Oral Presentation The 44th Lorne Conference on Protein Structure and Function 2019

Coupling directed evolution with NMR to probe ligand induced changes in α1A-adrenoceptor conformational equilibria and characterize fragment screening hits (#51)

Daniel J Scott 1 2 , Feng-Jie Wu 1 2 , Lisa M Williams 1 , Alaa Abdul-Ridha 1 , K. Johan Rosengren 3 , Ross AD Bathgate 1 2 , Paul R Gooley 2 4
  1. The Florey Institute of Neuroscience and Mental Health, Parkville, VIC, Australia
  2. Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville, VIC, Australia
  3. School of Biomedical Sciences, The University of Queensland, Brisbane, QLD, Australia
  4. Bio21 Molecular Science & Biotechnology Institute, Parkville

α1-adrenoceptors (α1-ARs) are G protein-coupled receptors (GPCR) that stimulate smooth muscle contraction in response to binding adrenaline and noradrenaline. The α1A-AR subtype is clinically targeted for treating hypertension and benign prostatic hyperplasia but is a putative drug target for neurodegenerative diseases. New subtype-selective tool compounds are required to probe the role of these receptors in the brain and to validate them as drug targets for neurodegenerative diseases. GPCRs are allosteric machines that sample multiple conformations existing in equilibrium. Agonist binding shifts the equilibrium to active states to promote G protein signaling. Recent crystal structures give us snapshots of inactive and active states, but not the dynamics that underlie GPCR activation. Studying the GPCR dynamics with methods such as NMR remains challenging because of the inherent instability of these membrane proteins in solution. Here we used directed evolution to engineer thermostabilised α1A-AR mutants that could be isotopically labeled with 13CH3-methionine to probe how ligands modulate the conformational equilibrium of this GPCR using NMR. Met292 sits in the orthosteric ligand binding pocket and its chemical shift was unique upon binding different ligands. Met203 on-the-other-hand, is located on the intracellular side of the receptor where G proteins interact. We found the resonance of Met203 shifted upfield in the presence of inverse agonists, downfield upon agonist binding and that the chemical shift changes correlated with ligand efficacy. The linear dependence of the chemical shifts is consistent with a conformational selection mechanism, while the resonance broadening in the presence of agonist suggests increased microsecond motion. We subsequently used this molecular efficacy ruler to validate the pharmacology of two novel hits from a trial fragment screen and the peptide toxin, τ-Tia. This study validates the current conformational equilibrium-based hypothesis of GPCRs and NMR for screening and characterizing novel ligands.