Architecture of the chloroplast PSI-NDH supercomplex inHordeum vulgare

Oxygen-containing photosynthesis uses solar energy todrive the oxidation of water and the reduction of carbon dioxide (CO₂)to produce oxygen and carbohydrates, which are essential for most life onearth. In the photoreaction, photosystem I (PSI) and photosystem II (PSII)convert light energy into ATP and NADPH, which are used to fix CO₂ to form carbohydrates in the Calvin-Benson cycle. There are two types ofphotosynthetic electron transfer pathways in the photoreaction process: linearand cyclic electron transfer. NDH-mediated PSI cyclic electron transfergenerates a transmembrane proton gradient for synthesizing more ATP withoutproducing NADPH, playing an important role in supplying sufficient ATP for carbonfixation. Dr.Zhang's group determined the high-resolution structure of higherplant PSI-NDH complex, which reveals the structural basis of PSI-NDH-mediatedregulation of photosynthetic cyclic electron transport. Our results show thatthe PSI-NDH complex contains 2 PSI-LHCI, 1 NDH, and an unknown protein USP. Itcontains 55 protein subunits, 298 chlorophyll molecules, 67 carotenoidmolecules, and 25 lipid molecules. The molecular weight is about 1.6 MDa; The resultsalso reveal the precise location and structural characteristics of the specialantenna subunits Lhca5 and Lhca6 in PSI-LHCI. These two unique LHCI subunitsmediate the interactions between PSI-LHCI and NDH; The new subunits interacttightly with the intramembrane subunits of NDH, maintaining the structuralstability of PSI-LHCI-NDH which are essential for complex assembly; inaddition, since the chloroplast NDH complex accepts electrons from Fd, thecombination of NDH and two PSIs will increase number of Fd may promote thetransfer of electrons from PSI to NDH. Especially under low light conditions, suchcombination can increase efficiency of electron transfer. These results are notonly important for understanding the mechanism of photosynthetic cyclicelectron transfer, for studying the adaptation of angiosperms to theterrestrial light environment during evolution, but also for improving thelight energy conversion, efficiency of CO₂ fixation and stressresistance of plants.