Structure of a novel photoreceptor, the BLUF domain of AppA from Rhodobacter sphaeroides

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 angstrom resolution. The domain demonstrates a new function for the common ferredoxin-like fold; two long a-helices flank the flavin, which is bound with its iscialloxazine ring perpendicular to a five-stranded beta-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 04 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.