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Photosynthetic light-harvesting complexes of higher plants, moss and green algae can undergo dynamic conformational transitions, which have been correlated to their ability to adapt to fluctuations in the light environment. Herein, we demonstrate the application of solid-state NMR spectroscopy on native, heterogeneous thylakoid membranes of Chlamydomonas reinhardtii (Cr) and on Cr Light-Harvesting Complex II (LHCII) in thylakoid lipid bilayers to detect LHCII conformational dynamics in its native membrane environment. We show that membrane-reconstituted LHCII contains selective sites that undergo fast, large-amplitude motions, including the phytol tails of two chlorophylls. Protein plasticity is also observed in the N-terminal stromal loop and in protein fragments facing the lumen, involving sites that stabilize the xanthophyll-cycle carotenoid violaxanthin and the two luteins. The results report on the intrinsic flexibility of LHCII pigment-protein complexes in a membrane environment, revealing putative sites for conformational switching. In thylakoid membranes, fast dynamics of protein and pigment sites is significantly reduced, which suggests that in their native organelle membranes, LHCII complexes are locked in specific conformational states.STATEMENT OF SIGNIFICANCEPhotosynthetic Light-Harvesting Complexes undergo dynamic conformational transitions that regulate the capacity of the light-harvesting antenna. We demonstrate the application of solid-state (ss)NMR spectroscopy to investigate the structural dynamics of LHCII, the most abundant LHC complex of plants and algae, in native membranes. Selective dynamic protein and pigment residues are identified that are putative sites for a conformational switch.

Photosynthetic light-harvesting antenna complexes (LHCs) of plants, moss and green algae form dynamic switches between light harvesting and excitation-quenched, dissipative states. This mechanism protects the photosynthetic apparatus under light stress via a photo protective membrane response. Herein, we demonstrate the application of solid-state NMR spectroscopy to wild type, heterogeneous thylakoid membranes of Chlamydomonas reinhardtii ( Cr ) and purified Cr Light harvesting Complex II (LHCII) reconstituted in thylakoid lipid membranes, to detect the conformational dynamics of LHCII under native conditions. We find that membrane-reconstituted LHCII contains sites that undergo fast, large-amplitude motions, including the phytol tails of two chlorophylls. Furthermore, plasticity is observed in the N-terminal stretch and in the trans-membrane helical edges facing the thylakoid lumen. In intact thylakoids, the dynamics of these protein and pigment sites is significantly reduced. We conclude that LHCIIs contain flexible sites but that their conformational dynamics are constrained in vivo , implying that changes in the physicochemical environment are required to enable switching between different conformational states. In situ NMR spectroscopy opens a new route to investigate the plasticity of light-harvesting complexes and their seminal role in biological regulation mechanisms such as membrane state transitions, non-photochemical quenching or post-translational modifications.

When exposed to saturating light conditions photosynthetic eukaryotes activate the xanthophyll cycle where the carotenoid violaxanthin is converted into zeaxanthin by the enzyme violaxanthin de-epoxidase (VDE). VDE protein sequence includes 13 cysteine residues, 12 of which are strongly conserved in both land plants and algae. Site directed mutagenesis of Arabidopsis thaliana VDE showed that all these 12 conserved cysteines have a major role in protein function and their mutation leads to a strong reduction of activity. VDE is also shown to be active in its completely oxidized form presenting six disulfide bonds. Redox titration showed that VDE activity is sensitive to variation in redox potential, suggesting the possibility that dithiol/disulfide exchange reactions may represent a mechanism for VDE regulation.