The heteromeric GABA-B receptor recognizes G-protein α subunit C-termini

Abstract

The recently cloned GABA-B receptors are related to the metabotropic glutamate receptors (mGlu receptors), the Ca 2+-sensing receptor and one group of vomeronasal receptors. The GABA-B receptors likely function in a heterodimeric form, constituted of GABA-BR1 and GABA-BR2. This novel feature in the G-protein coupled receptors (GPCRs) structure raises questions as to the mechanism of recognition of G-proteins by such receptors. In the present study we show that the GABA-BR1 and BR2 subunits form a functional receptor that recognizes the extreme C-termini of the Gαi and Gαo proteins when expressed in HEK293 cells. Indeed, heteromeric GABA-BR1/BR2 receptors do not activate PLC when co-expressed with Gαq, but do so when co-expressed with the chimeric Gαqi5 or Gαqo5 subunits, the Gαq subunit in which the 5 C-terminal residues are those of Gαi or Gαo, respectively. Interestingly, the heteromeric GABA-B receptor did not activate the chimeric Gαqz5 subunit that contains the 5 C-terminal residues of Gαz. Among the three residues that are distinct between Gαqo5 and Gαqz5 (at position −5, −4 and −1), the amino acid residue at position −4 of Gαo proteins is critical for specifying the coupling selectivity with the receptor and residue −5 influences the coupling efficacy. Interestingly, these findings correspond to data obtained with the mGluR2 receptor, a distant relative of GABA-B proteins. This shows that the same molecular determinants of the G-protein α-subunits are involved in the specific recognition of both the heteromeric GABA-B receptors and the other GPCRs.