Required for pre-mRNA splicing as component of the spliceosome (PubMed:12226669, PubMed:11991638, PubMed:22961380, PubMed:28502770, PubMed:28076346, PubMed:29360106, PubMed:29301961). Promotes exon-junction complex (EJC) assembly (PubMed:22959432, PubMed:22961380). 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 (PubMed:11991638, PubMed:22961380, PubMed:28502770, PubMed:28076346, PubMed:29360106, PubMed:29301961). Interacts with EIF4A3 and PRPF19 in an RNA-independent manner. Direct interaction with EIF4A3 is mediated by the MIF4G domain (PubMed:24218557). 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. Contaminating sequence. Potential poly-A sequence. Contaminating sequence. Potential poly-A sequence. Contaminating sequence. Potential poly-A sequence. Extended N-terminus. Truncated N-terminus.