Role of the Second and Third Intracellular Loops of Metabotropic Glutamate Receptors in Mediating Dual Signal Transduction Activation

Abstract

On the basis of sequence homology and structural similarities, metabotropic glutamate receptors (mGluRs), extracellular Ca 2؉ -sensing receptor, ␥-aminobutyric acid type B receptor, and pheromone receptors are enlisted in a distinct family within the larger G proteincoupled receptor superfamily. When expressed in heterologous systems, group I mGluRs can activate dual signal transduction pathways, phosphoinositides turnover and cAMP production. To investigate the structural basis of these coupling properties, we introduced single amino acid substitutions within the second and third intracellular loops (i2 and i3) of mGluR1␣. Metabotropic glutamate receptors (mGluRs)1 are coupled to heterotrimeric G proteins, and through this interaction they regulate the intracellular level of second messenger molecules such as inositol trisphosphate and cyclic AMP (cAMP). Furthermore, they have been shown to modulate the activity of voltage-sensitive Ca 2ϩ and K ϩ channels, G protein-regulated inward rectifier K ϩ channels, as well as GABA A , ␣-amino-3-hydroxy-5-methylisoxazole-4-propionate, and N-methyl-D-aspartate receptors (1-2). Thus far, eight mGluRs (mGluR1 through mGluR8; see Refs. 3-10) have been cloned. They have been categorized into three groups according to their sequence homology, agonist selectivity, and main signal transduction pathway activated in heterologous systems (1,(11)(12). Group I mGluRs (mGluR1 and -5) mobilize intracellular Ca 2ϩ by stimulating phosphoinositide (PI) turnover and promote cAMP accumulation; group II (mGluR2 and 3) and group III (mGluR4, -6 -8) receptors inhibit adenylyl cyclase. mGluRs, together with the parathyroid Ca 2ϩ -sensing receptors (PCaR1 (13)) and the GABA B receptor (14), form a separate family within the seven transmembrane domain G protein-coupled receptor (GPCR) superfamily (15-17). mGluRs are characterized by a large amino-terminal extracellular domain that comprises the glutamate binding site (18 -19). The intracellular loops connecting the putative membrane-spanning helices are also distinctive. In mGluRs, they are relatively small compared with those of seven transmembrane domain receptors belonging to the rhodopsin/␤-adrenergic family. Most importantly, there is no significant sequence homology between members of the mGluR family and other cloned GPCRs.The structure-function relationships supporting the coupling to G proteins have been well investigated for members of the rhodopsin/␤-adrenergic family. Although all the cytoplasmic domains of these receptors take part in G protein activation to some degree, i3 appears to harbor the structural elements that impart specificity to the interaction (20 -21).However, much less is known about the structural determinants of the coupling of mGluRs to G proteins. Recently, the analysis of chimeric receptors derived from the G i -coupled mGluR3 bearing different portions of the cytoplasmic domains of the G q -coupled mGluR1 has shown that i2 of group I receptors is necessary, but not sufficient, for the specific activation of pho...

Selectivity and Evolutionary Divergence of Metabotropic Glutamate Receptors for Endogenous Ligands and G Proteins Coupled to Phospholipase C or TRP Channels

“…10, B and C) identifies a subset of residues that may be important for activation of G q/11 by mGluR1 and mGluR5. The importance of the intracellular loops and C terminus has been well documented in previous reports, with the second intracellular loop reported to be necessary but not sufficient for G protein specificity (44,48).…”

The role of members of the pertussis toxin-sensitive family of G proteins in coupling receptors to the activation of the G protein-gated inwardly rectifying potassium channel

“…The splice variants GABA B 1a and GABA B 1b differ by a 117-aa stretch that is missing at the N terminus of GABA B 1b: this region contains amino acid motifs, termed ''Sushi repeats,'' thought to be involved in proteinprotein interactions (54). The regions involved in coupling the metabotropic glutamate receptors to G proteins are thought to be the second and third intracellular loops (55)(56)(57), so it might be likely that the same applies to GABA B receptors (58). However, we have clearly shown that GABA B 1aϩ2 receptors have a different G ␣ coupling profile to the GABA B 1bϩ2 receptors.…”

The cyclic AMP phenotype of fragile X and autism

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