The cryo-EM structure of the UPF-EJC complex shows UPF1 poised toward the RNA 3' end.

Nonsense-mediated mRNA decay (NMD) is a eukaryotic surveillance pathway that degrades aberrant mRNAs containing premature termination codons (PTCs). NMD is triggered upon the assembly of the UPF surveillance complex near a PTC. In humans, UPF assembly is prompted by the exon junction complex (EJC). We investigated the molecular architecture of the human UPF complex bound to the EJC by cryo-EM and using positional restraints from additional EM, MS and biochemical interaction data. The heptameric assembly is built around UPF2, a scaffold protein with a ring structure that closes around the CH domain of UPF1, keeping the helicase region in an accessible and unwinding-competent state. UPF2 also positions UPF3 to interact with the EJC. The geometry is such that this transient complex poises UPF1 to elicit helicase activity toward the 3' end of the mRNP.