STRUCTURE AT 0.85 ANGSTROM RESOLUTION OF AN EARLY PROTEIN PHOTOCYCLE INTERMEDIATE

Protein photosensors from all kingdoms of life(1,2) use bound organic molecules, known as chromophores, to detect light, A specific double b ond within each chromophore is isomerized by light, triggering slower changes in the protein as a whole, The initial movements of the chromo phore, which can occur in femtoseconds, are tightly constrained by the surrounding protein, making it difficult to see how isomerization can occur, be recognized, and be appropriately converted into a protein-w ide structural change and biological signal, Here we report how this d ilemma is resolved in the photoactive yellow protein (PYP). We trapped a key early intermediate in the light cycle of PYP at temperatures be low -100 degrees C, and determined its structure at better than 1 Angs trom resolution, The 4-hydroxycinnamoyl chromophore(3,4) isomerizes by flipping its thioester linkage with the protein, thus avoiding collis ions resulting from large-scale movement of its aromatic ring during t he initial light reaction, A protein-to-chromophore hydrogen bond that is present in both the preceding dark state(5) and the subsequent sig nalling state(6) of the photosensor breaks, forcing one of the hydroge n-bonding partners into a hydrophobic pocket. The isomerized bond is d istorted into a conformation resembling that in the transition state. The resultant stored energy is used to drive the PYP light cycle. Thes e results suggest a model for phototransduction, with implications for bacteriorhodopsin(7,8), photoactive proteins(1,2), PAS domains(9), an d signalling proteins.