Signaling states of rhodopsin - Retinal provides a scaffold for activatingproton transfer switches

The G-protein-coupled receptor rhodopsin is activated by photoconversion of its covalently bound ligand Il-cis-retinal to the agonist all-trans-retina l. After light-induced isomerization and early photointermediates, the rece ptor reaches a G-protein-dependent equilibrium between active and inactive conformations distinguished by the protonation of key opsin residues. In th is report, we study the role of the 9-methyl group of retinal, one of the c rucial steric determinants of light activation. We find that when this grou p is removed, the protonation equilibrium is strongly shifted to the inacti ve conformation. The residually formed active species is very similar to th e active form of normal rhodopsin, metarhodopsin II. It has a deprotonated Schiff base, binds to the retinal G-protein transducin, and is favored at a cidic pH. Our data show that the normal proton transfer reactions are inhib ited in 9-demethyl rhodopsin but are still mandatory for receptor activatio n, We propose that retinal and its 9-methyl group act as a scaffold for ops in to adjust key proton donor and acceptor side chains for the proton trans fer reactions that stabilize the active conformation. The mechanism may als o be applicable to related receptors and may thus explain the partial agoni sm of certain ligands.