Required for pre-mRNA splicing as component of the spliceosome (PubMed:11991638). Plays a redundant role with MAGOHB as core component of the exon junction complex (EJC) and in the nonsense-mediated decay (NMD) pathway (PubMed:23917022). The EJC is a dynamic structure consisting of core proteins and several peripheral nuclear and cytoplasmic associated factors that join the complex only transiently either during EJC assembly or during subsequent mRNA metabolism. The EJC marks the position of the exon-exon junction in the mature mRNA for the gene expression machinery and the core components remain bound to spliced mRNAs throughout all stages of mRNA metabolism thereby influencing downstream processes including nuclear mRNA export, subcellular mRNA localization, translation efficiency and nonsense-mediated mRNA decay (NMD). The MAGOH-RBM8A heterodimer inhibits the ATPase activity of EIF4A3, thereby trapping the ATP-bound EJC core onto spliced mRNA in a stable conformation. The MAGOH-RBM8A heterodimer interacts with the EJC key regulator PYM1 leading to EJC disassembly in the cytoplasm and translation enhancement of EJC-bearing spliced mRNAs by recruiting them to the ribosomal 48S preinitiation complex. Involved in the splicing modulation of BCL2L1/Bcl-X (and probably other apoptotic genes); specifically inhibits formation of proapoptotic isoforms such as Bcl-X(S); the function is different from the established EJC assembly. Heterodimer with RBM8A (PubMed:12730685, PubMed:12781131, PubMed:23917022). Core component of the mRNA splicing-dependent exon junction complex (EJC); the core complex contains CASC3, EIF4A3, MAGOH or MAGOHB, and RBM8A (PubMed:11991638, PubMed:16170325, PubMed:16314458, PubMed:23917022, PubMed:16923391, PubMed:16931718, PubMed:19033377, PubMed:20479275). Interacts with PYM1; the interaction is direct and dissociates the EJC from spliced mRNAs (PubMed:14968132, PubMed:18026120, PubMed:19410547). Identified in a complex composed of the EJC core, UPF3B and UPF2. The EJC core can also interact with UPF3A (in vitro) (PubMed:20479275). Identified in the spliceosome C complex (PubMed:11991638). Detected in granule-like structures in the dendroplasm (By similarity). Travels to the cytoplasm as part of the exon junction complex (EJC) bound to mRNA. Colocalizes with the core EJC, ALYREF/THOC4, NXF1 and UAP56 in the nucleus and nuclear speckles (PubMed:19324961). Ubiquitous. Belongs to the mago nashi family.