title: Light-induced trimer to monomer transition in the main light-harvesting antenna complex of plants: thermo-optic mechanism.
abstract: The main chlorophyll a/b light-harvesting complex of photosystem II, LHCIIb, has earlier been shown to be capable of undergoing light-induced reversible structural changes and chlorophyll a fluorescence quenching in a way resembling those observed in granal thylakoids when exposed to excess light [Barzda, V., et al. (1996) Biochemistry 35, 8981-8985]. The nature and mechanism of this unexpected structural flexibility has not been elucidated. In this work, by using density gradient centrifugation and nondenaturing green gel electrophoresis, as well as absorbance and circular dichroic spectroscopy, we show that light induces a significant degree of monomerization, which is in contrast with the preferentially trimeric organization of the isolated complexes in the dark. Monomerization is accompanied by a reversible release of Mg ions, most likely from the outer loop of the complexes. These data, as well as the built-in thermal and light instability of the trimeric organization, are explained in terms of a simple theoretical model of thermo-optic mechanism, effect of fast thermal transients (local T-jumps) due to dissipated photon energies in the vicinity of the cation binding sites, which lead to thermally assisted elementary structural transitions. Disruption of trimers to monomers by excess light is not confined to isolated trimers and lamellar aggregates of LHCII but occurs in photosystem II-enriched grana membranes, intact thylakoid membranes, and whole plants. As indicated by differences in the quenching capability of trimers and monomers, the appearance of monomers could facilitate the nonphotochemical quenching of the singlet excited state of chlorophyll a. The light-induced formation of monomers may also be important in regulated proteolytic degradation of the complexes. Structural changes driven by thermo-optic mechanisms may therefore provide plants with a novel mechanism for regulation of light harvesting in excess light.
authors.affiliation: Institutes of Plant Biology and Biophysics, Biological Research Center, Hungarian Academy of Sciences, P.O. Box 521, H-6701 Szeged, Hungary. Gyozo@nucleus.szbk.u-szeged.hu
authors.forename: Gyozo
authors.initials: G
authors.lastname: Garab
authors.forename: Zoltán
authors.initials: Z
authors.lastname: Cseh
authors.forename: László
authors.initials: L
authors.lastname: Kovács
authors.forename: Subramanyam
authors.initials: S
authors.lastname: Rajagopal
authors.forename: Zsuzsanna
authors.initials: Z
authors.lastname: Várkonyi
authors.forename: Mark
authors.initials: M
authors.lastname: Wentworth
authors.forename: László
authors.initials: L
authors.lastname: Mustárdy
authors.forename: András
authors.initials: A
authors.lastname: Dér
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Ruban
authors.forename: Elemér
authors.initials: E
authors.lastname: Papp
authors.forename: Andreas
authors.initials: A
authors.lastname: Holzenburg
authors.forename: Peter
authors.initials: P
authors.lastname: Horton
mesh_terms: D029681:Arabidopsis Proteins / Q000737:chemistry / Q000302:isolation & purification; D002499:Centrifugation, Density Gradient; D002734:Chlorophyll / Q000737:chemistry; D000077194:Chlorophyll A; D002942:Circular Dichroism; D015180:Electrophoresis, Gel, Two-Dimensional; D004591:Electrophoresis, Polyacrylamide Gel; D008027:Light; D045342:Light-Harvesting Protein Complexes; D018532:Pisum sativum; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000302:isolation & purification; D045332:Photosystem II Protein Complex; D013053:Spectrophotometry; D018724:Spinacia oleracea; D013816:Thermodynamics; D020524:Thylakoids / Q000737:chemistry
chemical_list: D029681:Arabidopsis Proteins; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D000077194:Chlorophyll A