Plays a role in nonsense-mediated mRNA decay. Recruits UPF1 to cytoplasmic mRNA decay bodies. Together with SMG5 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. Part of a complex that contains SMG5, SMG7, PPP2CA, a short isoform of UPF3A (isoform UPF3AS, but not isoform UPF3AL) and phosphorylated UPF1 (PubMed:14636577). Interacts with DHX34; the interaction is RNA-independent (PubMed:25220460). Predominantly cytoplasmic, and nuclear. Shuttles between nucleus and cytoplasm. Extended N-terminus.