Substrate-specific adapter of a BCR (BTB-CUL3-RBX1) E3 ubiquitin-protein ligase complex, which mediates the ubiquitination of target proteins, leading to their degradation by the proteasome. Protein modification; protein ubiquitination. Forms pentamers. Component of a complex composed of 5 subunits of KCTD9 and 5 CUL3.