title: Isolation of the elusive supercomplex that drives cyclic electron flow in photosynthesis.
abstract: Photosynthetic light reactions establish electron flow in the chloroplast's thylakoid membranes, leading to the production of the ATP and NADPH that participate in carbon fixation. Two modes of electron flow exist-linear electron flow (LEF) from water to NADP(+) via photosystem (PS) II and PSI in series and cyclic electron flow (CEF) around PSI (ref. 2). Although CEF is essential for satisfying the varying demand for ATP, the exact molecule(s) and operational site are as yet unclear. In the green alga Chlamydomonas reinhardtii, the electron flow shifts from LEF to CEF on preferential excitation of PSII (ref. 3), which is brought about by an energy balancing mechanism between PSII and PSI (state transitions). Here, we isolated a protein supercomplex composed of PSI with its own light-harvesting complex (LHCI), the PSII light-harvesting complex (LHCII), the cytochrome b(6)f complex (Cyt bf), ferredoxin (Fd)-NADPH oxidoreductase (FNR), and the integral membrane protein PGRL1 (ref. 5) from C. reinhardtii cells under PSII-favouring conditions. Spectroscopic analyses indicated that on illumination, reducing equivalents from downstream of PSI were transferred to Cyt bf, whereas oxidised PSI was re-reduced by reducing equivalents from Cyt bf, indicating that this supercomplex is engaged in CEF (Supplementary Fig. 1). Thus, formation and dissociation of the PSI-LHCI-LHCII-FNR-Cyt bf-PGRL1 supercomplex not only controlled the energy balance of the two photosystems, but also switched the mode of photosynthetic electron flow.
authors.affiliation: Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan.
authors.forename: Masakazu
authors.initials: M
authors.lastname: Iwai
authors.forename: Kenji
authors.initials: K
authors.lastname: Takizawa
authors.forename: Ryutaro
authors.initials: R
authors.lastname: Tokutsu
authors.forename: Akira
authors.initials: A
authors.lastname: Okamuro
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Takahashi
authors.forename: Jun
authors.initials: J
authors.lastname: Minagawa
mesh_terms: D000255:Adenosine Triphosphate / Q000096:biosynthesis / Q000378:metabolism; D016825:Chlamydomonas reinhardtii / Q000201:enzymology / Q000378:metabolism*; D045346:Cytochrome b6f Complex / Q000378:metabolism; D004579:Electron Transport; D004583:Electrons; D005287:Ferredoxin-NADP Reductase / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism; D046912:Multiprotein Complexes / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism*; D010084:Oxidation-Reduction; D010788:Photosynthesis / Q000502:physiology*; D045331:Photosystem I Protein Complex / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000378:metabolism
chemical_list: D045342:Light-Harvesting Protein Complexes; D046912:Multiprotein Complexes; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D000255:Adenosine Triphosphate; D045346:Cytochrome b6f Complex; D005287:Ferredoxin-NADP Reductase