Part of pre- and post-splicing multiprotein mRNP complexes. Auxiliary component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junction on mRNAs. The EJC is a dynamic structure consisting of core proteins and several peripheral nuclear and cytoplasmic associated factors that join the complex only transiently either during EJC assembly or during subsequent mRNA metabolism. Component of the ASAP and PSAP complexes which bind RNA in a sequence-independent manner and are proposed to be recruited to the EJC prior to or during the splicing process and to regulate specific excision of introns in specific transcription subsets. The ASAP complex can inhibit RNA processing during in vitro splicing reactions. The ASAP complex promotes apoptosis and is disassembled after induction of apoptosis. Enhances the formation of the ATP-dependent A complex of the spliceosome. Involved in both constitutive splicing and, in association with SRP54 and TRA2B/SFRS10, in distinctive modulation of alternative splicing in a substrate-dependent manner. Involved in the splicing modulation of BCL2L1/Bcl-X (and probably other apoptotic genes); specifically inhibits formation of proapoptotic isoforms such as Bcl-X(S); the activity is different from the established EJC assembly and function. Participates in mRNA 3'-end cleavage. Involved in UPF2-dependent nonsense-mediated decay (NMD) of mRNAs containing premature stop codons. Also mediates increase of mRNA abundance and translational efficiency. Binds spliced mRNA 20-25 nt upstream of exon-exon junctions. Found in mRNA splicing-dependent exon junction complexes (EJC). Found in a post-splicing complex with NXF1, RBM8A, UPF1, UPF2, UPF3A, UPF3B and RNPS1. Component of the heterotrimeric ASAP (apoptosis- and splicing-associated protein) and PSAP complexes consisting of RNPS1, SAP18 and either ACIN1 or PNN, respectively; the ASAP and PSAP complexes probably are formed mutually exclusive. Component of the active spliceosome. Associates with polysomes. Interacts with the cleaved p110 isoform of CDC2L1, CSNK2A1, PNN, SART3, SRP54, SRRM1 and TRA2B/SFRS10. Nucleocytoplasmic shuttling protein. Colocalizes with the core EJC, ALYREF/THOC4, NXF1 and UAP56 in the nucleus and nuclear speckles. Ubiquitous. The RRM domain is required for the formation of the ASAP complex. Phosphorylated on one or more of the four Ser/Thr residues (Ser-43, Thr-49, Ser-52 or Ser-53). Ser-53 phosphorylation site is important for splicing and translation stimulation activity in vitro. Belongs to the splicing factor SR family.