Involved in nonsense-mediated decay (NMD) of mRNAs containing premature stop codons by associating with the nuclear exon junction complex (EJC) and serving as link between the EJC core and NMD machinery. Recruits UPF2 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. However, UPF3A is shown to be only marginally active in NMD as compared to UPF3B. Binds spliced mRNA upstream of exon-exon junctions. In vitro, weakly stimulates translation. Found in a post-splicing messenger ribonucleoprotein (mRNP) complex. Associates with the exon junction complex (EJC). Interacts with UPF2 and RBM8A. Interacts with DHX34; the interaction is RNA-independent (PubMed:25220460). Shuttling between the nucleus and the cytoplasm. Isoform 1 is strongly expressed in testis, uterus, muscle, fetal brain and spinal cord. Isoform 2 is strongly expressed in fetal brain and spinal cord. Belongs to the RENT3 family.