Blue-Light-Induced Unfolding of the Jα Helix Allows for the Dimerization of Aureochrome-LOV from the Diatom <i>Phaeodactylum tricornutum</i>

Abstract

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 Jα helix of other LOV proteins to the terminus, raising the question of the role of Jα in aureochrome signaling. In phototropins, signaling proceeds from LOV2 via dissociation and unwinding of the Jα helix to the C-terminal kinase effector domain. In contrast, other LOV proteins have been demonstrated to activate the effector without the unfolding of Jα. We investigated the LOV domain of aureochrome1a from the diatom Phaeodactylum tricornutum both with and without the Jα helix. Fourier transform infrared difference spectroscopy provides evidence that the Jα 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-Jα than LOV. We therefore postulate that the Jα helix plays an important role in aureochrome signaling.