SMG7 acts as a molecular link between mRNA surveillance and mRNA decay.

Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that eliminates mRNAs containing premature termination codons (PTCs). The proteins UPF1, SMG5, SMG6, and SMG7 are essential NMD factors in metazoa. SMG5 and SMG7 form a complex with UPF1 and interact with each other via their N-terminal domains. Here we show that SMG5 and SMG7 colocalize in cytoplasmic mRNA decay bodies, while SMG6 forms separate cytoplasmic foci. When SMG7 is tethered to a reporter transcript, it elicits its degradation, bypassing the requirement for a PTC, UPF1, SMG5, or SMG6. This activity is mediated by the C-terminal domain of SMG7. In contrast, SMG5 requires SMG7 to trigger mRNA decay and to localize to decay bodies. Our findings indicate that SMG7 provides a link between the NMD and the mRNA degradation machinery by interacting with SMG5 and UPF1 via its N-terminal domain and targeting bound transcripts for decay via its C-terminal domain.