Received for publication June 28, 2004.
Revised September 24, 2004.
Accepted for publication October 1, 2004.

Activation of metabotropic glutamate receptor 1 dimers requires glutamate binding in both subunits

Abstract

Group I metabotropic glutamate receptors (mGluRs) form stable, disulfide-linked homodimers. Lack of a verifiably monomeric mGluR1 mutant has led to difficulty in assessing the role of dimerization in the molecular mechanism of mGluR1 activation. The related GABA_B receptor exhibits striking intradimer crosstalk (ligand binding at one subunit effectively produces G protein activation at the other), but it is unclear whether group I mGluRs exhibit analogous crosstalk. Signaling of heterologously expressed mGluR1 was examined in isolated rat sympathetic neurons by measuring glutamate-mediated inhibition of native calcium currents. To examine mGluR1 activity when only one dimer subunit has access to glutamate ligand, wild type mGluR1 was coexpressed with mGluR1 Y74A, a mutant with impaired glutamate binding, and the activity of the 'heterodimer' (mutant/wild type) was examined. The mGluR1 Y74A mutant alone had a dose response curve that was shifted by about 2 orders of magnitude. The half maximal dose of glutamate shifted from 1.3 µM (wild type mGluR1) to about 450 µM (mGluR1 Y74A). However, the maximal effect was similar. Wild type mGluR1 was expressed with excess Y74A mGluR1 to generate a receptor population consisting largely of mutant homodimers and mutant/wild type heterodimers but without detectable wild type homodimers. Under these conditions, no glutamate mediated calcium current inhibition was observed below ~300 µM glutamate, although wild type mGluR1 protein was detectable with immunofluorescence. These data suggest that mutant/wild type heterodimeric receptors are inactive at ligand concentrations favoring glutamate association with receptor dimers at only one subunit.

Key words: G protein, calcium channel, crosstalk, mGluR, rat, sympathetic neurons

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