Regulator of actin cytoskeleton dynamics underlying cell motility and adhesion. Functions as a component of the WAVE complex, which activates actin nucleating machinery Arp2/3 to drive lamellipodia formation (By similarity). Acts as regulator and substrate of nonreceptor tyrosine kinases ABL1 and ABL2 involved in processes linked to cell growth and differentiation. Positively regulates ABL1-mediated phosphorylation of ENAH, which is required for proper polymerization of nucleated actin filaments at the leading edge (By similarity). Contributes to the regulation of actin assembly at the tips of neuron projections. In particular, controls dendritic spine morphogenesis and may promote dendritic spine specification toward large mushroom-type spines known as repositories of memory in the brain (PubMed:15572692). In hippocampal neurons, may mediate actin-dependent BDNF-NTRK2 early endocytic trafficking that triggers dendrite outgrowth (PubMed:27605705). Participates in ocular lens morphogenesis, likely by regulating lamellipodia-driven adherens junction formation at the epithelial cell-secondary lens fiber interface (PubMed:15572692). Also required for nascent adherens junction assembly in epithelial cells (By similarity). Component of the WAVE complex composed of ABI2, CYFIP1 or CYFIP2, BRK1, NCKAP1 and WASF1/WAVE1. Within the complex, a heterodimer containing NCKAP1 and CYFIP1 interacts with a heterotrimer formed by WASF1/WAVE1, ABI2 and BRK1. CYFIP2 binds to activated RAC1 which causes the complex to dissociate, releasing activated WASF1 (By similarity). Interacts (via SH3 domain) with ABL1 and ABL2 (By similarity). Localized to the protruding lamellipodia and filopodia tips (By similarity). Present at nascent adherens junctions where it clusters adjacent to the tips of F-actin protrusions (By similarity). Expresses in embryonic and adult brain. In adult brain prominently expressed in the neocortex, hippocampus and dentate gyrus. Detected at 10 dpc in developing brain, and expression is more prominent in the neuroepithelium compared to the surrounding tissue. At 12 dpc expression is enhanced throughout the CNS and is detected along the full length of the spinal chord. At 16 dpc expression remains enhanced in the CNS, and is particularly prominent in the olfactory bulb (PubMed:10995551). Also highly expressed in dorsal root ganglia (PubMed:10995551). At 18.5 dpc is prominently expressed in the marginal zone of the cortex, an area rich in neuronal and glial cell projections (at protein level) (PubMed:15572692). Detected at 16.5 dpc in ocular lens, specifically localized where the tips of migrating secondary fibers forms adherens junctions with anterior epithelial cells (at protein level) (PubMed:15572692). The SH3 domain is critical for binding to ABL1 and ABL2. Phosphorylated by ABL1. Knockout mice are born at the expected Mendelian rate. Mutant mice have grossly distorted lenses due to defective orientation and migration of secondary lens fibers associated with impaired anterior and posterior sutures formation. Mice exhibit neuroanatomical abnormalities characterized by defective dendritic spine morphology and misoriented cortical and hippocampal neurons, causing profound deficits in learning and memory. Belongs to the ABI family.