title: Different dark conformations function in color-sensitive photosignaling by the sensory rhodopsin I-HtrI complex.
abstract: The haloarchaeal phototaxis receptor sensory rhodopsin I (SRI) in complex with its transducer HtrI delivers an attractant signal from excitation with an orange photon and a repellent signal from a second near-UV photon excitation. Using a proteoliposome system with purified SRI in complex with its transducer HtrI, we identified by site-directed fluorescence labeling a site (Ser(155)) on SRI that is conformationally active in signal relay to HtrI. Using site-directed spin labeling of Ser(155)Cys with a nitroxide side chain, we detected a change in conformation following one-photon excitation such that the spin probe exhibits a splitting of the outer hyperfine extrema (2A'(zz)) significantly smaller than that of the electron paramagnetic resonance spectrum in the dark state. The dark conformations of five mutant complexes that do not discriminate between orange and near-UV excitation show shifts to lower or higher 2A'(zz) values correlated with the alterations in their motility behavior to one- and two-photon stimuli. These data are interpreted in terms of a model in which the dark complex is populated by two conformers in the wild type, one that inhibits the CheA kinase (A) and the other that activates it (R), shifted in the dark by mutations and shifted in the wild-type SRI-HtrI complex in opposite directions by one-photon and two-photon reactions.
authors.affiliation: Center for Membrane Biology, Department of Biochemistry & Molecular Biology, University of Texas Medical School, Houston, Texas 77030, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Sasaki
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Phillips
authors.forename: Xinpu
authors.initials: X
authors.lastname: Chen
authors.forename: Ned
authors.initials: N
authors.lastname: Van Eps
authors.forename: Ah-Lim
authors.initials: AL
authors.lastname: Tsai
authors.forename: Wayne L
authors.initials: WL
authors.lastname: Hubbell
authors.forename: John L
authors.initials: JL
authors.lastname: Spudich
mesh_terms: D019843:Archaeal Proteins / Q000737:chemistry* / Q000235:genetics; D025602:Halorhodopsins / Q000737:chemistry* / Q000235:genetics; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics; D009154:Mutation; D011487:Protein Conformation; D025603:Sensory Rhodopsins / Q000737:chemistry* / Q000235:genetics; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D019843:Archaeal Proteins; D025602:Halorhodopsins; D008565:Membrane Proteins; C061497:SRI protein, Halobacterium; D025603:Sensory Rhodopsins; C082504:htrI protein, Halobacterium salinarium