Thermodynamic Architecture and Conformational Plasticity of GPCRs

ABSTRACT G-protein-coupled receptors (GPCRs) are ubiquitous integral membrane proteins involved in diverse cellular signaling processes and consequently serve as crucial drug targets. Here, we carry out the first large-scale ensemble thermodynamic study of 45 different ligand-free GPCRs employing a structure-based statistical mechanical framework and identify extensive conformational plasticity encompassing the seven transmembrane (TM) helices. Multiple partially structured states or intermediates co-exist in equilibrium in the native ensemble, with the TM helices 1, 6 and 7 displaying varied degrees of structure, and TM3 exhibiting the maximal stability. Active state GPCRs are characterized by reduced conformational heterogeneity with altered coupling-patterns distributed not just locally but throughout the structural scaffold. Strongly coupled residues are distributed across the structure in an anisotropic manner accounting for only 13% of the residues, highlighting that a large number of residues in GPCRs are inherently dynamic to enable structural motions critical for function. Our work thus uncovers the thermodynamic hallmarks of GPCR structure and activation, and how differences quantifiable only via higher-order coupling free energies provide insights into their exquisite structural specialization and the fluid nature of the intramolecular interaction network. The intricate landscapes and perturbation methodologies presented here lay the foundation for understanding allosteric mechanisms in GPCRs, location of structural-functional hot-spots, and effects of disease-causing mutations.