The RNA-binding protein Y14 binds preferentially to mRNAs produced by splicing and is a component of a multiprotein complex that assembles approximately 20 nucleotides upstream of exon-exon junctions. This complex probably has important functions in post-splicing events including nuclear export and nonsense-mediated decay of mRNA. We show that Y14 binds to two previously reported components, Aly/REF and RNPS1, and to the mRNA export factor TAP. Moreover, we identified magoh, a human homolog of the Drosophila mago nashi gene product, as a novel component of the complex. Magoh binds avidly and directly to Y14 and TAP, but not to other known components of the complex, and is found in Y14-containing mRNPs in vivo. Importantly, magoh also binds to mRNAs produced by splicing upstream (approximately 20 nucleotides) of exon- exon junctions and its binding to mRNA persists after export. These experiments thus reveal specific protein-protein interactions among the proteins of the splicing-dependent mRNP complex and suggest an important role for the highly evolutionarily conserved magoh protein in this complex.