Heterodimerization of a functional GABA(B) receptor is mediated by parallel coiled-coil α-helices

A detailed understanding of GABA(B) receptor assembly is an important issue in view of its role as attractive target for treatment of epilepsy, anxiety, depression, cognitive defects, and nociceptive disorders. Heteromerization of GABA(B)-R1 and GABA(B)-R2 subunits is a prerequisite for the formation of a functional GABA(B) receptor. Each individual subunit contains one stretch of ~30 amino acid residues within its intracellular C- terminal domain that mediates heteromer formation. To investigate the mechanism of the GABA(B)-R1/GABA(B)-R2 interaction and to assess the subunit stoichiometry of the complex, recombinant polypeptide chain fragments containing the heteromerization site were produced by heterologous gene expression in Escherichia coli. When mixed in equimolar amounts, these peptides preferentially formed parallel coiled-coil heterodimers under physiological buffer conditions. This demonstrates that the short C-terminal regions are sufficient to determine the specificity of interaction between GABA(B) receptor subunits. In contrast, isolated GABA(B)-R1 peptides folded into relatively unstable homodimers, whereas GABA(B)-R2 peptides were largely unstructured. Together with the data reported in the literature, the results presented here indicate that the functional GABA(B) receptor is a heterodimer assembled by parallel coiled-coil α-helices.