Characterization of the dimerization of metabotropic glutamate receptors using an N-terminal truncation of mGluR1α

The metabotropic glutamate receptor mGluR1 alpha in membranes isolated both from rat brain and from cell lines transfected with cDNA coding for the receptor migrates as a disulphide-bonded dimer on sodium dodecyl sulphate-polyacrylamide gels. Dimerization of mGluR1 alpha takes place in the endoplasmic reticulum because it is not prevented by exposing transfected human embryonic kidney (HEK) 293 cells to the drug brefeldin A, a drug that prevents egress of proteins from the endoplasmic reticulum, Dimerization was also not dependent on protein glycosylation as it was not prevented by treatment of the cells with tunicamycin. Using a mammalian expression vector containing the N-terminal domain of mGluR1 alpha, truncated just before the first transmembrane domain (NT-mGluR1 alpha), we show that the N-terminal domain is secreted as a soluble disulphide-bonded dimeric protein. In addition, the truncated N-terminal domain can form heterodimers with mGluR1 alpha when both proteins are cotransfected into HEK 293 cells. However, mGluR1 alpha and its splice variant mGluR1 beta did not form heterodimers in doubly transfected HEK 293 cells. These results show that although the N-terminal domain of mGluR1 alpha is sufficient for dimer formation, other domains in the molecule must regulate the process.