Journal of Biological Chemistry
Volume 288, Issue 50, 13 December 2013, Pages 35714-35725
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Protein Structure and Folding
Structure of the Cyanobacterial Phytochrome 2 Photosensor Implies a Tryptophan Switch for Phytochrome Signaling*

https://doi.org/10.1074/jbc.M113.510461Get rights and content
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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.

Cyanobacteria
Protein Conformation
Signal Transduction
Signaling
Structural Biology
Biliprotein
c-di-GMP Signaling
Photochromicity
Phytochrome
Red Light Photoreceptor

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The atomic coordinates and structure factors (code 4BWI) have been deposited in the Protein Data Bank (http://wwpdb.org/).

*

This work was supported by Deutsche Forschungsgemeinschaft Grants ES152/9-1 and MR81/3-1 and the LOEWE Center for Synthetic Microbiology (to L.-O. E.).

This article contains supplemental Figs. S1 and S2 and Tables S1 and S2.