Drebrins (developmentally regulated brain proteins) are cytoplasmic proteins that bind F-actin in the brain and are involved in cell migration, extension of neuronal processes and plasticity of dendrites. HIP-55 (HPK1-interacting protein of 55 kDa), also known as ABP1, SH3P7 or DBNL (drebrin-like), is a 430 amino acid cytoplasmic protein that belongs to the ABP1 family. HIP-55 binds to F-actin but is not involved in actin polymerization, capping or bundling. In addition to containing an ADF-H domain, HIP-55 also consists of a SH3 domain, which mediates interaction with SHANK2, SHANK3 and PRAM-1. HIP-55 acts as an actin-binding adapter protein and as a common effector of antigen receptor-signaling pathways in leukocytes. As a key component of the immunological synapse, HIP-55 regulates T-cell activation by bridging TCRs and the actin cytoskeleton to gene activation and endocytic processes. HIP-55 is degraded by caspases during apoptosis.