Component of the SIN3-repressing complex. Enhances the ability of SIN3-HDAC1-mediated transcriptional repression. When tethered to the promoter, it can direct the formation of a repressive complex to core histone proteins. 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 mRNA processing during in vitro splicing reactions. The ASAP complex promotes apoptosis and is disassembled after induction of apoptosis. Involved in the splicing modulation of BCL2L1/Bcl-X (and probably other apoptotic genes); specifically inhibits the formation of proapoptotic isoforms such as Bcl-X(S); the activity is different from the established EJC assembly and function (By similarity). Found in a mRNA splicing-dependent exon junction complex (EJC). 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. For the ASAP complex, the association of SAP18 seems to require a preformed RNPS1:ACIN1 complex. Forms a complex with SIN3A and HDAC1. Interacts with SUFU. Shuttles between the nucleus and the cytoplasm (By similarity). Colocalizes with ACIN1 and SRSF2 in nuclear speckles (By similarity). Expressed in all tissues tested; highest levels in the brain, kidney and muscle; lowest levels in lung spleen, liver, intestine and testis, and moderate levels in salivary gland and heart. Belongs to the SAP18 family.