Role of Dimerization in Modulating Sensitivity and Dynamic Range of Ligand Sensing in Family CG Protein Coupled Receptors

Recent research on addiction has paid increasing attention to the neurotransmitter glutamate. A primary class of glutamate receptors are metabotropic glutamate receptors (mGluRs), belonging to family CG protein-coupled receptors. They mediate slower, modulatory glutamate transmission. Considerable efforts thus have been directed at targeting mGluRs for treatments of drug addiction and many other diseases, such as Alzheimer’s disease, fragile X mental retardation, and schizophrenia.

Metabotropic glutamate receptor I (mGluR1) is a typical mGluR family member, existing as a homodimer linked by a single disulfide bridge in the extracellular ligand-binding domain (LBD), and extensive non-covalent interactions at the dimer interface. The LBD functions as a sensor of glutamate. Each subunit in the LBD dimer adopts a Venus flytrap (VFT) fold with one glutamate binding site in its cleft. The dimeric LBD exhibits negative cooperativity for glutamate binding, with an elusive molecular mechanism. We thus aim to understand the underlying mechanism of the cooperativity and how homodimerization modulate the sensitivity and dynamic range of ligand sensing for mGluR1. We will (1) express and purify the extracellular LBD of native dimeric mGluR1 and monomeric mGluR1 with mutations at the cysteine site of the disulfide bridge, (2) use multiangle light scattering to quantify the monomer/dimer equilibrium in the mGluR1 LBD mutants, and (3) use fluorescence spectroscopy to measure glutamate response for wild type and mutated mGluR1 LBD to assess the role of dimerization in the molecular mechanism of glutamate binding. The study will in turn guide the design of drugs that are precise modulators of the conformational equilibria of receptors and downstream responses of the cells.