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. In cooperation with UPF2 stimulates both ATPase and RNA helicase activities of UPF1. Binds spliced mRNA upstream of exon-exon junctions. In vitro, stimulates translation; the function is independent of association with UPF2 and components of the EJC core. Found in a post-splicing messenger ribonucleoprotein (mRNP) complex. Core component of the mRNA splicing-dependent exon junction complex (EJC); the core complex contains CASC3, EIF4A3, MAGOH or MAGOHB, and RBM8A. The EJC core components EIF4A3 and the MAGOH-RBM8A dimer form a composite binding site for UPF3B which overlaps with the EJC binding site for WIBG (PubMed:16601204, PubMed:18066079, PubMed:23917022, PubMed:20479275). Interacts with EST1A, UPF2 and RBM8A (PubMed:12554878, PubMed:18066079, PubMed:15004547). Interacts with CPSF6 (PubMed:19864460). Interacts with DHX34; the interaction is RNA-independent. Shuttling between the nucleus and the cytoplasm. Expressed in testis, uterus, prostate, heart, muscle, brain, spinal cord and placenta. The disease is caused by variants affecting the gene represented in this entry. Belongs to the RENT3 family.