Symposium I: Tropisms
10001: Phototropins: Photoreceptors that provide a novel photochemical mechanism for signaling
Briggs, Winslow R., email@example.com||Authors||Briggs, Winslow R. (A) |
(A): Briggs, Winslow R., Carnegie Institution of Washington, Department of Plant Biology|
Phototropin1 and phototropin 2 are blue-light receptors that are known to mediate phototropism, stomatal opening, chloroplast movements, rapid inhibition of hypocotyl growth, and leaf expansion. The phototropins are chromoproteins that contain two very similar domains each of which binds a molecule of FMN, and a downstream serine/threonine kinase domain. The FMN molecules are each contained within a cage of beta sheets and alpha helices in domains designated LOV1 and LOV2 (similar domains in other signaling proteins are activated by Light, Oxygen or Voltage, hence LOV). Just downstream from LOV2 is an amphipathic alpha helix that in the dark state is tightly appressed to the surface of the LOV2 domain. Blue light absorption by the FMN chromophores leads to the formation of a covalent bond between a highly conserved cysteine within the LOV domain and the C(4a) carbon of the FMN isoalloxazine ring. The formation of this cysteinyl adduct leads to a change both in the conformation of the flavin and the LOV domain itself, bringing about an unfolding of the amphipathic alpha helix. The consequence of this unfolding is activation of the kinase domain and hierarchical phosphorylation. At least in one system association of a 14-3-3 protein with the phosphorylated phototropin has been shown by others to be an early step in signaling. This light-activated protein unfolding represents a unique photochemical mechanism for enzyme activation and the initiation of signal transduction. There is no amphipathic alpha helix downstream from LOV1 and LOV1 likely plays a role in modulating the signal generated by photoexcitation of LOV2.