Structure of a minimal photosystem I from the green alga Dunaliella salina

Abstract

Solar energy harnessed by oxygenic photosynthesis supports most of the life forms on Earth. In eukaryotes, photosynthesis occurs in chloroplasts and is achieved by membrane-embedded macromolecular complexes that contain core and peripheral antennae with multiple pigments. The structure of photosystem I (PSI) comprises the core and light-harvesting (LHCI) complexes, which together form PSI–LHCI. Here we determined the structure of PSI–LHCI from the salt-tolerant green alga Dunaliella salina using X-ray crystallography and electron cryo-microscopy. Our results reveal a previously undescribed configuration of the PSI core. It is composed of only 7 subunits, compared with 14–16 subunits in plants and the alga Chlamydomonas reinhardtii, and forms the smallest known PSI. The LHCI is poorly conserved at the sequence level and binds to pigments that form new energy pathways, and the interactions between the individual Lhca1–4 proteins are weakened. Overall, the data indicate the PSI of D. salina represents a different type of the molecular organization that provides important information for reconstructing the plasticity and evolution of PSI.

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Fig. 1: The structure of the mini-PSI.
Fig. 2: Molecular organization of the mini-PSI.
Fig. 3: The altered conformations of the phylloquinone PQN2001/A.
Fig. 4: Energy transfer pathways between LHCI and PSI.

Data availability

All data generated or analysed during this study are included in this Article and its Extended data and Supplementary tables. The cryo-EM map has been deposited into the Electron Microscopy Data Bank with accession code EMD-4883. The atomic models have been deposited in the PDB under accession codes 6QPH and 6RHZ.

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Acknowledgements

The cryo-EM data were collected at the ESRF beamline CM01, experiment MX-2090. We thank G. Effantin for the outstanding support with the grid screening and data collection; the staff scientists of the SciLifeLab cryo-EM facility, funded by the Knut and Alice Wallenberg, Erling-Persson Family and Kempe foundations. The X-ray diffraction data were collected at the SLS and ESRF. We thank the staff scientists for excellent guidance and assistance; A. Kaplan for providing the green alga Chlorella ohadii. This work was supported by the Israel Science Foundation (569/17), Joint UGC-ISF (2716/17), German-Israeli Foundation for Scientific Research and Development (1483), Swedish Foundation for Strategic Research (FFL15:0325), Ragnar Söderberg Foundation (M44/16), Swedish Research Council (NT_2015-04107), European Research Council (ERC-2018-StG-805230) and Knut and Alice Wallenberg Foundation (2018.0080). A.A. is supported by the EMBO Young Investigator Program.

Author information

A.P.-B., D.K., I.C., S.Y.N.-E., A.A. and N.N. performed the research. A.P.-B., A.A. and N.N. analysed the data. A.P.-B., A.A. and N.N. wrote the manuscript. All of the authors discussed and commented on the final manuscript.

Correspondence to Alexey Amunts or Nathan Nelson.

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Competing interests

The authors declare no competing interests.

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Peer review information Nature Plants thanks Poul Erik Jensen, Mei Li and the other, anonymous, reviewers for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 X-ray crystal structure determination.

(a) Polypeptide composition of D. salina mini-PSI used for crystallization, and comparison with related organisms. The experiments described were independently repeated at least three times (b) activity in NADP photoreduction. (c) Crystals of PSI. Over one hundred of similar wells were generated in the crystallization plates. (d) A representative diffraction pattern showing single spots reaching ~ 3 Å resolution anisotropically.

Extended Data Fig. 2

Cryo-EM data processing workflow.

Extended Data Fig. 3 Quality of the cryo-EM map.

(a) The final map colored by local resolution and viewed from the surface and cut- through. Examples of the best (b) and worst (c) modeled densities for ligands.

Extended Data Fig. 4 Unassigned densities.

Six discrete weak densities were identified in the membrane bilayer that could not be unambiguously assigned. The densities likely represent non-protein components that do not affect the overall conclusions.

Extended Data Fig. 5 Purification profile of PSI-LHCI from D. salina thylakoids.

MW; lane 1: thylakoid membranes after solubilization; lane 2: dark-green fraction of TOYOPEARL DEAE-650C; lane 3: fraction eluted from sucrose gradient; lane 4: elution from FPLC SOURCE 15Q; lane 5: after second sucrose gradient. A subunit distribution of PSI is indicated according to MS data. The experiment was repeated at least three times.

Extended Data Fig. 6 Mini-PSI analysis.

(a) Mild solubilization of the thylakoid membrane with two detergents. Lane 1: MW, lane 2: D. salina thylakoid membranes solubilized in α-DDM, lane 3: D. salina thylakoid membranes solubilized in β-DDM. Both preparations indicate that PSI is found in two different forms. The experiment was repeated twice. (b) Comparison of the mildly solubilized D. salina PSI with purified mini-PSI indicates that the complexes correspond to each other, and small differences in migration can be explained by presence/absence of lipids. Lane 1: MW, lane 2: D. salina thylakoids membranes solubilized in β-DDM, lane 3: purified D. salina PSI-LCHI. This experiment was repeated 5 times. (c) Solubilized membranes were applied on a sucrose gradient 15 to 50% The indicated bands 1 and 2, representing two forms of PSI were run on 20% polyacrylamide urea gel for protein separation. (d) Quantification of the bands from (c). The data suggests that fraction 2 is enriched with mini-PSI, lacking some of the core subunit. The experiment was repeated 5 times with similar results.

Extended Data Fig. 7 Major energy pathways.

(a) Major energy pathways are shown with dashed lines, distances are magnesium to magnesium (Å). The newly identified potential energy pathways are shown in orange and black. (b) Names and locations of the pathways are shown in the table and the newly identified potential energy pathways are shown in orange as highlighted in A.

Extended Data Fig. 8 Identification of new carotenes in Lhca3.

Chlorophylls are shown in green, conserved carotenes are colored in dark orange, represented in yellow are the newly identified carotenes in D. salina and in dark grey is chlorophyll a313 present in plant (PDBID:4KX8) but not in D. salina.

Extended Data Fig. 9 Identification of Lhca3 Chl a607.

Comparison of the density for the highly conserved Chl a607 between (a) the current work, (b) 3.3 Å resolution structure of plant PSI-LHCI-LHCII9, (c) 2.8 Å resolution X-ray crystal structure of plant PSI-LHCI12.

Extended Data Fig. 10 Comparison of the structures and sequences of LHCI between green algae.

The superposition between LHCI proteins of D. salina and C. reinhardtii shows that C-terminus extensions of Lhca7 and Lhca3 responsible for the binding the outer belt are missing in D. salina.

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Perez-Boerema, A., Klaiman, D., Caspy, I. et al. Structure of a minimal photosystem I from the green alga Dunaliella salina. Nat. Plants 6, 321–327 (2020). https://doi.org/10.1038/s41477-020-0611-9

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