Photosystem I (PSI) is a pigment-protein complex required for the light-dependent reactions of photosynthesis and participates in light-harvesting and redox-driven chloroplast metabolism. Assembly of PSI into supercomplexes with light harvesting complex (LHC) II, cytochrome b6f (Cytb6f) or NAD(P)H dehydrogenase complex (NDH) has been proposed as a means for regulating photosynthesis. However, structural details about the binding positions in plant PSI are lacking. We analyzed large data sets of electron microscopy single particle projections of supercomplexes obtained from the stroma membrane of Arabidopsis thaliana. By single particle analysis, we established the binding position of Cytb6f at the antenna side of PSI. The rectangular-shaped Cytb6f dimer binds at the side where Lhca1 is located. The complex binds with its short side rather than its long side to PSI, which may explain why these supercomplexes are difficult to purify and easily disrupted. Refined analysis of the interaction between PSI and the NDH complex indicates that in total up to 6 copies of PSI can arrange with one NDH complex. Most PSI-NDH supercomplexes appeared to have 1-3 PSI copies associated. Finally, the PSI-LHCII supercomplex was found to bind an additional LHCII trimer at two positions on the LHCI side in Arabidopsis. The organization of PSI, either in a complex with NDH or with Cytb6f, may improve regulation of electron transport by the control of binding partners and distances in small domains.

• MeSH
• Arabidopsis metabolism   MeSH
• Chlorophyll metabolism   MeSH
• Chloroplasts metabolism   MeSH
• Photosynthesis physiology   MeSH
• Photosystem I Protein Complex metabolism   MeSH
• Cytochrome b6f Complex metabolism   MeSH
• NADH Dehydrogenase metabolism   MeSH
• Oxidation-Reduction MeSH
• Light MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Electron Transport physiology   MeSH
• Thylakoids metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Cyclic electron transport (CET) around photosystem I (PSI) plays an important role in balancing the ATP/NADPH ratio and the photoprotection of plants. The NAD(P)H dehydrogenase complex (NDH) has a key function in one of the CET pathways. Current knowledge indicates that, in order to fulfill its role in CET, the NDH complex needs to be associated with PSI; however, until now there has been no direct structural information about such a supercomplex. Here we present structural data obtained for a plant PSI-NDH supercomplex. Electron microscopy analysis revealed that in this supercomplex two copies of PSI are attached to one NDH complex. A constructed pseudo-atomic model indicates asymmetric binding of two PSI complexes to NDH and suggests that the low-abundant Lhca5 and Lhca6 subunits mediate the binding of one of the PSI complexes to NDH. On the basis of our structural data, we propose a model of electron transport in the PSI-NDH supercomplex in which the association of PSI to NDH seems to be important for efficient trapping of reduced ferredoxin by NDH.

• MeSH
• Microscopy, Electron MeSH
• Ferredoxins metabolism   MeSH
• Photosystem I Protein Complex chemistry   isolation & purification   metabolism   MeSH
• Hordeum chemistry   enzymology   radiation effects   MeSH
• Plant Leaves chemistry   enzymology   radiation effects   MeSH
• Models, Molecular MeSH
• NAD metabolism   MeSH
• NADPH Dehydrogenase chemistry   isolation & purification   metabolism   MeSH
• Native Polyacrylamide Gel Electrophoresis MeSH
• Oxidation-Reduction MeSH
• Light MeSH
• Light-Harvesting Protein Complexes chemistry   isolation & purification   metabolism   MeSH
• Electron Transport MeSH
• Thylakoids metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH