Structure of the Cyanobacterial Phytochrome 2 Photosensor Implies a Tryptophan Switch for Phytochrome Signaling

Background: Phytochromes are red/far-red photoreceptors using a bilin chromophore. Results: Compared with Cph1, the Cph2 bilin-binding site differs around the propionates, but utilizes an otherwise conserved tongue for sealing the chromophore from solvent. Conclusion: The tongue signals via a tryptophan switch within the tongue-GAF domain interface. Significance: The first structure of a Cph2-type phytochrome indicates a common mechanism for photoswitching in all canonical phytochromes. 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- crystal structure of its red/far-red responsive photosensory module in the P-r 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 P-r P-fr 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.