Conformational Fluctuations in G-Protein-Coupled Receptors

G-protein\textbf{-}coupled receptors (GPCRs) comprise almost 50{\%} of pharmaceutical drug targets, where rhodopsin is an important prototype and occurs naturally in a lipid membrane. Rhodopsin photoactivation entails 11-\textit{cis} to all-\textit{trans} isomerization of the retinal cofactor, yielding an equilibrium between inactive Meta-I and active Meta-II states. Two important questions are: (1) Is rhodopsin is a simple two-state switch? Or (2) does isomerization of retinal unlock an activated conformational ensemble? For an ensemble-based activation mechanism (EAM) [1] a role for conformational fluctuations is clearly indicated. Solid-state NMR data together with theoretical molecular dynamics (MD) simulations detect increased local mobility of retinal after light activation [2]. Resultant changes in local dynamics of the cofactor initiate large-scale fluctuations of transmembrane helices that expose recognition sites for the signal-transducing G-protein. Time-resolved FTIR studies and electronic spectroscopy further show the conformational ensemble is strongly biased by the membrane lipid composition, as well as pH and osmotic pressure [3]. A new flexible surface model (FSM) describes how the curvature stress field of the membrane governs the energetics of active rhodopsin, due to the spontaneous monolayer curvature of the lipids [4]. Furthermore, influences of osmotic pressure dictate that a large number of bulk water molecules are implicated in rhodopsin activation. Around 60 bulk water molecules activate rhodopsin, which is much larger than the number of structural waters seen in X-ray crystallography, or inferred from studies of bulk hydrostatic pressure. Conformational selection and promoting vibrational motions of rhodopsin lead to activation of the G-protein (transducin). Our biophysical data give a paradigm shift in understanding GPCR activation. The new view is: dynamics and conformational fluctuations involve an ensemble of substates that activate the cognate G-protein in the amplified visual response.\\[4pt] [1] A. V. Struts et al. (2011) \textit{Nat. Struct. Mol. Biol.} \textbf{18}, 392.\\[0pt] [2] A. V. Struts et al. (2011)~\textit{PNAS}$~$\textbf{108}, 8263.\\[0pt] [3] M. Mahalingam et al. (2008) \textit{PNAS} \textbf{105}, 17795.\\[0pt] [4] M. F. Brown (2012) \textit{Biochemistry} \textbf{51}, 9782.