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 homology and show different structural features compared with most other G protein-coupled receptors (GPCRs). In particular, some isoforms of the phospholipase C (PLC)-coupled mGluRs (mGluR1a, mGluR5a, and mGluR5b) have a surprisingly long carboxyl-terminal intracellular domain of more than 350 residues, whereas the splice variants mGluR1b and mGluR1c have a much shorter carboxyl terminus. In the current study, the different splice variants of mGluR1 were expressed in porcine kidney epithelial (LLC-PK1) or the human embryonic kidney (HEK 293) cells, and their levels of expression were examined with the use of Western blot analysis. Expression of the short isoforms mGluR1b and mGluR1c did not modify the basal inositol phosphate production. In contrast, expression to similar levels of mGluR1a resulted in a 2-fold increase in the basal inositol phosphate formation. This increase in basal PLC activity was due to neither the presence of a low concentration of glutamate in the incubation medium nor a modification of the PLC pathway, resulting, for 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 these molecules act as inverse agonists. Taken together, these results indicate that the long carboxyl-terminal domain confers a small agonist-independent activity to mGluR1. This indicates that, as already observed for other GPCRs, little constitutive activity of wild-type mGluRs can be detected. Our results also add to the splice variants and further suggest that the long carboxyl-terminal domain of mGluR1a confers better coupling efficiency to the G proteins.