title: Structure of the cyanobacterial phytochrome 2 photosensor implies a tryptophan switch for phytochrome signaling.
abstract: Phytochromes are highly versatile photoreceptors, which occur ubiquitously in plants as well as in many light-responsive microorganisms. Here, photosynthetic cyanobacteria utilize up to three different phytochrome architectures, where only the plant-like and the single-domain cyanobacteriochromes are structurally characterized so far. Cph2 represents a third group in Synechocystis species and affects their capability of phototaxis by controlling c-di-GMP synthesis and degradation. The 2.6-Å crystal structure of its red/far-red responsive photosensory module in the Pr state reveals a tandem-GAF bidomain that lacks the figure-of-eight knot of the plant/cph1 subfamily. Its covalently attached phycocyanobilin chromophore adopts a highly tilted ZZZssa conformation with a novel set of interactions between its propionates and the GAF1 domain. The tongue-like protrusion from the GAF2 domain interacts with the GAF1-bound chromophore via its conserved PRXSF, WXE, and W(G/A)G motifs. Mutagenesis showed that the integrity of the tongue is indispensable for Pr → Pfr photoconversion and involves a swap of the motifs' tryptophans within the tongue-GAF1 interface. This "Trp switch" is supposed to be a crucial element for the photochromicity of all multidomain phytochromes.
authors.affiliation: From the Department of Chemistry, Biomedical Research Centre, Philipps-Universität, D-35032 Marburg, Germany.
authors.forename: Katrin
authors.initials: K
authors.lastname: Anders
authors.forename: Grazia
authors.initials: G
authors.lastname: Daminelli-Widany
authors.forename: Maria Andrea
authors.initials: MA
authors.lastname: Mroginski
authors.forename: David
authors.initials: D
authors.lastname: von Stetten
authors.forename: Lars-Oliver
authors.initials: LO
authors.lastname: Essen
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010788:Photosynthesis; D052980:Phycobilins / Q000378:metabolism; D010798:Phycocyanin / Q000378:metabolism; D010834:Phytochrome / Q000737:chemistry* / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D015398:Signal Transduction; D046939:Synechocystis / Q000166:cytology* / Q000378:metabolism*; D014364:Tryptophan
chemical_list: D001426:Bacterial Proteins; C413622:Cph2 protein, bacteria; D052980:Phycobilins; D010798:Phycocyanin; D010834:Phytochrome; C011069:phycocyanobilin; D014364:Tryptophan
keywords: Biliprotein; Cyanobacteria; Photochromicity; Phytochrome; Protein Conformation; Red Light Photoreceptor; Signal Transduction; Signaling; Structural Biology; c-di-GMP Signaling