Characterization of the Dimerization of Metabotropic Glutamate Receptors Using an N‐Terminal Truncation of mGluR1α

Abstract

The metabotropic glutamate receptor mGluR1α 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 mGluR1α 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 mGluR1α, truncated just before the first transmembrane domain (NT-mGluR1α), 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 mGluR1α when both proteins are cotransfected into HEK 293 cells. However, mGluR1α and its splice variant mGluR1β did not form heterodimers in doubly transfected HEK 293 cells. These results show that although the N-terminal domain of mGluR1α is sufficient for dimer formation, other domains in the molecule must regulate the process.