Mapping of calmodulin and Gβγ binding domains within the C-terminal region of the metabotropic glutamate receptor 7A

Ca2+/calmodulin (Ca2+/CaM) and the beta gamma subunits of heterotrimeric G-proteins (G beta gamma) have recently been shown to interact in a mutually exclusive fashion with the intracellular C terminus of the presynaptic metabotropic glutamate receptor 7 (mGluR 7). Here, we further characterized the core CaM and G beta gamma binding sequences. In contrast to a previous report, we find that the CaM binding motif localized in the N-terminal region of the cytoplasmic tail domain of mGluR 7 is conserved in the related group III mGluRs 4A and 8 and allows these receptors to also bind Ca2+/CaM. Mutational analysis of the Ca2+/CaM binding motif is consistent with group III receptors containing a conventional CaM binding site formed by an amphipathic alpha -helix. Substitutions adjacent to the core CaM target sequence selectively prevent G beta gamma binding, suggesting that the CaM-dependent regulation of signal transduction involves determinants that overlap with but are different from those mediating G beta gamma recruitment. In addition, we present evidence that G beta gamma uses distinct nonoverlapping interfaces for interaction with the mGluR 7 C-terminal tail and the effector enzyme adenylyl cyclase II, respectively. Although G beta gamma -mediated signaling is abolished in receptors lacking the core CaM binding sequence, alpha subunit activation, as assayed by agonist-dependent GTP gammaS binding, was not affected. This suggests that Ca2+/CaM may alter the mode of group III mGluR signaling from mono- (a) to bidirectional (alpha and beta gamma) activation of downstream effector cascades.