Function : 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 (PubMed:11691839).