title: Retinal counterion switch in the photoactivation of the G protein-coupled receptor rhodopsin.
abstract: The biological function of Glu-181 in the photoactivation process of rhodopsin is explored through spectroscopic studies of site-specific mutants. Preresonance Raman vibrational spectra of the unphotolyzed E181Q mutant are nearly identical to spectra of the native pigment, supporting the view that Glu-181 is uncharged (protonated) in the dark state. The pH dependence of the absorption of the metarhodopsin I (Meta I)-like photoproduct of E181Q is investigated, revealing a dramatic shift of its Schiff base pKa compared with the native pigment. This result is most consistent with the assignment of Glu-181 as the primary counterion of the retinylidene protonated Schiff base in the Meta I state, implying that there is a counterion switch from Glu-113 in the dark state to Glu-181 in Meta I. We propose a model where the counterion switch occurs by transferring a proton from Glu-181 to Glu-113 through an H-bond network formed primarily with residues on extracellular loop II (EII). The resulting reorganization of EII is then coupled to movements of helix III through a conserved disulfide bond (Cys110-Cys187); this process may be a general element of G protein-coupled receptor activation.
authors.affiliation: Department of Chemistry, University of California, Berkeley, CA 94720, USA.
authors.forename: Elsa C Y
authors.initials: EC
authors.lastname: Yan
authors.forename: Manija A
authors.initials: MA
authors.lastname: Kazmi
authors.forename: Ziad
authors.initials: Z
authors.lastname: Ganim
authors.forename: Jian-Min
authors.initials: JM
authors.lastname: Hou
authors.forename: Douhai
authors.initials: D
authors.lastname: Pan
authors.forename: Belinda S W
authors.initials: BS
authors.lastname: Chang
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Sakmar
authors.forename: Richard A
authors.initials: RA
authors.lastname: Mathies
mesh_terms: D000818:Animals; D002417:Cattle; D019204:GTP-Binding Proteins / Q000737:chemistry* / Q000502:physiology; D018698:Glutamic Acid / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008027:Light; D008956:Models, Chemical; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D012160:Retina / Q000378:metabolism*; D012243:Rhodopsin / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000502:physiology; D013059:Spectrum Analysis, Raman; D013696:Temperature; D014466:Ultraviolet Rays
chemical_list: D007477:Ions; D018698:Glutamic Acid; C013110:metarhodopsins; D012243:Rhodopsin; D019204:GTP-Binding Proteins