Small Angle Neutron and X-Ray Scattering Reveal Conformational Differences in Detergents Affecting Rhodopsin Activation

Abstract

Understanding G-protein-coupled receptor (GPCR) activation plays a crucial role in the development of novel improved molecular drugs. During photoactivation, the retinal chromophore of the visual GPCR rhodopsin isomerizes from the 11-cis conformation to the all-trans conformation, yielding an equilibrium between inactive Meta-I and active Meta-II states [1]. The principal goals of this work are to address whether the activation of rhodopsin leads to a single state or a conformational ensemble, and how the protein organizational structure changes with the detergent environment. We used small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) to answer the above questions. Both SANS and SAXS are powerful techniques to study the macromolecular structures in solution within the length scale from angstroms to several nanometers. In our experiments, rhodopsin is solubilized in CHAPS detergent, which favors the inactive Meta-I state. By contrast, dodecylmaltoside (DDM) detergent stabilizes the active Meta-II state [2]. Notably SANS with contrast-variation enables the separate study of the protein structure within the detergent assembly [3], and suggests a looser structure of rhodopsin in DDM versus CHAPS micelles. Such results are consistent with the SAXS data fitted by either a core-shell ellipsoid or core-shell cylindrical model, describing a monolayer of detergent molecules surrounding the rhodopsin molecule. Moreover, the SAXS experiments with different rhodopsin to detergent ratios delineate the role of the detergent in stabilization of the protein in solution. Our combined approach of SANS and SAXS studies reveals the protein structural changes associated with GPCR activation in the case of visual rhodopsin. [1] A. V. Struts et al. (2011) PNAS 108, 8263-8268. [2] A. V. Struts et al. (2014) Meth. Mol. Biol. in press. [3] R. K. Le et al. (2014) Arch. Biochem. Biophys. 550-551, 50-57.