Exon junction complex components are specifically required to process the large MAP kinase transcript in Drosophila

he exon junction complex (EJC) is assembled on spliced mRNAs upstream of exon-exon junctions, and can regulate their subsequent translation, localization, or degradation. We isolated mutations in Drosophila mago nashi (mago), which encodes a core EJC subunit, based on their unexpectedly specific effects on photoreceptor differentiation. Loss of Mago prevents Epidermal growth factor receptor signaling, due to a large reduction in MAPK mRNA levels. MAPK expression also requires the EJC subunits Y14 and eIF4AIII, and EJC-associated splicing factors. Mago depletion does not affect MAPK transcription or mRNA stability, but leads to abnormal splicing. Mago is required for MAPK expression from an exogenous promoter only when the template contains introns. MAPK is the major target of mago in eye development; in cultured cells, Mago knockdown disproportionately affects other large genes located in heterochromatin. These data support a novel nuclear role for EJC components in splicing a specific subset of introns.