CHANGES IN THE CARBOXYL-TERMINAL DOMAIN OF METABOTROPIC GLUTAMATE-RECEPTOR-1 BY ALTERNATIVE SPLICING GENERATE RECEPTORS WITH DIFFERING AGONIST-INDEPENDENT ACTIVITY

The metabotropic glutamate receptors (mGluRs) share no sequence homolo gy and show different structural features compared with most other G p rotein-coupled receptors (GPCRs). In particular, some isoforms of the phospholipase C (PLC)-coupled mGluRs (mGluR1a, mGluR5a, and mGluR5b) h ave a surprisingly long carboxyl-terminal intracellular domain of more than 350 residues, whereas the splice variants mGluR1b and mGluR1c ha ve a much shorter carboxyl terminus. In the current study, the differe nt splice variants of mGluR1 were expressed in porcine kidney epitheli al (LLC-PK1) or the human embryonic kidney (HEK 293) cells, and their levels of expression were examined with the use of Western blot analys is. Expression of the short isoforms mGluR1b and mGluR1c did not modif y the basal inositol phosphate production. In contrast, expression to similar levels of mGluR1a resulted in a 2-fold increase in the basal i nositol phosphate formation. This increase in basal PLC activity was d ue to neither the presence of a low concentration of glutamate in the incubation medium nor a modification of the PLC pathway, resulting, fo r example, from the constant activation of mGluR1a by glutamate during the culture. Surprisingly, none of the known competitive antagonists of mGluR1 inhibited the basal PLC activity, indicating that none of th ese molecules act as inverse agonists. Taken together, these results i ndicate that the long carboxyl-terminal domain confers a small agonist -independent activity to mGluR1. This indicates that, as already obser ved for other GPCRs, little constitutive activity of wildtype mGluRs c an be detected. Our results also add to the functional differences alr eady observed among the mGluR1 splice variants and further suggest tha t the long carboxyl-terminal domain of mGluR1a confers better coupling efficiency to the G proteins.