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

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.

Automated summary

Visual rhodopsin absorbs light and takes in approximately 80 water molecules to form a solvent-swollen active state, which is essential for activating the photoreceptor and is supported by molecular dynamics simulations. Force-based measurements show that the expansion of rhodopsin occurs through changes in cavity volumes and increased hydration in the active Meta II state. Furthermore, the binding and release of the C-terminal helix of transducin is coupled to hydration changes, potentially influencing visual signal amplification through a dynamic allosteric mechanism involving lipid and water interactions in the catalytic G-protein cycle.