Kinetic, Energetic, and Mechanical Differences between Dark-State Rhodopsin and Opsin

SummaryRhodopsin, the photoreceptor pigment of the retina, initiates vision upon photon capture by its covalently linked chromophore 11-cis-retinal. In the absence of light, the chromophore serves as an inverse agonist locking the receptor in the inactive dark state. In the absence of chromophore, the apoprotein opsin shows low-level constitutive activity. Toward revealing insight into receptor properties controlled by the chromophore, we applied dynamic single-molecule force spectroscopy to quantify the kinetic, energetic, and mechanical differences between dark-state rhodopsin and opsin in native membranes from the retina of mice. Both rhodopsin and opsin are stabilized by ten structural segments. Compared to dark-state rhodopsin, the structural segments stabilizing opsin showed higher interaction strengths and mechanical rigidities and lower conformational variabilities, lifetimes, and free energies. These changes outline a common mechanism toward activating G-protein-coupled receptors. Additionally, we detected that opsin was more pliable and frequently stabilized alternate structural intermediates.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (258 K)Download as PowerPoint slideHighlights► Mapping the stability of opsin from the rod outer segment of the mouse eye ► Compared to dark-state rhodopsin opsin increases mechanical stability and rigidity ► Compared to dark-state rhodopsin opsin reduces lifetime and free energy ► Compared to dark-state rhodopsin opsin is pliable and adopts alternate conformations