Structural, Energetic, and Mechanical Perturbations in Rhodopsin Mutant That Causes Congenital Stationary Night Blindness

Several point mutations in rhodopsin cause retinal diseases including congenital stationary night blindness and retinitis pigmentosa. The mechanism by which a single amino acid residue substitution leads to dysfunction is poorly understood at the molecular level. A G90D point mutation in rhodopsin causes constitutive activity and leads to congenital stationary night blindness. It is unclear which perturbations the mutation introduces and how they can cause the receptor to be constitutively active. To reveal insight into these mechanisms, we characterized the perturbations introduced into dark state G90D rhodopsin from a transgenic mouse model expressing exclusively the mutant rhodopsin in rod photoreceptor cells. UV-visible absorbance spectroscopy revealed hydroxylamine accessibility to the chromophore-binding pocket of dark state G90D rhodopsin, which is not detected in dark state wild-type rhodopsin but is detected in light-activated wild-type rhodopsin. Single-molecule force spectroscopy suggested that the structural changes introduced by the mutation are small. Dynamic single-molecule force spectroscopy revealed that, compared with dark state wild-type rhodopsin, the G90D mutation decreased energetic stability and increased mechanical rigidity of most structural regions in the dark state mutant receptor. The observed structural, energetic, and mechanical changes in dark state G90D rhodopsin provide insights into the nature of perturbations caused by a pathological point mutation. Moreover, these changed properties observed for dark state G90D rhodopsin are consistent with properties expected for an active state.