PUB - Publikationen an der Universität Bielefeld

Aureochromes have recently been shown to act as blue-light-regulated transcription factors in the stramenopile alga Vaucheria frigida. They comprise a light-, oxygen-, or voltage-sensitive (LOV) domain as a sensory module with flavin mononucleotide (FMN) as a chromophore and a basic region leucine zipper (bZIP) domain as an effector. Aureochromes are the only members of a large LOV protein family, where the effector domain is located N-terminal to the sensor domain. This domain inversion positions the linking J alpha helix of other LOV proteins to the terminus, raising the question of the role of J alpha in aureochrome signaling. In phototropins, signaling proceeds from LOV2 via dissociation and unwinding of the J alpha helix to the C-terminal kinase effector domain. In contrast, other LOV proteins have been demonstrated to activate the effector without the unfolding of J alpha. We investigated the LOV domain of aureochrome la from the diatom Phaeodactylum tricornutum both with and without the J alpha helix. Fourier transform infrared difference spectroscopy provides evidence that the J alpha helix unfolds upon illumination. This unfolding is prerequisite for light-induced dimerization of LOV. Under illumination, full conversion to the dimer was observed by size exclusion chromatography. In the absence of the helix, a monomer was detected in the dark and in the light. As a further effect, the recovery of the dark state is 6-fold slower in LOV-J alpha than LOV. We therefore postulate that the J alpha helix plays an important role in aureochrome signaling.