| Light is the original source of energy for almost all current forms of life. Most organisms also use light to determine their daily and annual rhythms.;Photoreceptors form the interface between the physical light environment and the biological signaling processes in a cell. This location at the intersection of physics and biology poses problems that are unique among sensors. The light signal is very short lived and is destroyed immediately when the receptor interacts with the signal. The response to the capture of a photon therefore has to be extremely rapid. Yet at the same time the encounter has to result in a structural signal that can be passed on to a signal transduction cascade that employs regular biochemical mechanisms.;To understand how a photoreceptor is able to accomplish those two tasks we have determined the structure of three structures (ground state, early intermediate, signaling intermediate) that occur during the light cycle of Photoactive Yellow Protein (PYP) using ultra high-resolution, freeze trapping and true time-resolved crystallographic techniques. The three structures show that PYP first uses the captured photon to cause trans-to- cis isomerization of its 4-hydroxy cinnamic acid chromophore in an extremely rapid step involving only minimal atomic movements. In the second step, this change in the chromophore configuration is recognized by the protein, causing rearrangements in the active site region that propagate to the protein surface where they can be recognized by a downstream signaling partner. Based on these results we proposed a general model for protein light cycles and tested this model through the generation of site-directed mutations of key active site residues. |
