A trapped double bond-photoisomerization intermediate in a bacterial photoreceptor

The GAF3 domain of cyanobacteriochrome Slr1393 (Synechocystis PCC6803) with an in vivo assembled phycocyanobilin (PCB) chromophore has been crystallized in parental state (1.8 A) and photoproduct state (1.86 A), identified by 15-Z and 15-E chromophore configuration. Comparison of both structures for the same protein allows precise determination of structural changes after photo-activation. The chromophore photoisomerization causes an outward movement and partial helix formation of a formerly unstructured loop. A tryptophan residue located in this loop, in π-π stacking distance to PCB in the dark state, moves away by 14 A opening the binding cleft for the entry of water molecules. Also the in vitro assembled protein (chromophore addition to apo-protein) has been crystallized (1.6 A resolution). Most importantly, an intermediate structure was solved (2.1 A) with the protein in photoproduct conformation and the chromophore already isomerized into the parental 15-Z configuration, thereby giving insight into chromophore-initiated conformational protein changes.