Conservation of core complex subunits shaped the structure and function of photosystem I in the secondary endosymbiont alga Nannochloropsis gaditana.
Photosystem I (PSI) is a pigment protein complex catalyzing the light-driven electron transport from plastocyanin to ferredoxin in oxygenic photosynthetic organisms. Several PSI subunits are highly conserved in cyanobacteria, algae and plants, whereas others are distributed differentially in the various organisms. Here we characterized the structural and functional properties of PSI purified from the heterokont alga Nannochloropsis gaditana, showing that it is organized as a supercomplex including a core complex and an outer antenna, as in plants and other eukaryotic algae. Differently from all known organisms, the N. gaditana PSI supercomplex contains five peripheral antenna proteins, identified by proteome analysis as type-R light-harvesting complexes (LHCr4-8). Two antenna subunits are bound in a conserved position, as in PSI in plants, whereas three additional antennae are associated with the core on the other side. This peculiar antenna association correlates with the presence of PsaF/J and the absence of PsaH, G and K in the N. gaditana genome and proteome. Excitation energy transfer in the supercomplex is highly efficient, leading to a very high trapping efficiency as observed in all other PSI eukaryotes, showing that although the supramolecular organization of PSI changed during evolution, fundamental functional properties such as trapping efficiency were maintained.
Dipartimento di Biologia, Università di Padova, Via U. Bassi 58/B, 35121, Padova, Italy.
Alessandro
A
Alboresi
Department of Physics and Astronomy and Institute for Lasers, Life and Biophotonics, Faculty of Sciences, VU University Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam, the Netherlands.
Clotilde
C
Le Quiniou
Electron Microscopy Group, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 7, 9747 AG, Groningen, the Netherlands.
Sathish K N
SK
Yadav
Institute of Plant Biology and Biotechnology, University of Münster, Münster, 48143, Germany.
Martin
M
Scholz
Dipartimento di Biologia, Università di Padova, Via U. Bassi 58/B, 35121, Padova, Italy.
Andrea
A
Meneghesso
Dipartimento di Biologia, Università di Padova, Via U. Bassi 58/B, 35121, Padova, Italy.
Caterina
C
Gerotto
Dipartimento di Biologia, Università di Padova, Via U. Bassi 58/B, 35121, Padova, Italy.
Diana
D
Simionato
Institute of Plant Biology and Biotechnology, University of Münster, Münster, 48143, Germany.
Michael
M
Hippler
Electron Microscopy Group, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 7, 9747 AG, Groningen, the Netherlands.
Egbert J
EJ
Boekema
Department of Physics and Astronomy and Institute for Lasers, Life and Biophotonics, Faculty of Sciences, VU University Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam, the Netherlands.
Roberta
R
Croce
Dipartimento di Biologia, Università di Padova, Via U. Bassi 58/B, 35121, Padova, Italy.
Tomas
T
Morosinotto
D000595:Amino Acid Sequence; D017124:Conserved Sequence; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure; D008954:Models, Biological; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000378:metabolism* / Q000648:ultrastructure; D010860:Pigments, Biological / Q000378:metabolism; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D058009:Stramenopiles / Q000378:metabolism*; D013559:Symbiosis; D020524:Thylakoids / Q000378:metabolism
D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D010860:Pigments, Biological; D021122:Protein Subunits

Nannochloropsis
; Heterokonta; Light-harvesting complex; electron microscopy; photosynthesis; photosynthetic apparatus; photosystem