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. Homopentamer; forms an open pentamer (By similarity). In contrast to other BTB domain-containing proteins, does not interact with CUL3 (By similarity). Interacts as a tetramer with GABRB1 and GABRB2 (PubMed:20400944). Colocalizes with GABRB1. Expressed in the brain, mainly in the hippocampus.