Dynamics of Ligand Binding Sites and Chloride Penetration in a Bitter Taste GPCR

Taste perception strongly influences food choice, drug compliance, and dietary behavior. Bitter taste receptors belong to a subfamily of Family A GPCRs, while having unique features and deviations from conserved motifs. Recent CryoEM structures revealed that the human bitter taste receptor TAS2R14 contains not only the canonical extracellular binding site, but also a novel intracellular site, raising questions about their interplay. Using molecular dynamics simulations, we examined how binding of aristolochic acid in the intracellular site and cholesterol at the extracellular site, alone or together, affects receptor dynamics and pocket communication. We observed that cholesterol binding in the extracellular site expanded the intracellular pocket volume and induced conformational changes in TM6, whereas aristolochic acid binding in the intracellular site had no such effect on the extracellular site. Notably, in cholesterol-only simulations, a chloride ion entered from the intracellular side and interacted with Arginine 55 (BW position 2.50), revealing an inverted ion-binding pattern to Family A GPCRs, where aspartate 2.50 is known to bind a sodium ion to stabilize an inactive state. Our results suggest chloride-dependent and cholesterol-dependent modulation mechanisms in the dual-site dynamics of TAS2R14.