CCIPE, UPR 9023, 141 rue de la Cardonille 34094 Montpellier cedex 05 France,
We cloned a Drosophila metabotropic glutamate receptor (mGluR) cDNA, that we called DmGluRA. This G-protein coupled receptor displays 45% amino acid sequence identity with the nearest mammalian mGluRs. Eight metabotropic receptors (mGluR1-8) have been cloned in mammals. Sequence comparisons have shown that these receptors have no homology with previously known G protein coupled receptors (GPCRs) and constitute with the recently cloned calcium-sensing receptors, a new family of GPCRs. We have determined a precise pharmacology (agonists and antagonists) of DmGluRA. Our results show that the pharmacological profile is very similar to that of group II mammalian mGluRs (mGluR2-3). This is remarkable knowing that the amino-acid identity of DmGluRA with mGluR2/3 in the supposed glutamate binding domain -i.e. the region homologous to bacterial periplasmic proteins - is only 45%. This means that the critical amino-acids that determine the pharmacophore of mGluR2/3 are conserved in DmGluRA. In order to study the structure-activity relationships of mGluRs, we have constructed chimeric receptors between DmGluRA and the mammalian receptor mGluR1. The chimeric receptor that possesses the extracellular domain of mGluR1, and the 7 transmembrane domain of DmGluRA, has the same pharmacology as mGluR1, and the G-protein selectivity of DmGluRA. The reciprocal chimeric receptor has the same pharmacology as DmGluRA, and the G-protein selectivity of mGluR1. These results clearly demonstrate that the extracellular domain and 7TMD of two distant members of this receptor family are interchangeable, the former being responsible for the ligand recognition, the latter for the G-protein activation. The activity of glutamate and DmGluRA agonists and antagonists on the behavior of larvae ( locomotion and feeding) are currently analysed.