Required for pre-mRNA splicing as component of the spliceosome. Promotes exon-junction complex (EJC) assembly. Hinders EIF4A3 from non-specifically binding RNA and escorts it to the splicing machinery to promote EJC assembly on mature mRNAs. Through its role in EJC assembly, required for nonsense-mediated mRNA decay. Component of the pre-catalytic spliceosome B and the catalytic spliceosome C complexes. Interacts with EIF4A3 and PRPF19 in an RNA-independent manner. Direct interaction with EIF4A3 is mediated by the MIF4G domain. Full interaction with EIF4A3 occurs only when EIF4A3 is not part of the EJC and prevents EIF4A3 binding to RNA. Concentrates around speckles, which are sites of pre-mRNA synthesis and processing, where it colocalizes with EJC core proteins. Belongs to the CWC22 family.