In vitro assays of rod and cone opsin activity: retinoid analogs as agonists and inverse agonists

Upon absorption of a photon, the bound 11-cis-retinoid isomerizes to the all-trans form resulting in a protein conformational change that enables it to activate its G protein, transducin, to begin the visual signal transduction cascade. The native ligand, 11-cis-retinal, acts as an inverse agonist to both the apoproteins of rod and cone visual pigments (opsins); all-trans-retinal is an agonist. Truncated analogs of retinal have been used to characterize structure-function relationships with rod opsins, but little has been done with cone opsins. Activation of transducin by an opsin is one method to characterize the conformational state of the opsin. This chapter describes an in vitro transducin activation assay that can be used with cone opsins to determine the degree to which different ligands can act as an agonist or an inverse agonist to gain insight into the ligand-binding pocket of cone opsins and differences between the different classes of opsins. The understanding of the effects of ligands on cone opsin activity can potentially be applied to future therapeutic agents targeting opsins.