Abstract
The Monod-Wyman-Changeux (MWC) model was initially proposed to describe the allosteric properties of regulatory enzymes and subsequently extended to receptors. Yet despite GPCRs representing the largest family of receptors and drug targets, no study has systematically evaluated the MWC mechanism as it applies to GPCR allosteric ligands. We reveal how the recently described allosteric modulator, benzyl quinolone carboxylic acid (BQCA), behaves according to a strict, two-state MWC mechanism at the M1 muscarinic acetylcholine receptor (mAChR). Despite having a low affinity for the M1 mAChR, BQCA demonstrated state dependence, exhibiting high positive cooperativity with orthosteric agonists in a manner that correlated with efficacy but negative cooperativity with inverse agonists. The activity of BQCA was significantly increased at a constitutively active M1 mAChR but abolished at an inactive mutant. Interestingly, BQCA possessed intrinsic signaling efficacy, ranging from near-quiescence to full agonism depending on the coupling efficiency of the chosen intracellular pathway. This latter cellular property also determined the difference in magnitude of positive cooperativity between BQCA and the orthosteric agonist, carbachol, across pathways. The lack of additional, pathway-biased, allosteric modulation by BQCA was confirmed in genetically engineered yeast strains expressing different chimeras between the endogenous yeast G(pa1) protein and human Gα subunits. These findings define a chemical biological framework that can be applied to the study and classification of allosteric modulators across different GPCR families.
Highlights
The Monod-Wyman-Changeux (MWC) mechanism is the preeminent conformational selection model for allosteric proteins
The key statements of the MWC model are that allosteric proteins are oligomeric, they possess an axis of symmetry, they can exist in an equilibrium between two distinct states in the absence of ligand, and they possess multiple ligand recognition sites, binding to which stabilizes a subset of conformational states at the expense of others
We propose that the interaction between benzyl quinolone carboxylic acid (BQCA) and the M1 muscarinic acetylcholine receptor (mAChR) represents an unprecedented example of GPCR allosteric modulation that is entirely consistent with a strict, two-state MWC mechanism
Summary
The Monod-Wyman-Changeux (MWC) mechanism is the preeminent conformational selection model for allosteric proteins. The key statements of the MWC model are that allosteric proteins are oligomeric, they possess an axis of symmetry, they can exist in an equilibrium between (at least) two distinct states in the absence of ligand, and they possess multiple ligand recognition sites, binding to which stabilizes a subset of conformational states at the expense of others In terms of chemical biology and pharmacology, the conformational selection mechanism that underpins the MWC, referred to as a “two-state model” [3, 4], predicts that it should be possible to discover orthosteric, but allosteric, drugs that preferentially favor either active or inactive receptor states and can selectively modulate the properties of co-bound ligands in a manner that correlates with the efficacy (positive, negative, or neutral) of such ligands. It is envisaged that the approaches outlined can be broadly applicable in the evaluation of mechanism of action of various allosteric ligands of other GPCR families
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