Fig. 3.

The tongue and the chromophore binding pocket. (A) Space-filling model of Cph1 (Left) in comparison with known bacteriophytochrome structures (12, 13). The PCB chromophore (cyan) is completely sealed from solvent access by the tongue (dark red) in contrast to the exposed biliverdin (green) in the incomplete bidomains. (B and C) The tripartite PCB-binding pocket of Cph1 comprising the GAF-domain (orange), the tongue-like protrusion from the PHY domain (red) and the N-terminal α₁-helix (green). Waters are shown as red spheres. (B) Edge-on view of the pocket highlighting the collinear arrangement of the N-terminal α₁-helix and α₇-helix of the GAF domain and their interaction with the chromophore and the tongue. (C) The conformation of the PCB chromophore (cyan) within the PCB-binding site adopts a ZZZssa configuration similar to that of BV in bacteriophytochromes (12, 13). For clarity, α₈-helix of the GAF domain as well as Tyr-263 and Phe-475 have been omitted.