ATP-dependent RNA helicase (PubMed:22961380). Core component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junctions on mRNAs (PubMed:14973490, PubMed:22961380). Involved in exon definition of genes containing long introns, including the rolled/MAPK gene (PubMed:20946982, PubMed:20946983). Has a role in oskar mRNA localization at the posterior pole of the developing oocyte (PubMed:14973490). ATP + H2O = ADP + H(+) + phosphate Part of the mRNA splicing-dependent exon junction complex (EJC) complex; the core complex contains btz/CASC3, eIF4AIII, mago and tsu/RBM8A (PubMed:14973490, PubMed:22961380). Interacts with btz/CASC3 and mago (PubMed:14973490). Interacts with ncm/CWC22 (PubMed:22961380). Localizes to the posterior pole of the oocyte during stages 1-9 of oogenesis, where it colocalizes with mago. Belongs to the DEAD box helicase family.