Web Release Date: May 10,
Structure of a Novel Photoreceptor, the BLUF Domain of AppA from Rhodobacter
sphaeroides
and
Departments of Biology and Chemistry, Indiana University, Bloomington, Indiana 47405, Consortium for Advanced Radiation Sources, Department of Biochemistry and Molecular Biology, and Institute for Biophysical Dynamics, University of Chicago, Chicago, Illinois 60637, and Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, Japan
Received February 14, 2005
Revised Manuscript Received April 13, 2005
Abstract:
The flavin-binding BLUF domain of AppA represents a new class of blue light photoreceptors
that are present in a number of bacterial and algal species. The dark state X-ray structure of this domain
was determined at 2.3 Å resolution. The domain demonstrates a new function for the common ferredoxin-like fold; two long 
-helices flank the flavin, which is bound with its isoalloxazine ring perpendicular to
a five-stranded 
-sheet. The hydrogen bond network and the overall protein topology of the BLUF domain
(but not its sequence) bear some resemblance to LOV domains, a subset of PAS domains widely involved
in signaling. Nearly all residues conserved in BLUF domains surround the flavin chromophore, many of
which are involved in an intricate hydrogen bond network. Photoactivation may induce a rearrangement
in this network via reorientation of the Gln63 side chain to form a new hydrogen bond to the flavin O4
position. This shift would also break a hydrogen bond to the Trp104 side chain, which may be critical in
induction of global structural change in AppA.
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