Mago knockdown in Drosophila S2R+ cells
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.