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 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; ACIN1 confers RNA-binding to the complex. The ASAP complex can inhibit RNA 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 formation of proapoptotic isoforms such as Bcl-X(S); the activity is different from the established EJC assembly and function. Induces apoptotic chromatin condensation after activation by CASP3. Regulates cyclin A1, but not cyclin A2, expression in leukemia cells. Found in a mRNA splicing-dependent exon junction complex (EJC). Component of the heterotrimeric ASAP (apoptosis- and splicing-associated protein) complexes consisting of RNPS1, SAP18 and different isoforms of ACIN1; the association of SAP18 seems to require a preformed RNPS1:ACIN1 complex. Interacts with API5. Interacts with SRPK2 in a phosphorylation-dependent manner. Phosphorylation on Ser-1180 by SRPK2 redistributes it from the nuclear speckles to the nucleoplasm. Ubiquitous. The Ser-1180 phosphorylated form (by SRPK2) is highly expressed and phosphorylated in patients with myeloid hematologic malignancies. Phosphorylation on Ser-1180 by SRPK2 up-regulates its stimulatory effect on cyclin A1. Undergoes proteolytic cleavage; the processed form is active, contrary to the uncleaved form. Structural and functional studies of the ASAP complex have been conducted with a chimeric complex involving a conserved fragment of Drosophila melanogaster Acinus/hkl.