Auxiliary subunit of GABA-B receptors that determine the pharmacology and kinetics of the receptor response. Increases agonist potency and markedly alter the G-protein signaling of the receptors by accelerating onset and promoting desensitization (By similarity). Homopentamer; forms an open pentamer (PubMed:28963344). In contrast to other BTB domain-containing proteins, does not interact with CUL3 (PubMed:28963344). Interacts as a tetramer with GABRB1 and GABRB2.