Activation mechanisms of G proteins by seven-transmembrane receptors : A view from rhodopsin studies

Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan E-mail : shichida@photo2.biophys.kyoto-u.ac.jp Phone : 81-75-753-4213 Fax : 81-75-753-4210 Manuscript received 8 November 2000 Signal transduction cascades consisting of seven-transmembrane receptors, heterotrimeric G proteins and effector enzymes are ubiquitous in intracellular signalling. The visual transduction cascade in rod photoreceptor cells is the one most exten­sively investigated, where the G protein-coupled receptor rhodopsin receives a light signal and transfers the signal to the retinal G protein transducin. Light absorption causes conformational changes in the protein moiety of rhodopsin through cis-trans isomerization of the II-cis-retinal chromophore and finally leads to the formation of the active state. This article reviews recent spectroscopic and biochemical studies that enable us to elucidate the mechanism for activation of rhodopsin and that for the interactions between photoactivated rhodopsin and G protein (transducin). It also summarizes the features of receptor-G protein interaction surfaces on three types of G protein (Gi/o, Gs and Gq/11). The interaction surfaces arc widely diversified but the mechanisms of interaction have several points similar to those between rhodopsin and transducin. Thus, the rhodopsin-transducin system is one of the model systems to provide understanding of other GPCR-G protein sys­tems