Plays a role in nonsense-mediated mRNA decay. Does not have RNase activity by itself. Promotes dephosphorylation of UPF1. Together with SMG7 is thought to provide a link to the mRNA degradation machinery involving exonucleolytic pathways, and to serve as an adapter for UPF1 to protein phosphatase 2A (PP2A), thereby triggering UPF1 dephosphorylation. Necessary for TERT activity (By similarity). Interacts with TERT, PPP2CA and SMG1. Part of a complex that contains SMG1, SMG5, SMG7, PPP2CA, a short isoform of UPF3A (isoform UPF3AS, but not isoform UPF3AL) and phosphorylated UPF1. Not detected in complexes that contain unphosphorylated UPF1. Detected in cytoplasmic mRNA decay bodies.