Core component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junctions on mRNAs (PubMed:14973490, PubMed:24967911). Involved in exon definition of genes containing long introns, including the rolled/MAPK gene (PubMed:20946982, PubMed:20946983). The mago-tsu heterodimer interacts with the EJC key regulator Pym leading to EJC disassembly in the cytoplasm (PubMed:24967911). Has a role in oskar mRNA localization to the posterior pole of the developing oocyte, and may also be involved in polarization of the oocyte microtubule cytoskeleton (PubMed:8026338, PubMed:9272960). Heterodimer with tsu/RBM8A (PubMed:12704080, PubMed:12730685, PubMed:14968132). Part of the mRNA splicing-dependent exon junction complex (EJC) complex; the core complex contains btz/CASC3, eIF4AIII, mago and tsu/RBM8A (PubMed:14973490). Interacts with Pym (via N-terminus); the interaction is direct (PubMed:24967911, PubMed:14968132). Interacts with eIF4AIII (PubMed:14973490). Part of the EJC assembled on mRNAs in the nucleus and remains part of the complex when mRNA moves into the cytoplasm. Expressed both maternally and zygotically (PubMed:8026338). Localizes to the posterior pole of the oocyte during stage 9 of oogenesis (PubMed:14973490). 'Mago nashi' means 'without grandchildren' in Japanese. Belongs to the mago nashi family.