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). . This is the function of KCTD12 (potassium channel tetramerization domain containing 12, Ensembl gene identifier ENSG00000178695).