Curated Optogenetic Publication Database

Abstract: Light-oxygen-voltage (LOV) receptors sense blue light through the photochemical generation of a covalent adduct between a flavin-nucleotide chromophore and a strictly conserved cysteine residue. Here we show that, after cysteine removal, the circadian-clock LOV-protein Vivid still undergoes light-induced dimerization and signalling because of flavin photoreduction to the neutral semiquinone (NSQ). Similarly, photoreduction of the engineered LOV histidine kinase YF1 to the NSQ modulates activity and downstream effects on gene expression. Signal transduction in both proteins hence hinges on flavin protonation, which is common to both the cysteinyl adduct and the NSQ. This general mechanism is also conserved by natural cysteine-less, LOV-like regulators that respond to chemical or photoreduction of their flavin cofactors. As LOV proteins can react to light even when devoid of the adduct-forming cysteine, modern LOV photoreceptors may have arisen from ancestral redox-active flavoproteins. The ability to tune LOV reactivity through photoreduction may have important implications for LOV mechanism and optogenetic applications.

Abstract: Light, oxygen, or voltage (LOV) protein domains are present in many signaling proteins in bacteria, archaea, protists, plants, and fungi. The LOV protein VIVID (VVD) of the filamentous fungus Neurospora crassa enables the organism to adapt to constant or increasing amounts of light and facilitates proper entrainment of circadian rhythms. Here, we determined the crystal structure of the fully light-adapted VVD dimer and reveal the mechanism by which light-driven conformational change alters the oligomeric state of the protein. Light-induced formation of a cysteinyl-flavin adduct generated a new hydrogen bond network that released the amino (N) terminus from the protein core and restructured an acceptor pocket for binding of the N terminus on the opposite subunit of the dimer. Substitution of residues critical for the switch between the monomeric and the dimeric states of the protein had profound effects on light adaptation in Neurospora. The mechanism of dimerization of VVD provides molecular details that explain how members of a large family of photoreceptors convert light responses to alterations in protein-protein interactions.