Structural Model of Ligand-G Protein-coupled Receptor (GPCR) Complex Based on Experimental Double Mutant Cycle Data

Abstract

The snake toxin MT7 is a potent and specific allosteric modulator of the human M1 muscarinic receptor (hM1). We previously characterized by mutagenesis experiments the functional determinants of the MT7-hM1 receptor interaction (FruchartGaillard, C., Mourier, G., Marquer, C., Stura, E., Birdsall, N. J., and Servent, D. (2008) Mol. Pharmacol. 74, 1554 -1563) and more recently collected evidence indicating that MT7 may bind to a dimeric form of hM1 (Marquer, C., Fruchart-Gaillard, C., Mourier, G., Grandjean, O., Girard, E., le Maire, M., Brown, S., and Servent, D. (2010) Biol. Cell 102, 409 -420). To structurally characterize the MT7-hM1 complex, we adopted a strategy combining double mutant cycle experiments and molecular modeling calculations. First, thirty-three ligand-receptor proximities were identified from the analysis of sixty-one double mutant binding affinities. Several toxin residues that are more than 25 Å apart still contact the same residues on the receptor. As a consequence, attempts to satisfy all the restraints by docking the toxin onto a single receptor failed. The toxin was then positioned onto two receptors during five independent flexible docking simulations. The different possible ligand and receptor extracellular loop conformations were described by performing simulations in explicit solvent. All the docking calculations converged to the same conformation of the MT7-hM1 dimer complex, satisfying the experimental restraints and in which (i) the toxin interacts with the extracellular side of the receptor, (ii) the tips of MT7 loops II and III contact one hM1 protomer, whereas the tip of loop I binds to the other protomer, and (iii) the hM1 dimeric interface involves the transmembrane helices TM6 and TM7. These results structurally support the high affinity and selectivity of the MT7-hM1 interaction and highlight the atypical mode of interaction of this allosteric ligand on its G proteincoupled receptor target. G protein-coupled receptors (GPCRs)5 constitute the largest family of membrane proteins (1) and represent the most important class of targets for current therapeutic agents (2, 3). It is therefore of particular interest to structurally characterize these receptors with the goal of understanding at the atomic level the molecular bases of their ligand recognition and functional versatility. In the last few years, several GPCR x-ray structures were solved, highlighting structural differences between different receptor families and various receptor states (4 -13). Despite these encouraging successes, the determination of experimental three-dimensional structures of GPCR-ligand complexes remains a challenge, especially in the case of allosteric modulators. Therefore, we have chosen an alternative way, based on mutagenesis and pharmacological studies combined with molecular modeling and docking procedures, to highlight the molecular bases of the interaction between a GPCR and a highly specific allosteric peptide ligand.Furthermore, we have addressed the question of the oligomeric state of the ...

Structural Insight into Specificity of Interactions between Nonconventional Three-finger Weak Toxin from Naja kaouthia (WTX) and Muscarinic Acetylcholine Receptors

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The Prevalence, Maintenance, and Relevance of G Protein–Coupled Receptor Oligomerization

“…Perhaps the best example of this to date is not for a small-molecule ligand, but for the snake toxin MT7 (Marquer et al, 2011). Here, both extensive mutagenesis studies and a series of studies that indicated that the toxin favors either the formation or stability of such a complex of the M 1 muscarinic receptor have been produced.…”

Peptide and Protein Neurotoxin Toolbox in Research on Nicotinic Acetylcholine Receptors

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