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.  Here we show experimentally for the first time that ~80 water molecules flood rhodopsin upon light absorption to form a solvent-swollen active state.  An influx of mobile water is sufficient for activating the photoreceptor, and 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.