Involved in nonsense-mediated decay (NMD) of mRNAs containing premature stop codons by associating with the nuclear exon junction complex (EJC). Recruited by UPF3B associated with the EJC core at the cytoplasmic side of the nuclear envelope and the subsequent formation of an UPF1-UPF2-UPF3 surveillance complex (including UPF1 bound to release factors at the stalled ribosome) is believed to activate NMD. In cooperation with UPF3B stimulates both ATPase and RNA helicase activities of UPF1. Binds spliced mRNA. {ECO:0000269|PubMed:11163187, ECO:0000269|PubMed:16209946, ECO:0000269|PubMed:18066079}. This is the function of UPF2 (UPF2 regulator of nonsense mediated mRNA decay, Ensembl gene identifier ENSG00000151461).