of mRNAs with premature translation-termination codons (PTCs), but its meaning has recently been extended to be a translation-dependent post-transcriptional regulator of gene expression affecting 3%-10% of all mRNAs. The degradation of NMD target mRNAs involves both exonucleolytic and endonucleolytic pathways in mammalian cells. While the latter is mediated by the endonuclease SMG6, the former pathway has been reported to require a complex of SMG5-SMG7 or SMG5-PNRC2 binding to UPF1. However, the existence, dominance, and mechanistic details of these exonucleolytic pathways are divisive. Therefore, we have investigated the possible exonucleolytic modes of mRNA decay in NMD by examining the roles of UPF1, SMG5, SMG7, and PNRC2 using a combination of functional assays and interaction mapping. Confirming previous work, we detected an interaction between SMG5 and SMG7 and also a functional need for this complex in NMD. In contrast, we found no evidence for the existence of a physical or functional interaction between SMG5 and PNRC2. Instead, we show that UPF1 interacts with PNRC2 and that it triggers 5'-3' exonucleolytic decay of reporter transcripts in tethering assays. PNRC2 interacts mainly with decapping factors and its knockdown does not affect the RNA levels of NMD reporters. We conclude that PNRC2 is probably an important mRNA decapping factor but that it does not appear to be required for NMD.

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Dissecting the functions of SMG5, SMG7, and PNRC2 in nonsense-mediated mRNA decay of human cells

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