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). Heterodimer (via RRM domain) with mago (PubMed:11691839, PubMed:12730685, PubMed:12704080). 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 throughout development. Localizes to the posterior pole of the oocyte during stages 1-9 of oogenesis. The RNA recognition motif (RRM) is involved in the interaction with mago. The RNA-binding activity of such domains is therefore uncertain. Developing photoreceptor cell clusters express reduced levels of rolled/MAPK and fail to differentiate normally. 'Tsunagi' means 'connection' or 'link' in Japanese. Belongs to the RBM8A family.