Activation of the G‐Protein‐Coupled Receptor Rhodopsin by Water

Abstract

Visual rhodopsin is an important archetype for G‐protein‐coupled receptors, which are membrane proteins implicated in cellular signal transduction. Herein, we show experimentally that approximately 80 water molecules flood rhodopsin upon light absorption to form a solvent‐swollen active state. An influx of mobile water is necessary for activating the photoreceptor, and this finding is supported by molecular dynamics (MD) simulations. Combined force‐based measurements involving osmotic and hydrostatic pressure indicate the expansion occurs by changes in cavity volumes, together with greater hydration in the active metarhodopsin‐II state. Moreover, we discovered that binding and release of the C‐terminal helix of transducin is coupled to hydration changes as may occur in visual signal amplification. Hydration–dehydration explains signaling by a dynamic allosteric mechanism, in which the soft membrane matter (lipids and water) has a pivotal role in the catalytic G‐protein cycle.

Membrane Curvature Revisited—the Archetype of Rhodopsin Studied by Time-Resolved Electronic Spectroscopy

“…At the protein level, the reason for the coupling between rhodopsin activation and negative bilayer curvature can be attributed to an overall expansion of rhodopsin during formation of the MII state, mediated by an outward movement of helix H6 by 5 A ˚and an extension of helix H5 compared with the dark state (75,76). This volumetric expansion has been experimentally shown by the effects of hydrostatic pressure (77,78), and osmotic pressure (37,79), as well as neutron scattering studies (80,81). It follows that a combination of mesoscopic-scale bilayer deformation properties in addition to specific lipid interactions modulates rhodopsin activation (63)(64)(65)(66)(67)82,83).…”

“…Electronic spectra were quantitatively analyzed for metarhodopsin I (MI) and metarhodopsin II (MII) fractions by fitting a linear combination of MI and MII basis spectra to the experimental spectrum, giving the fraction of MII denoted as q. To account for light scattering changes after bleaching, the experimental difference spectrum was subjected to an inverse-square wavelength scattering correction using the following equation (37):…”

Concerted conformational dynamics and water movements in the ghrelin G protein-coupled receptor

“…This is fully consistent with previous work demonstrating the role of water molecules in the activation process of other GPCRs such as the GLP-1 receptor ( Zhao et al, 2020 ; Wootten et al, 2016a ; Wootten et al, 2016b ). Whether our data reflects the fact that water molecules are allosteric players in the activation process, as demonstrated for other proteins including rhodopsin ( Chawla et al, 2021 ), or that the movements of water molecules compensate the changes in the intramolecular voids within the different states involved in receptor activation remains an open question.…”

Ion-water coupling controls class A GPCR signal transduction pathways

“…In the sodium-independent, visual class A GPCR rhodopsin, two protonation switches are known to initiate activation. The first protonation event is associated with the lightinduced transition of the retinal Schiff base, central to the receptor similar to Asp 2.50 in non-visual receptors, while the additional protonation occurs at the D 3.49 RY motif (51,52). The coordinated action of such a conserved twin-protonation switch during activation of class A GPCRs would likely re- quire a mechanism to relay protons, and exchange information about the states of the two distal sites.…”