| Summary: | Histamine receptor 2 (HR<sub>H2</sub>) blockers are used to treat peptic ulcers and gastric reflux. Chlorquinaldol and chloroxine, which contain an 8-hydroxyquinoline (8HQ) core, have recently been identified as blocking HR<sub>H2</sub>. To gain insight into the mode of action of 8HQ-based blockers, here, we leverage an HR<sub>H2</sub>-based sensor in yeast to evaluate the role of key residues in the HR<sub>H2</sub> active site on histamine and 8HQ-based blocker binding. We find that the HR<sub>H2</sub> mutations D98A, F254A, Y182A, and Y250A render the receptor inactive in the presence of histamine, while HR<sub>H2</sub>:D186A and HR<sub>H2</sub>:T190A retain residual activity. Based on molecular docking studies, this outcome correlates with the ability of the pharmacologically relevant histamine tautomers to interact with D98 via the charged amine. Docking studies also suggest that, unlike established HR<sub>H2</sub> blockers that interact with both ends of the HR<sub>H2</sub> binding site, 8HQ-based blockers interact with only one end, either the end framed by D98/Y250 or T190/D186. Experimentally, we find that chlorquinaldol and chloroxine still inactivate HR<sub>H2</sub>:D186A by shifting their engagement from D98 to Y250 in the case of chlorquinaldol and D186 to Y182 in the case of chloroxine. Importantly, the tyrosine interactions are supported by the intramolecular hydrogen bonding of the 8HQ-based blockers. The insight gained in this work will aid in the development of improved HR<sub>H2</sub> therapeutics. More generally, this work demonstrates that Gprotein-coupled receptor (GPCR)-based sensors in yeast can help elucidate the mode of action of novel ligands for GPCRs, a family of receptors that bind 30% of FDA therapeutics.
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