The C‐terminus of the metabotropic glutamate receptor 1b regulates dimerization of the receptor

Abstract

The Group C G protein-coupled receptors include the metabotropic glutamate receptors (mGluRs), the GABA B receptor, the calcium sensor and several taste receptors, most of which are obligate dimers, indeed recent work has shown that dimerization is necessary for the activation of these receptors. Consequently factors that regulate their ability to homo-or heterodimerize are important. The Group 1 mGluRs include mGluR1 and mGluR5 both of which have splice variants with altered C-termini. In this study, we show that mGluR1b is a dimer and that it does not efficiently heterodimerize with mGluR1a, unlike the two splice variants of mGluR5 that can heterodimerize. Mutation of a positively charged motif (RRKK) at the C-terminus of the mGluR1b tail permits mGluR1b to heterodimerize with mGluR1a. Coexpression of mGluR1a and mGluR1b in COS-7 cells results in the accumulation of mGluR1b in intracellular inclusions that do not contain mGluR1a. This behaviour is mimicked by a chimera of the lymphocyte antigen CD2 with the C-terminus of mGluR1b (pCD1b) and depends on the presence of the RRKK motif. These accumulations are immunoreactive for endoplasmic reticulum (ER) markers, but not Golgi and ERGIC markers. This segregation of mGluR1b from other ER proteins may contribute to its failure to dimerize with mGluR1a.

Surface expression of metabotropic glutamate receptor variants mGluR1a and mGluR1b in transfected HEK293 cells

“…This is in accordance with results from previous studies (Mary et al, 1998;Mary et al, 1997;Pin et al, 1994;Remelli et al, 2008). We used mGluR1a mutant deficient in G-protein coupling (mGluR1aF/P) and co-expressed it with mGluR1b subunit.…”

“…Most likely explanation is that the mGluR1a Ctail includes sequence that neutralizes the RRKK motif either directly or it may interact with another protein that masks the basic residues motif (Mary et al, 1998;Remelli et al, 2008). Moreover, our results suggest that within the splice variants heterodimers single copy of the long mGluR1a C-tail may hinder RRKK motive of its own polypeptide chain and also of adjacent subunit that lacks such neutralization sequence.…”

“…For the release from the ER, this sequence has to be neutralized by interactions with other proteins or by sequences positioned on its own polypeptide chain. Such mechanism of neutralization of the RRKK motif by sequence from its own polypeptide chain was proposed for the long tail of mGluR1a (Ciruela et al, 1999;Mary et al, 1998;Mundell et al, 2004;Remelli et al, 2008), based on data obtained both in transfected cells and in neurons. Indeed, the RRKK motif has pronounced effect on the cell surface targeting and limits specific signaling properties of the short variants only (Das and Banker, 2006;Francesconi and Duvoisin, 2002;Mary et al, 1998;Pin et al, 1994;Pin et al, 1992;Prezeau et al, 1996).…”

Cooperative Signaling between Homodimers of Metabotropic Glutamate Receptors 1 and 5

“…2B when these receptors were expressed together, the partial glutamate-response curve indicates a high-efficacy response with a potency intermediate to that of mGluR1b and mGluR1 Y74A when expressed alone, validating our prediction. Thus, these data support the finding (Remelli et al, 2008) that mGluR1a and mGluR1b fail to dimerize.…”

“…We next tested whether mGluR1 Y74A (constructed from the mGluR1a variant), which appeared to dimerize with wildtype mGluR1a in our functional assay (Kammermeier and Yun, 2005), would interact with mGluR1b, which cannot form dimers with mGluR1a (Remelli et al, 2008). Thus, our model predicts that the glutamate concentration response of the mGluR1 Y74A/mGluR1b combination should mimic that of mGluR1 Y74A/mGluR5 (high efficacy, intermediate potency), rather than that of mGluR1 Y74A/mGluR1a (high efficacy, potency nearly identical to the YA mutant alone).…”