Mechanism of rhodopsin activation as examined with ring-constrained retinal analogs and the crystal structure of the ground state protein

The guanine nucleotide-binding protein (G-protein)coupled receptor superfam ily (GPCR) is comprised of a large group of membrane proteins involved in a wide range of physiological signaling processes. The functional switch fro m a quiescent to an active conformation is at the heart of GPCR action. The GPCR rhodopsin has been studied extensively because of its key role in sco topic vision. The ground state chromophore, Il-cis-retinal, holds the trans membrane region of the protein in the inactive conformation, Light induces cis-trans isomerization and rhodopsin activation. Here we show that rhodops in regenerated with a ring-constrained 11-cis-retinal analog undergoes phot oisomerization; however, it remains marginally active because isomerization occurs without the chromophore-induced conformational change of the opsin moiety, Modeling the locked chromophore analogs in the active site of rhodo psin suggests that the beta -ionone ring rotates but is largely confined wi thin the binding site of the natural Il-cis-retinal chromophore, This const raint is a result of the geometry of the stable 11-cis-locked configuration of the chromophore analogs. These results suggest that the native chromoph ore cis-trans isomerization is merely a mechanism for repositioning of the beta -ionone ring which ultimately leads to helix movements and determines receptor activation.