Most casual observers have likely noticed that seedlings on a windowsill will grow toward the light. This phenomenon, known as phototropism, is a manifestation of a sensitive system plants have for detecting light. This light sensing system guides seedlings through the soil and has profound influences on their development during the critical stage of seedling establishment and later as the leaves adapt to changes in the light environment.
Briggs's research group has discovered the two-member family of protein molecules that serves as the detector and decoder of the blue photons on which the seedling cues to determine the direction of light. The molecule, known as phototropin, is now being intensively studied because of its unique properties by chemists and biophysicists as well as plant biologists.
Professor Briggs began experimenting on how plants detect the direction and intensity of light in the 1950's, but he certainly was not the first to be drawn to this fascinating example of sensory biology. For example, Charles Darwin and his son were drawn to the phenomenon and performed some classic experiments that paved the way for further studies, including Briggs' successful approaches. The Darwins could not have imagined that the topic would in the 21st century be studied at the molecular level as it is now.
Professor Briggs will present his group's latest findings on the topic at the ASPB Annual meeting during the Major Symposium on Tropisms 4 p.m. to 6:30 p.m. Saturday, July 24, 2004 in Coronado Ballrooms L-T at Disney's Coronado Springs Resort & Convention Center.
The title and abstract of Professor Briggs presentation July 24 are at: http://abstracts.aspb.org/pb2004/public/S01/9179.html and follow:
Abs # 10001: Phototropins: Photoreceptors that provide a novel photochemical mechanism for signaling.
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.
Presenter and Author: Briggs, Winslow R., briggs@stanford.edu phone number 650-325-1521, ext. 207 Carnegie Institution of Washington, Department of Plant Biology