Light-induced structural changes of apo-protein and chromophore in the BLUF domain of AppA for a signaling state

Abstract

AppA is a novel blue-light receptor controlling photosynthesis gene expression in purple bacteria. Light-induced FTIR spectrum of the AppA showed simple features, which were composed of two sets of derivative-shaped sharp bands at 1709(-)/1695(+) and 1632(+)/1619(-) cm^-1. We reconstituted the BLUF domain from free FAD and an apo-protein isotopically labeled with ¹³C, and compared the spectra with those for uniformly ¹⁵N-, ¹³C-labeled or unlabeled sample. It was shown that the light-induced FTIR spectrum of the unlabeled BLUF domain of AppA was predominantly composed of multiple apo-protein bands, while a C(4)=O stretching of an isoalloxazine ring was the only band exclusively assigned to FAD. The results showed that relatively large structural changes occur in the protein backbone of the BLUF domain of AppA upon illumination. These changes will be discussed in relation to the mechanistic role of the BLUF domain in the process of blue-light perception by AppA.