title: HAMP domain signal relay mechanism in a sensory rhodopsin-transducer complex.
abstract: The phototaxis receptor complex composed of sensory rhodopsin II (SRII) and the transducer subunit HtrII mediates photorepellent responses in haloarchaea. Light-activated SRII transmits a signal through two HAMP switch domains (HAMP1 and HAMP2) in HtrII that bridge the photoreceptive membrane domain of the complex and the cytoplasmic output kinase-modulating domain. HAMP domains, widespread signal relay modules in prokaryotic sensors, consist of four-helix bundles composed of two helices, AS1 and AS2, from each of two dimerized transducer subunits. To examine their molecular motion during signal transmission, we incorporated SRII-HtrII dimeric complexes in nanodiscs to allow unrestricted probe access to the cytoplasmic side HAMP domains. Spin-spin dipolar coupling measurements confirmed that in the nanodiscs, SRII photoactivation induces helix movement in the HtrII membrane domain diagnostic of transducer activation. Labeling kinetics of a fluorescein probe in monocysteine-substituted HAMP1 mutants revealed a light-induced shift of AS2 against AS1 by one-half α-helix turn with minimal other changes. An opposite shift of AS2 against AS1 in HAMP2 at the corresponding positions supports the proposal from x-ray crystal structures by Airola et al. (Airola, M. V., Watts, K. J., Bilwes, A. M., and Crane, B. R. (2010) Structure 18, 436-448) that poly-HAMP chains undergo alternating opposite interconversions to relay the signal. Moreover, we found that haloarchaeal cells expressing a HAMP2-deleted SRII-HtrII exhibit attractant phototaxis, opposite from the repellent phototaxis mediated by the wild-type di-HAMP SRII-HtrII complex. The opposite conformational changes and corresponding opposite output signals of HAMP1 and HAMP2 imply a signal transmission mechanism entailing small shifts in helical register between AS1 and AS2 alternately in opposite directions in adjacent HAMPs.
authors.affiliation: Center for Membrane Biology, Department of Biochemistry and Molecular Biology, Houston, Texas 77030, USA.
authors.forename: Jihong
authors.initials: J
authors.lastname: Wang
authors.forename: Jun
authors.initials: J
authors.lastname: Sasaki
authors.forename: Ah-Lim
authors.initials: AL
authors.lastname: Tsai
authors.forename: John L
authors.initials: JL
authors.lastname: Spudich
mesh_terms: D019843:Archaeal Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002338:Carotenoids / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018360:Crystallography, X-Ray; D006216:Halobacteriaceae / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D046912:Multiprotein Complexes / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D025604:Rhodopsins, Microbial / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013329:Structure-Activity Relationship
chemical_list: D019843:Archaeal Proteins; D046912:Multiprotein Complexes; D025604:Rhodopsins, Microbial; C434450:phototaxis receptor sensory rhodopsin II, Natronobacterium pharaonis; D002338:Carotenoids