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Metabotropic glutamate receptor (mGluR) type 7-mediated neurotransmission depends critically on its regulation by associated molecules, such as kinases, phosphatases and structural proteins. The splice variants mGluR7a and mGluR7b are defined by different intracellular C-termini, and simultaneous or exclusive binding of interacting proteins to these domains modulates mGluR7-mediated signalling. However, molecular determinants defining binding regions for associated proteins within mGluR7 C-termini are mostly unknown. In the present study, we have mapped the binding domains of four proteins [filamin A, protein phosphatase (PP) 1C, protein interacting with protein kinase C (PICK) 1 and syntenin] interacting with the mGluR7b variant, and show that the alternatively spliced distal part of the mGluR7b C-terminus was sufficient for the interactions. By individual substitution of all mGluR7b isoform-specific amino acids with alanine and construction of a series of deletion constructs, residues important for the interactions were identified and binding regions could be defined. Interestingly, mGluR7b contains an unusual PP1C-binding motif, located at the N-terminus of the binding domains for PICK1 and syntenin. Consistently, binding of PP1C and PICK1 or PP1C and syntenin to mGluR7b was not competitive. Furthermore, PICK1, but not PP1C, interacted physically with syntenin. Our results represent a molecular description of the binding mechanisms of four mGluR7-associated proteins, and indicate the formation of ternary protein complexes composed of mGluR7b, PP1C, PICK1 and syntenin.

The modulation of neurotransmitter receptors by kinases and phosphatases represents a key mechanism in controlling synaptic signal transduction. However, molecular determinants involved in the specific targeting and interactions of these enzymes are largely unknown. Here, we identified both catalytic ␥-isoforms of protein phosphatase 1C (PP1␥1 and PP1␥2) as binding partners of the group I metabotropic glutamate receptors type 1a, 5a, and 5b in yeast cells and pull-down assays, using recombinant and native protein preparations. The tissue distribution of interacting proteins was compared, and protein phosphatase 1C was detected in dendrites of retinal bipolar cells expressing the respective interacting glutamate receptors. We mapped interacting domains within binding partners and identified five amino acids in the intracellular C termini of the metabotropic glutamate receptors type 1a, 5a, 5b, and 7b being both necessary and sufficient to bind protein phosphatase 1C. Furthermore, we show a dose-dependent competition of these C termini in binding the enzyme. Based on our data, we investigated the structure of the identified amino acids bound to protein phosphatase 1C by homology-based molecular modeling. In summary, these results provide a molecular description of the interaction between protein phosphatase 1C and metabotropic glutamate receptors and thereby increase our understanding of glutamatergic signal transduction.The correct targeting and localization of proteins to synaptic specializations represents an important biological mechanism to regulate neuronal excitability. Increasing evidence underlines the importance of macromolecular signaling complexes, where functionally related proteins such as ion channels, neurotransmitters receptors, kinases, and phosphatases are arranged in close vicinity and are physically anchored to the synaptic cytoskeleton. Therefore, identification and characterization of interactions between synaptically localized proteins adds substantially to our understanding of molecular mechanisms of neurotransmission.Receptors for glutamate are divided in ion channel-associated (ionotropic) receptors of the AMPA-, Kainate-, and NMDAtype and in G-protein coupled (metabotropic) receptors. Although ionotropic glutamate receptors mediate fast synaptic transmission, metabotropic glutamate receptors (mGluRs) 1 modulate neuronal excitability and development, synaptic plasticity, transmitter release, and memory function using a variety of intracellular second messenger systems (1). The eight known members of this protein family are sub-divided into three groups, based on sequence homology, associated second messenger systems, and pharmacological properties (2). mGluR1 and mGluR5 (group I) stimulate phospholipase C, are selectively activated by quisqualate, and are generally expressed perisynaptically at postsynaptic sites (3-7). mGluR2 and mGluR3 (group II) are negatively coupled to adenylyl cyclase and do not show a specific preference for pre-or postsynaptic neurons (8 -10). mGluR4, mGluR7, and...

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