Curated Optogenetic Publication Database

Abstract: Phototropins are light-activated kinases important for plant responses to blue light. Light initiates signaling in these proteins by generating a covalent protein-flavin mononucleotide (FMN) adduct within sensory Per-ARNT-Sim (PAS) domains. We characterized the light-dependent changes of a phototropin PAS domain by solution nuclear magnetic resonance spectroscopy and found that an alpha helix located outside the canonical domain plays a key role in this activation process. Although this helix associates with the PAS core in the dark, photoinduced changes in the domain structure disrupt this interaction. We propose that this mechanism couples light-dependent bond formation to kinase activation and identifies a signaling pathway conserved among PAS domains.

Abstract: For single-cell and multicellular systems to survive, they must accurately sense and respond to their cellular and extracellular environment. Light is a nearly ubiquitous environmental factor, and many species have evolved the capability to respond to this extracellular stimulus. Numerous photoreceptors underlie the activation of light-sensitive signal transduction cascades controlling these responses. Here, we review the properties of the light, oxygen, or voltage (LOV) family of blue-light photoreceptor domains, a subset of the Per-ARNT-Sim (PAS) superfamily. These flavin-binding domains, first identified in the higher-plant phototropins, are now shown to be present in plants, fungi, and bacteria. Notably, LOV domains are coupled to a wide array of other domains, including kinases, phosphodiesterases, F-box domains, STAS domains, and zinc fingers, which suggests that the absorption of blue light by LOV domains regulates the activity of these structurally and functionally diverse domains. LOV domains contain a conserved molecular volume extending from the flavin cofactor, which is the locus for light-driven structural change, to the molecular surface. We discuss the role of this conserved volume of structure in LOV-regulated processes.

Abstract: Phototropin is the designation originally assigned to a recently characterized chromoprotein that serves as a photoreceptor for phototropism. Phototropin is a light-activated autophosphorylating serine/threonine kinase that binds two flavin mononucleotide (FMN) molecules that function as blue light-absorbing chromophores. Each FMN molecule is bound in a rigid binding pocket within specialized PAS (PER-ARNT-SIM superfamily) domains, known as LOV (light, oxygen, or voltage) domains. This article reviews the detailed photobiological and biochemical characterization of the light-activated phosphorylation reaction of phototropin and follows the sequence of events leading to the cloning, sequencing, and characterization of the gene and the subsequent biochemical characterization of its encoded protein. It then considers recent biochemical and photochemical evidence that light activation of phototropin involves the formation of a cysteinyl adduct at the C(4a) position of the FMN chromophores. Adduct formation causes a major conformational change in the chromophores and a possible conformational change in the protein moiety as well. The review concludes with a brief discussion of the evidence for a second phototropin-like protein in Arabidopsis and rice. Possible roles for this photoreceptor are discussed.