Core component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junctions on mRNAs. Involved in exon definition of genes containing long introns, including the rolled/MAPK gene. The mago-tsu heterodimer interacts with the EJC key regulator Pym leading to EJC disassembly in the cytoplasm. 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