Benjamin Perman, Vukica rajer, Zhong Ren, Tsu-yi Teng, Claude Pradervand, Thomas Ursby, Dominique Bourgeois, Friederich Schotte, Michael Wulff, Remco Kort, Klaas Hellingwerf, Keith Moffat ^*

Photoactive yellow protein (PYP) is a member of the xanthopsin family of eubacterial blue-light photoreceptors. On absorption of light, PYP enters a photocycle that ultimately transduces the energy contained in a light signal into an altered biological response. Nanosecond time-resolved x-ray crystallography was used to determine the structure of the short-lived, red-shifted, intermediate state denoted [pR], which develops within 1 nanosecond after photoelectronic excitation of the chromophore of PYP by absorption of light. The resulting structural model demonstrates that the [pR] state possesses the cis conformation of the 4-hydroxyl cinnamic thioester chromophore, and that the process of trans to cis isomerization is accompanied by the specific formation of new hydrogen bonds that replace those broken upon excitation of the chromophore. Regions of flexibility that compose the chromophore-binding pocket serve to lower the activation energy barrier between the dark state, denoted pG, and [pR], and help initiate entrance into the photocycle. Direct structural evidence is provided for the initial processes of transduction of light energy, which ultimately translate into a physiological signal.

B. Perman, Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, IL 60637, USA.
V. rajer, Z. Ren, T.-y. Teng, C. Pradervand, K. Moffat, Department of Biochemistry and Molecular Biology and the Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA.
T. Ursby, Molecular Biophysics, Chemical Center, Lund University, Post Office Box 124, S-221 00 Lund, Sweden.
D. Bourgeois, F. Schotte, M. Wulff, European Synchrotron Radiation Facility, 38043 Grenoble Cedex, France.
R. Kort and K. Hellingwerf, Laboratory for Microbiology, E. C. Slater Institute, 1018 WS Amsterdam, Netherlands.
^*   To whom correspondence should be addressed. E-mail: moffat{at}cars.uchicago.edu