Heterodimerization is required for the formation of a functional GABA(B) receptor

GABA (gamma-aminobutyric acid) is the main inhibitory neurotransmitter in t he mammalian central nervous system, where it exerts its effects through io notropic (GABA(A/C)) receptors to produce fast synaptic inhibition and meta botropic (GABA(B)) receptors to produce slow prolonged inhibitory signals. The gene encoding a GABA(B) receptor (GABA(B)R1) has been cloned(1); howeve r, when expressed in mammalian cells this receptor is retained as an immatu re glycoprotein on intracellular membranes(2) and exhibits low affinity for agonists compared with the endogenous receptor on brain membranes. Here we report the cloning of a complementary DNA encoding a new subtype of the GA BA(B) receptor (GABA(B)R2), which we identified by mining expressed-sequenc e-tag databases. Yeast two-hybrid screening showed that this new GABA(B)R2- receptor subtype forms heterodimers with GABA(B)R1 through an interaction a t their intracellular carboxy-terminal tails. Upon expression with GABA(B)R 2 in HEK293T cells, GABA(B)R1 is terminally glycosylated and expressed at t he cell surface. Go-expression of the two receptors produces a fully functi onal GABA(B) receptor at the cell surface; this receptor binds GABA with a high affinity equivalent to that of the endogenous brain receptor. These re sults indicate that, in vivo, functional brain GABA(B) receptors may be het erodimers composed of GABA(B)R1 and GABA(B)R2.