Light-stable rhodopsin. I. A rhodopsin analog reconstituted with a nonisomerizable 11-cis retinal derivative

Abstract

With the aim of preparing a light-stable rhodopsin-like pigment, an analog, II, of 11-cis retinal was synthesized in which isomerization of the C₁₁-C₁₂ cis-double bond is blocked by a cyclohexene ring built around the C₁₀ to C₁₃-methyl. The analog II formed a rhodopsin-like pigment (rhodopsin-II) with opsin expressed in COS-1 cells and with opsin from rod outer segments. The rate of rhodopsin-II formation from II and opsin was ~10 times slower than that of rhodopsin from 11-cis retinal and opsin. After solubilization in dodecyl maltoside and immunoaffinity purification, rhodopsin-II displayed an absorbance ratio (A_280nm/A_512nm) of 1.6, virtually identical with that of rhodopsin. Acid denaturation of rhodopsin-II formed a chromophore with λ_max, 452 nm, characteristic of protonated retinyl Schiff base. The ground state properties of rhodopsin-II were similar to those of rhodopsin in extinction coefficient (41,200 M^-1 cm^-1) and opsin-shift (2600 cm^-1). Rhodopsin-II was stable to hydroxylamine in the dark, while light-dependent bleaching by hydroxylamine was slowed by ~2 orders of magnitude relative to rhodopsin. Illumination of rhodopsin-II for 10 s caused ~3 nm blue-shift and 3% loss of visible absorbance. Prolonged illumination caused a maximal blue-shift up to ~20 nm and ~40% loss of visible absorbance. An apparent photochemical steady state was reached after 12 min of illumination. Subsequent acid denaturation indicated that the retinyl Schiff base linkage was intact. A red-shift (~12 nm) in λ_max and a 45% recovery of visible absorbance was observed after returning the 12-min illuminated pigment to darkness. Rhodopsin-II showed marginal light-dependent transducin activation and phosphorylation by rhodopsin kinase.