Structural and functional properties of metarhodopsin III: Recent spectroscopic studies on deactivation pathways of rhodopsin

The activation of rhodopsin has been the focus of researchers over the past decades, revealing many aspects of the activation pathways of this prototypical G protein-coupled receptor on a molecular level, starting with the light-dependent isomerization of its retinal chromophore from 11-cis to all-trans and leading eventually to the large scale helix movements in the transition to the active receptor state, Meta II. Comparatively little is known, however, on the deactivation pathways of the light receptor, which represent essential steps in maintaining a functional photoreceptor cell. Rhodopsin's active receptor species, Meta II, decays by two fundamentally different pathways, either forming the apoprotein opsin by release of the activating all-trans retinal ligand from its binding pocket, or by a thermal isomerization of this ligand to a less activating species in the transition to metarhodopsin III (Meta III). Both decay products, opsin and Meta III, are largely inactive under physiological conditions, yet they do not restore the complete inactivity of the dark state. Although some properties of Meta III have been described already in the 1960s, its molecular nature and the pathways of its formation have remained rather obscure. In this review, we focus on recent studies from our laboratories, which have provided a major progress in our understanding of the Meta III deactivation pathway and its potential physiological roles. Using Fourier-transform infrared (FTIR) difference spectroscopy in combination with a variety of other spectroscopic and biochemical techniques and quantum chemical calculations, we have developed a general picture of the interplay between the retinal ligand and the receptor protein, which is compared to similar reaction mechanisms in invertebrate photoreceptors and microbial retinal proteins.