RESUMO
Ca(2+)/calmodulin (Ca(2+)/CaM) and the betagamma subunits of heterotrimeric G-proteins (Gbetagamma) 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 Gbetagamma 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 Ca(2+)/CaM. Mutational analysis of the Ca(2+)/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 Gbetagamma binding, suggesting that the CaM-dependent regulation of signal transduction involves determinants that overlap with but are different from those mediating Gbetagamma recruitment. In addition, we present evidence that Gbetagamma uses distinct nonoverlapping interfaces for interaction with the mGluR 7 C-terminal tail and the effector enzyme adenylyl cyclase II, respectively. Although Gbetagamma-mediated signaling is abolished in receptors lacking the core CaM binding sequence, alpha subunit activation, as assayed by agonist-dependent GTPgammaS binding, was not affected. This suggests that Ca(2+)/CaM may alter the mode of group III mGluR signaling from mono- (alpha) to bidirectional (alpha and betagamma) activation of downstream effector cascades.
Assuntos
Calmodulina/metabolismo , Proteínas Heterotriméricas de Ligação ao GTP/metabolismo , Receptores de Glutamato Metabotrópico/química , Sequência de Aminoácidos , Sítios de Ligação , Cálcio/metabolismo , Dados de Sequência MolecularRESUMO
Group III metabotropic glutamate receptors (mGluRs) serve as presynaptic receptors that mediate feedback inhibition of glutamate release via a Ca(2+)/calmodulin (CaM)-dependent mechanism. In vitro phosphorylation of mGluR7A by protein kinase C (PKC) prevents its interaction with Ca(2+)/CaM. In addition, activation of PKC leads to an inhibition of mGluR signaling. Here, we demonstrate that disrupting CaM binding to mGluR7A by PKC in vitro is due to phosphorylation of a highly conserved serine residue, S862. We propose charge neutralization of the CaM binding consensus sequence resulting from phosphorylation to constitute a general mechanism for the regulation of presynaptic mGluR signaling.
Assuntos
Calmodulina/metabolismo , Sequência Conservada , Proteína Quinase C/metabolismo , Receptores de Glutamato Metabotrópico/metabolismo , Serina/metabolismo , Sequência de Aminoácidos , Sítios de Ligação , Dados de Sequência Molecular , Fosforilação , Receptores de Glutamato Metabotrópico/genética , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismo , Serina/genéticaRESUMO
Group III metabotropic glutamate receptors (mGluRs) are highly enriched in the presynaptic terminals of glutamatergic synapses where they mediate feedback inhibition of neurotransmitter release. Here, we used the yeast two-hybrid system to identify a direct interaction of the C-terminal tail region of mGluR7 with the rat homologue of the protein kinase C substrate PICK1. This interaction is specifically mediated by the very C-terminal amino acids of the receptor and can be reconstituted in human embryonic kidney 293 cells by transfection of full-length mGluR7 and PICK1 cDNAs. Quantitative beta-galactosidase assays revealed that among the different group III mGluRs, mGluR7 is the major PICK1 binding partner although other subfamily members can also interact with PICK1. These data indicate that PDZ domain-containing proteins might contribute to the presynaptic localization of group III mGluRs.
Assuntos
Proteínas de Transporte/química , Proteínas de Transporte/metabolismo , Proteínas Nucleares/química , Proteínas Nucleares/metabolismo , Receptores de Glutamato Metabotrópico/química , Receptores de Glutamato Metabotrópico/metabolismo , Sequência de Aminoácidos , Animais , Sítios de Ligação , Caenorhabditis elegans , Proteínas de Ciclo Celular , Linhagem Celular , Clonagem Molecular , Proteínas do Citoesqueleto , Humanos , Rim , Camundongos , Dados de Sequência Molecular , Fragmentos de Peptídeos/química , Fragmentos de Peptídeos/metabolismo , Ratos , Proteínas Recombinantes de Fusão/metabolismo , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Alinhamento de Sequência , Homologia de Sequência de Aminoácidos , TransfecçãoRESUMO
Glutamatergic neurotransmission is controlled by presynaptic metabotropic glutamate receptors (mGluRs). A subdomain in the intracellular carboxyl-terminal tail of group III mGluRs binds calmodulin and heterotrimeric guanosine triphosphate-binding protein (G protein) betagamma subunits in a mutually exclusive manner. Mutations interfering with calmodulin binding and calmodulin antagonists inhibit G protein-mediated modulation of ionic currents by mGluR 7. Calmodulin antagonists also prevent inhibition of excitatory neurotransmission via presynaptic mGluRs. These results reveal a novel mechanism of presynaptic modulation in which Ca(2+)-calmodulin is required to release G protein betagamma subunits from the C-tail of group III mGluRs in order to mediate glutamatergic autoinhibition.