Transcription coregulator that can have both coactivator and corepressor functions (PubMed:15031292, PubMed:18468999, PubMed:18922798, PubMed:25342469, PubMed:33938178). Adapter protein that forms a transcriptionally active complex with the gamma-secretase-derived amyloid precursor protein (APP) intracellular domain (PubMed:15031292, PubMed:18468999, PubMed:18922798, PubMed:25342469). Plays a central role in the response to DNA damage by translocating to the nucleus and inducing apoptosis (PubMed:15031292, PubMed:18468999, PubMed:18922798, PubMed:25342469). May act by specifically recognizing and binding histone H2AX phosphorylated on 'Tyr-142' (H2AXY142ph) at double-strand breaks (DSBs), recruiting other pro-apoptosis factors such as MAPK8/JNK1 (PubMed:19234442). Required for histone H4 acetylation at double-strand breaks (DSBs) (PubMed:19234442). Its ability to specifically bind modified histones and chromatin modifying enzymes such as KAT5/TIP60, probably explains its transcription activation activity (PubMed:33938178). Functions in association with TSHZ3, SET and HDAC factors as a transcriptional repressor, that inhibits the expression of CASP4 (PubMed:19343227). Associates with chromatin in a region surrounding the CASP4 transcriptional start site(s) (PubMed:19343227). Involved in hippocampal neurite branching and neuromuscular junction formation, as a result plays a role in spatial memory functioning (By similarity). Plays a role in the maintenance of lens transparency (By similarity). May play a role in muscle cell strength (By similarity). Component of a complex, at least composed of APBB1, RASD1/DEXRAS1 and APP (PubMed:18922798, PubMed:18833287, PubMed:18468999). Interacts (via PID domain 2) with APP (with the intracellular domain of the amyloid-beta precursor protein) (PubMed:18833287, PubMed:18468999). Interacts (via PID domain 2) with RASD1/DEXRAS1; impairs the transcription activation activity (PubMed:18922798). Interacts (via PID domain 1) with KAT5/TIP60 (PubMed:33938178). Interacts (via the WW domain) with the proline-rich region of APBB1IP (By similarity). Interacts with TSHZ1 and TSHZ2 (By similarity). Interacts (via the WW domain) with histone H2AX (when phosphorylated on 'Tyr-142') and the proline-rich region of ENAH (PubMed:17686488). Interacts with MAPK8 (PubMed:19234442). Interacts (via PID domain 1) with TSHZ3 (via homeobox domain) (PubMed:19343227). Interacts with SET (PubMed:19343227). Found in a trimeric complex with HDAC1 and TSHZ3; the interaction between HDAC1 and APBB1 is mediated by TSHZ3 (PubMed:19343227). Interacts (via WWW domain) with NEK6 (PubMed:17512906). Interacts (via WWW domain) with ABL1 (PubMed:15031292). Interacts with RNF157 (PubMed:25342469). Colocalizes with TSHZ3 in axonal growth cone (By similarity). Colocalizes with TSHZ3 in the nucleus (PubMed:19343227). In normal conditions, it mainly localizes to the cytoplasm, while a small fraction is tethered to the cell membrane via its interaction with APP (PubMed:18468999). Following exposure to DNA damaging agents, it is released from cell membrane and translocates to the nucleus (PubMed:18468999). Nuclear translocation is under the regulation of APP (PubMed:18468999). Colocalizes with NEK6 at the nuclear speckles (PubMed:17512906). Phosphorylation at Ser-610 by SGK1 promotes its localization to the nucleus (By similarity). Highly expressed in brain; strongly reduced in post-mortem elderly subjects with Alzheimer disease. Expressed preferentially in the brain. Phosphorylation at Ser-610 by SGK1 promotes its localization to the nucleus (By similarity). Phosphorylated following nuclear translocation (PubMed:15031292, PubMed:18922798). Phosphorylation at Tyr-547 by ABL1 enhances transcriptional activation activity and reduces the affinity for RASD1/DEXRAS1 (PubMed:15031292, PubMed:18922798). Acetylation at Lys-204 and Lys-701 by KAT5 promotes its transcription activator activity. Polyubiquitination by RNF157 leads to degradation by the proteasome (PubMed:25342469). Extended N-terminus.