As part of the CUL3(KBTBD6/7) E3 ubiquitin ligase complex functions as a substrate adapter for the RAC1 guanine exchange factor (GEF) TIAM1, mediating its 'Lys-48' ubiquitination and proteasomal degradation (PubMed:25684205). By controlling this ubiquitination, regulates RAC1 signal transduction and downstream biological processes including the organization of the cytoskeleton, cell migration and cell proliferation (PubMed:25684205). Ubiquitination of TIAM1 requires the membrane-associated protein GABARAP which may restrict locally the activity of the complex (PubMed:25684205). Protein modification; protein ubiquitination. Core component of a BCR3 (BTB-CUL3-RBX1) E3 ubiquitin ligase complex, also named Cul3-RING ubiquitin ligase complex CUL3(KBTBD6/7), composed of CUL3, RBX1, KBTBD6 and KBTBD7 (PubMed:25684205). Interacts with GABARAP; the interaction is direct and is required for the ubiquitination of TIAM1 (PubMed:25684205). Interacts with GABARAPL1, GABARAPL2 and MAP1LC3B; the interaction is direct (PubMed:25684205). The ATG8 interaction motif (AIM) mediates interaction with proteins of the ATG8 family including GABARAP. The BTB domain is required for interaction with CUL3. The Kelch repeats mediate interaction with TIAM1, a CUL3(KBTBD6/7) E3 ubiquitin ligase substrate.