Involved in regulation of actin and microtubule organization. Part of a WAVE complex that activates the Arp2/3 complex. As component of the WAVE1 complex, required for BDNF-NTRK2 endocytic trafficking and signaling from early endosomes (By similarity). Homotrimer when in free form. Directly interacts with WASF2. Component of the WAVE1 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 WAVE1, ABI2 and BRK1. Belongs to the BRK1 family. Extended N-terminus.