Light-dependent activation of transducin by pineal opsin

Purpose: To express the chick pineal opsin, P-opsin, in a pineal cell line, purify the photopigment and characterize its interaction with rod transducin. Methods. A P-opsin cDNA, containing an epitope for a monoclonal antibody to bovine rhodopsin, was expressed in the CP3 cell line. Photopigment was generated by incubation with 11-cis retinal, solubilized in dodecyl maltoside and purified by immunoaffinity chromatography. UV/Vis spectra were measured and activation of purified bovine transducin (G_T) was assayed using a [³⁵S]GTPγS filter binding assay. Results: The UV/Vis spectrum of P-opsin had a maximum absorbance at 465 nm, which bleached to 380 nm when exposed to light. In the presence of 50 mM NH₂OH in the dark, P-opsin slowly bleached (half-time of ∼2 h). Light-exposed P-opsin stimulated guanyl nucleotide exchange on bovine rod G_T. In exhaustive binding assays, up to one mol of G_T was activated per mol of P-opsin, similar to that observed with purified cone opsins, but ∼100-fold less than with bovine rhodopsin The kinetics of P-opsin was comparable to rhodopsin. Half-maximal activation occured with ion 1.5 min. However, total turnovers were less for P-opsin: 0.8 mol G_T/mol P-opsin/min compared to 105 mol Gr/mol rhodopsin/min. The lifetime of the active species formed following photobleaching of P-opsin was 6 min, shorter than the 30 min found for rhodopsin. Conclusion: Pineal opsin is a photopigment that binds 11-cis retinal and activates transducin in a light dependent manner. P-opsin has spectral and biochemical characteristics that are intermediate between rod and cone opsins. These results indicate a role for P-opsin in pineal phototransduction and circadian phase-shifting.