Mapping of calmodulin and G beta gamma 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 meta botropic glutamate receptor 7 (mGluR 7). Here, we further characterized the core CaM and G beta gamma binding sequences. In contrast to a previous rep ort, 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. Muta tional analysis of the Ca2+/CaM binding motif is consistent with group III receptors containing a conventional CaM binding site formed by an amphipath ic alpha -helix. Substitutions adjacent to the core CaM target sequence sel ectively prevent G beta gamma binding, suggesting that the CaM-dependent re gulation 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 interfac es for interaction with the mGluR 7 C-terminal tail and the effector enzyme adenylyl cyclase II, respectively. Although G beta gamma -mediated signali ng is abolished in receptors lacking the core CaM binding sequence, alpha s ubunit activation, as assayed by agonist-dependent GTP gammaS binding, was not affected. This suggests that Ca2+/CaM may alter the mode of group III m GluR signaling from mono- (a) to bidirectional (alpha and beta gamma) activ ation of downstream effector cascades.