Crystallographic structure of a photoreceptor protein at 2.4 A resolution.

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Abstract
The first essential step in protein photoreception is the capture and storage of energy from a photon. We have recently identified and isolated, from the purple photoautotrophic bacterium, Ectothiorhodospira halophila, a 13,000-dalton photoactive yellow protein (PVP) that has a photocycle with kinetics similar to sensory rhodopsin and a very high quantum efficiency. To study the structural chemistry of protein photoreception, we determined, refined, and analyzed the crystallographic structure of PVP at 2.4 .ANG. resolution and report here that it is composed of two perpendicular antiparallel .beta.-sheets that enclose the chromophore. Each of the 10 .beta.-strands of PYP is connected directly to its nearest neighbor with +1 topology. Globally, an asymmetric distribution of side chains places aromatic and acidic side chains in an ellipsoidal band around the chromophore with a cluster of basic side chains on one side. Locally, the electron density maps plae an internal lysine and the chromophore in an apparent Schiff base linkage stablized by a buried glutamate and a tyrosine side chain. To our knowledge, the atomic resolution structure of a protein with a reversible photoisomerization has not been reported previously. Furthermore, PVP may also represent a class of proteins that bind conjugated molecules and interact with a secondary receptor system.