Light Signal Transduction Pathway from Flavin Chromophore to the Jα Helix of Arabidopsis Phototropin1

In the plant blue-light sensor phototropin, illumination of the chromophoric LOV domains causes activation of the serine/threonine kinase domain. Flavin mononucleotide (FMN) is a chromophore molecule in the two LOV domains (LOV1 and LOV2), but only LOV2 is responsible for kinase activation. Previous studies reported an important role of an additional helix connected to the C-terminal of LOV2 (J alpha helix) for the function of phototropin; however, it remains unclear how the J alpha helix affects light-induced structural changes in LOV2. In this study we compared light-induced protein structural changes of the LOV2 domain of Arabidopsis phot1 in the absence (LOV2-core) and presence (LOV2-J alpha) of the J alpha helix by Fourier-transform infrared spectroscopy. Prominent peaks were observed only in the amide-I region (1650 (-)/1625 (+) cm(-1)) of LOV2-J alpha at physiological temperatures (>= 260 K), corresponding to structural perturbation of the alpha-helix. The peaks were diminished by point mutation of functionally important amino acids such as Phe-556 between FMN and the beta-sheet, Gln-575 being hydrogen-bonded with FMN, and Ile-608 on the J alpha helix. We thus conclude that a light signal is relayed from FMN through these amino acids and eventually changes the interaction between LOV2-core and the J alpha helix in Arabidopsis phot1.