Functional water molecules in rhodopsin activation

G-protein-coupled receptors (GPCRs) are integral membrane proteins that mediate cellular response to an extensive variety of extracellular stimuli. Studies of rhodopsin, a prototype GPCR, have suggested that water plays an important role in mediating the activation of family A GPCRs. However, our understanding of the function of water molecules in the GPCR activation is still rather limited because resolving the functional water molecules solely based on the results from existing experiments is challenging. Using all-atom molecular dynamics simulations in combination with inhomogeneous fluid theory, we identify in this work the positioning of functional water molecules in the inactive state, the Meta II state, and the constitutive active state of rhodopsin, basing on the thermodynamic signatures of the water molecules. We find that one hydration site likely functions as a switch to regulate the distance between Glu181 and the Schiff base in the rhodopsin activation. We observe that water molecules adjacent to the "NpxxY" motif are not as stable in the Meta II state as in the inactive state as indicated by the thermodynamics signatures, and we rationalize that the behaviors of these water molecules are closely correlated with the rearrangement of the water-mediated hydrogen-bond network in the "NPxxY" motif, which is essential for mediating the activation of the receptor. We thereby propose a hypothesis of the water-mediated rhodopsin activation pathway.