C-terminal deletion of metabotropic glutamate receptor 1 selectively abolishes coupling to Galphaq.

Recent studies indicate that the intracellular C-terminus of Group I metabotropic glutamate receptors (mGlu(1) and mGlu(5) receptor) is important in G protein coupling. To determine the necessity of the C-tail, a deletion mutant of mGlu(1) receptor was constructed, which included the first 840 amino acids of the rat mGlu(1a) receptor (mGlu(1)-dCT). G protein coupling of the receptors was assessed by measuring glutamate mediated inhibition of native calcium currents when each receptor was expressed in isolated sympathetic neurons from the rat superior cervical ganglion. Wild type mGlu(1) receptor activates both the Galpha(i/o) and Galpha(q/11) protein families. Each pathway can be detected in superior cervical ganglion neurons as voltage dependent and voltage independent inhibition of the calcium currents, respectively. While wild type mGlu(1) receptor gave rise to a strong, mixed voltage dependent and independent calcium current inhibition, mGlu(1)-dCT exhibited a weaker inhibition that was strongly voltage dependent, indicating activation of Galpha(i/o) was predominant. Further, pertussis toxin treatment reduced the inhibition by wild type mGlu(1) receptor to a smaller, voltage independent inhibition as expected, but completely abolished signaling through mGlu(1)-dCT. Finally, to test whether mGlu(1)-dCT could produce any activation of Galpha(q/11), inhibition of the native superior cervical ganglion M-type potassium currents was examined. M-channels, inhibited by PIP(2) depletion, were strongly inhibited by glutamate in cells expressing wild type mGlu(1) receptor, but no inhibition was detectable in neurons expressing mGlu(1)-dCT. These data indicate that C-terminal deletion of mGlu(1) receptor selectively abolishes Galpha(q/11) coupling.