Arguments for an additional long-lived intermediate in the photocycle of the full-length aureochrome 1c receptor: A time-resolved small-angle X-ray scattering study.

Aureochromes (AUREO) act as blue-light photoreceptors in algae. They consist of a light-, oxygen-, voltage-sensitive (LOV) domain and a DNA-binding basic region/leucine zipper. Illumination of the flavin cofactor in LOV leads to the formation of an adduct, followed by global structural changes. Here, we first applied UV/vis spectroscopy to characterize the photocycle of full-length aureochrome 1c (PtAUREO1c) from the diatom Phaeodactylum tricornutum. With a time constant of 850 s and a quantum yield of 23%, PtAUREO1c reveals a faster recovery time and a much lower sensitivity toward light than PtAUREO1a, pointing to its role as a high light sensor in vivo. UV/vis spectroscopy offers details on the local recovery of the flavin chromophore. However, kinetic information on the global structural recovery of full-length AUREO or any other multidomain LOV protein is missing. This information is essential not least for the photoreceptors' applications as optogenetic devices. Therefore, we established a procedure to apply small-angle X-ray scattering on PtAUREO1c in a time-resolved manner employing an in-house setup. In combination with UV/vis spectroscopy under similar conditions, we revealed a discrepancy between the recovery of the global protein structure and the adduct lifetime. Accordingly, we propose to supplement the photocycle by an intermediate state (I447), which decays with a time constant of about 800 s and prolongs the lifetime of the signaling state.