The Role of Intrinsically Disordered Domains in Regulating G Protein-Coupled Receptor Signaling

The α_2A adrenergic receptor (α_2AAR) is a clinically important target for various diseases including hypertension, diabetes, and chronic pain. Here, using single-molecule fluorescence resonance energy transfer imaging, we show how agonist-specific activation dynamics in both structured transmembrane domain (TMD) and intrinsic disorder regions (IDRs) of α_2AAR lead to diverse signaling profiles. Through seven pairs of strategically designed fluorophore labels, we systematically investigate the real-time conformational changes of α_2AAR. Our study reveals unique TM6 dynamics in α_2AAR, featured by a high energy barrier for agonist-induced outward movements essential for activation. In contrast, we identify agonist-specific conformational dynamics of a partially disordered extracellular loop (ECL2), highlighting its role as a dynamic regulatory module that controls receptor function. Moreover, we characterize the conformational landscapes of the long third intracellular loop (ICL3), revealing its compact structural features and membrane-proximal localization in the basal state, where it acts as a negative allosteric regulator in transducer coupling. Furthermore, we identify multiple functional substates of ICL3 that are dynamically modulated by both kinase phosphorylation and drug efficacy. These findings offer previously underappreciated structural and dynamic insights into α_2AAR function governed by both TMD and IDRs and may open up new avenues for the development of better therapeutics.