The Study Of Monomer Selection Mechanism In The Activation Process Of Metabotropic Glutamate Receptor 2

G protein coupled receptors(GPCRs)are the largest family of cell surface receptors and major players in cellular communication.GPCRs superfamily comprises about800-1000 members.They participate in many physiological and pathological processes.There are many diseases associated with G protein coupled receptors,and about 40%of the drugs targeting GPCRs.Metabotropic glutamate receptors(mGluRs)are class C GPCRs activated by the excitatory neurotransmitter L-glutamate.mGluRs are mandatory dimers,with each subunit being composed of several domains.In vivo,mGluRs participate in many important physiological processes,such as learning,memory,synaptic plasticity,signaling transmission and are considered promising targets for the treatment of various brain disorders.It has been reported that only the mGluR4 coupled the G protein in the mGluR2/R4 heterodimer.While in the presence of positive allosteric modulator of mGluR2 or negative allosteric modulator of mGluR4,mGluR2 is responsible for coupling G protein.In the GABA_BR heterodimer,GB1 binds ligand and GB2 couples the G protein.While the mutant of GB1-ASA can express on the cell surface and couple the G protein.In the heterodimer taste receptor,only the T1R1 or T1R2 binds ligand and couples the G protein.This shows the monomer selection is ubiquitous in the class C GPCRs,and it is dynamical and controllable but the mechanism is still unclear.The monomer selection is that the receptor dimer selectively activates one monomer during the activation process,which in turn causes the G protein couples to the activated monomer.The monomer selection in mGluR2/R4 heterodimer can be regulated by allosteric modulator.Then the allosteric modulator mainly changes the conformation of the transmembrane domain,so we guess:whether the structure of the transmembrane domain can be changed by mutation to study monomer selection?In addition,the conformational change caused by the mutation is smaller than by the allosteric modulator.Therefore,it is more difficult to study the monomer selection in heterodimer than in homodimer.Therefore,in the study,we use rat metabotropic glutamate receptor 2(mGluR2)as prototypical receptor to explore the mechanism of monomer selection in the process of receptor activation.Based on the binding site of allosteric modulator,we screen8 sites in TM3-TM7 and find two key residues that participate in the monomer selection:L732 and F776.Mutated to alanine,L732A and F776A mutants have asymmetric monomer selection in the process of receptor activation.The non-activated subunit is involved in the activation of the receptor proved by the using of mGluR2 negative allosteric modulator,which inhibits the activity of non-activated subunit.Using the mGluR2 positive allosteric modulator,which reduce the activation barrier of wild type subunit,the monomer selection is reversed.Similarly,it shows the monomer selection is dynamical and controllable.Finally,we propose the“seesaw-like”dynamic monomer selection model based on the difference activation barriers of the two subunits in mGluR2dimer.The study of the monomer selection in the process of receptor activation will be helpful for understanding the activation mechanism of mGluRs,even class C GPCRs dimers.It will provide theoretical basis for the physiological,pathologic research and drug targets.