May act in negative regulation of cell growth and transformation by interacting with nonreceptor tyrosine kinases ABL1 and/or ABL2. May play a role in regulation of EGF-induced Erk pathway activation. Involved in cytoskeletal reorganization and EGFR signaling. Together with EPS8 participates in transduction of signals from Ras to Rac. In vitro, a trimeric complex of ABI1, EPS8 and SOS1 exhibits Rac specific guanine nucleotide exchange factor (GEF) activity and ABI1 seems to act as an adapter in the complex. Regulates ABL1/c-Abl-mediated phosphorylation of ENAH. Recruits WASF1 to lamellipodia and there seems to regulate WASF1 protein level. In brain, seems to regulate the dendritic outgrowth and branching as well as to determine the shape and number of synaptic contacts of developing neurons. Interacts with ABL1, ENAH, STX1A, SNAP25, VAMP2, EPS8, and through its N-terminus with WASF1. Part of a complex consisting of ABI1, STX1A and SNAP25. Part of a complex consisting of ABI1, EPS8 and SOS1 (By similarity). Interacts with SOS1, SOS2, GRB2, SPTA1 and the first SH3 domain of NCK1. Isoform 6 does not interact with NCK1. Component of the WAVE2 complex composed of ABI1, CYFIP1/SRA1, NCKAP1/NAP1 (NCKAP1l/HEM1 in hematopoietic cells) and WASF2/WAVE2 (PubMed:16417406). Interacts (via SH3 domain) with SHANK2 and SHANK3, but not SHANK1; the interaction is direct. Interacts with the heterodimer MYC:MAX; the interaction may enhance MYC:MAX transcriptional activity. Interacts with FNBP1L (via the SH3 domain), WASF2, and CDC42, but only in the presence of FNBP1L (PubMed:19798448). (Microbial infection) Interacts with human cytomegalovirus/HHV-5 protein UL135. Localized to protruding lamellipodia and filopodia tips. Also localized to neuronal growth cones and synaptosomes. May shuttle from the postsynaptic densities to the nucleus (By similarity). Additional isoforms seem to exist. Widely expressed, with highest expression in brain. The t-SNARE coiled-coil homology domain is necessary and sufficient for interaction with STX1A. Phosphorylated on tyrosine residues after serum stimulation or induction by v-Abl. Seems to be phosphorylated at Tyr-53 by ABL1, required for nuclear but not for synaptic localization. A chromosomal aberration involving ABI1 is a cause of acute leukemias. Translocation t(10;11)(p11.2;q23) with KMT2A/MLL1. ABI1 isoform 2 was found to be present in acute leukemia KMT2A/MLL1-ABI1 fusion transcript. Belongs to the ABI family.