Abstract
BLUF (sensor of Blue Light Using FAD) protein PixD controls phototactic movement of cells. The X-ray crystal structural analysis of the SyPixD of synechocystis sp. PCC6803 at 1.8 A resolution showed a high-ordered decameric structure formed of a pair of pentameric SyPixD rings. Photoexcitation of a FAD chromophore in PixD induces decomposition of the complex into dimmers. It is proposed that the light-induced decomposition regulates the optical response in cells. PELDOR (Pulsed ELectron elctron DOuble Resonance) was applied to the SyPixD multimers to determine the relative arrangement of each SyPixD monomer in the complex. A Pake's doublet type EPR signal was detected at 10 K after preillumination at 150 K for 30 min and indicated a magnetic dipole interaction between a neutral flabosemiquinone radical FADH and a neutral tyrosine radical Y8. PELDOR envelope signal induced by the selective excitation was modulated with the dipolar coupling frequency depending on the arrangements of radical pairs. We will discuss the distance and relative orientation between proteins in a SyPixD multimer.
