Dimerization Is Essential for Class C Receptor Function

The modular ligand binding VFT domains in class C receptors exhibit a rigid dimerization interface between the first lobes, consolidated in mGluR and Ca -sensing receptors by a cys-cys disulfide bond [53, 97] (for a review, see Reference 51). Constitutive N-terminal dimerization can be supported by other contacts, for example, within the C terminus. The coiled-coil interaction between the C-terminal tails of the GABAB subunits masks the endoplasmic reticulum retention signal of GABAB1 and allows surface delivery of the heterodimer -89, 90] . while C-terminal domains of glutamate receptors can prevent inappropriate heterodimerization [98].

The VFT dimer interface provides a rotational anchor, in which domain closure after agonist binding shortens the distance between the C termini of each module, bringing the attached respective 7TM domains closer together

[99]. FRET studies support such a rearrangement after activation of mGluR1

[100] - Intersubunit communication is critical for GABAB receptor activation since GABAB1 subunits do not function alone even when the C tail retention sequence is mutated to allow plasma membrane expression [ 89], while the GABAB2 subunit lacks the conserved VFT domain GABA binding site [101]. Moreover, receptor mutations that prevent GABAB1/B2 G protein signaling are effective only when applied to the GABAB2 and not the GABAB1 protomer [102,103]. GABAB2 also contains the binding site for the allosteric modulator CGP7630 [104]. Such evidence led to the proposal that binding of GABA to a single site in GABAB1 trans-activates its GABAB2 partner, responsible for G protein signaling. However, the stark division between "agonist binding" and "coupling" GABAB subunits is a simplification. The presence of the nonbind-ing GABAB2 VFT domain still facilities closure of the GABAB1 VFT and thereby enhances its affinity for GABA [105-107] - Equally, the presence of the GABAB1 7TM domain in the GABAB1/B2 heterodimer improves G protein coupling efficiency, and cis-activation of chimeric GABA receptor consisting of efficient binding (GABAB1 VFT) and activation (GABAB2 7TM) domains can only occur on coexpression with a GABAB2 VFT/GABAB1 7TM chimera. Thus, the critical element in GABAB receptor activation is not specific complementation between the "binding" and "activation" monomers. Rather, it is the formation of the GABAB dimer itself, which allows global rearrangement of the 7TM domains to be driven by closure of the linked VFT modules.

This activation theme runs through other class C receptors, including taste receptor heterodimers [108] and even mGluR homodimers, in which the orthosteric VFT binding sites are identical. Recently, an elegant investigation into the mechanisms of mGluR5 activation made use of the GABAB C tail quality control system, to express defined mGluR5 dimers at the cell surface with complementary mutations in one or other of the protomers [54] - These prevented glutamate binding in the VFT, G protein activation, or dissociated the coupling between the VFT and the 7TM domain in the same subunit; in addition, the binding site for the mGluR1 allosteric modulator Ro-01-6128 was introduced into one mGluR5 promoter, so that it could be locked in an active conformation. This study indicated first that the binding of glutamate to one VFT domain was sufficient for a response. Second, following glutamate binding, only one 7TM domain in the dimer reached an active conformation at a given time, but this activated protomer could either be in the opposing one (trans) from the ligand-bound VFT or in the same subunit (cis). In either case, the activation mechanism required communication between receptor protomers. As for GABAB receptors, the formation of mGluR5 dimers is therefore integral to their function.

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