Structure of the cyanobacterial phytochrome 2 photosensor implies a tryptophan switch for phytochrome signaling.

The Journal of biological chemistry

PubMedID: 24174528

Anders K, Daminelli-Widany G, Mroginski MA, von Stetten D, Essen LO. Structure of the cyanobacterial phytochrome 2 photosensor implies a tryptophan switch for phytochrome signaling. J Biol Chem. 2013;.
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)/AG motifs. Mutagenesis showed that the tongue's integrity 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 multi-domain phytochromes.