G-Protein-Coupled Receptors are Solvent-Swollen in the Functionally Active State

Abstract

G-protein-coupled receptors (GPCRs) are transmembrane receptors that play critical roles in a number of physiological signaling processes. In the standard biochemical model, GPCRs behave as primarily agonist-dependent bimodal switches, with little influence of the surrounding medium. However, using the visual receptor rhodopsin as a model GPCR, we show that water drives rhodopsin to a partially disordered, solvent-swollen conformational ensemble upon light absorption, rendering the standard model obsolete. We placed rhodopsin under varying degrees of osmotic stress using polyethylene glycol solutes and investigated the activation equilibrium response using UV-visible spectroscopy. Our results show a flood of ∼80 water molecules into the rhodopsin interior during photoactivation, a result supported by atomistic MD simulations [1]. Furthermore, the osmolyte effects on rhodopsin activation are size-dependent: large osmolytes back shift the equilibrium to inactive metarhodopsin-I (MI), while small osmolytes forward-shift the equilibrium to active metarhodopsin-II (MII). We attribute these size effects to varying degrees of osmolyte penetration into the rhodopsin core. Large polymers behave similarly to ideal osmolytes and dehydrate rhodopsin, while smaller polymers wriggle into the rhodopsin interior and stabilize the open MII conformation of rhodopsin. Besides osmotic pressure, the application of hydrostatic pressure also back shifts the metarhodopsin equilibrium but for fundamentally different reasons. Integrating the two force-based methods together with neutron scattering experiments [2] indicates that the active state of rhodopsin is more hydrated yet more locally collapsed. At the same time, the active GPCR undergoes volume fluctuations and solvent coupling, which give rise to greater thermal volume. The combined force-based results necessitate a new understanding of GPCR activation in which the surrounding soft matter is paramount in governing conformational energy landscapes. [1] N. Leioatts et al. (2014) Biochemistry 53, 376−385. [2] S.M.D.C. Perera et al. (2018) J. Phys. Chem. Lett.9, 7064−7071.