13C Isotopologue editing of FMN bound to phototropin domains.

The plant blue light receptor phototropin comprises a protein kinase domain and two FMN-binding LOV domains (LOV1 and LOV2). Blue light irradiation of recombinant LOV domains is conducive to the addition of a cysteinyl thiolate group to carbon 4a of the FMN chromophore, and spontaneous cleavage of that photoadduct completes the photocycle of the receptor. The present study is based on C-13 NMR signal modulation observed after reconstitution of LOV domains of different origins with random libraries of C-13-labeled FMN isotopologues. Using this approach, all C-13 signals of FMN bound to LOV1 and LOV2 domains of Avena sativa and to the LOV2 domain of the fern, Adiantum capillus-veneris, could be unequivocally assigned under dark and under blue light irradiation conditions. C-13 Chemical shifts of FMN are shown to be differently modulated by complexation with the LOV domains under study, indicating slight differences in the binding interactions of FMN and the apoproteins.