From Structure to Dynamics: Activation Mechanism of the G Protein-Coupled Bile Acid Receptor 1-Gs Complex

Abstract

The G protein-coupled bile acid receptor 1 (GPBAR1, also known as TGR5) is a key mediator of bile acid signaling, exerting its physiological effects through coupling with the stimulatory G protein (Gs). This interaction stabilizes the receptor’s active state and triggers downstream signaling; however, the dynamic features underlying GPBAR1 ligand recognition, activation, and G protein engagement remain poorly understood. In this study, we investigated the molecular basis of GPBAR1 receptor activation by combining microsecond-scale molecular dynamics simulations, principal component analysis, and network-based allosteric communication analysis, using lithocholic acid (LCA), the most potent endogenous agonist, as a reference compound. By comparing LCA-bound and ligand-free GPBAR1 in the presence and in the absence of Gs, we disentangled the respective contributions of agonist binding and G protein coupling to receptor activation. Our results show that LCA promotes activation-associated rearrangements of TM5, TM6, and key microswitches, reshaping the GPBAR1 conformational landscape independently of G protein recruitment. These ligand-induced changes strengthen the coupling interface with the α5 helix of Gαs and facilitate allosteric communication between the orthosteric and intracellular sites. While agonist binding is sufficient to trigger these rearrangements, Gs coupling further stabilizes more organized active-like conformational ensembles. Overall, our findings provide a dynamic mechanistic view of LCA-driven GPBAR1 activation and G-protein engagement, highlighting its role as a molecular effector of bile acid signaling and offering molecular details relevant to GPBAR1-targeted drug discovery.