Blue Light-Induced Conformational Changes in a Light-Regulated Transcription Factor, Aureochrome-1

Aureochrome-1 (AUREO1) is a blue light (BL) receptor that mediates the branching response in the stramenopile alga, Vaucheria frigida. AUREO1 harbors a basic leucine zipper (bZIP) domain at the N-terminus and a light-oxygen-voltage-sensing (LOV) domain within the C-terminal region, and has been suggested to function as a light-regulated transcription factor. To understand the molecular mechanism of AUREO1, we have prepared three recombinant proteins: a full-length AUREO1 (FL), an N-terminal truncated construct containing bZIP and LOV (ZL) and a LOV-only (LOV) construct. The constructs showed the same absorption and fluorescent spectra in the dark state and underwent the characteristic cyclic reaction as previously observed in LOV domains upon BL excitation. FL and ZL bound to DNA in a sequence-specific manner. BL appeared to induce a shift of the alpha-helical structure of the LOV domain to a beta-sheet structure, but did not alter the hydrodynamic radius (R-H) of this domain. ZL formed a dimer possibly through disulfide linkages in the bZIP and the linker region between bZIP and LOV. BL induced an approximately 5% increase in the R-H of ZL, although its secondary structure was unchanged. These results support a schema where BL-induced changes in the LOV domain may cause conformational changes in the bZIP and/or the linker of a dimeric ZL molecule. Since a 5% increase of the R-H was also observed with the FL construct, BL may induce global conformational changes similar to those observed for ZL, and formation of the FL dimer may facilitate DNA binding.