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Molecular Mechanisms of Nonphotochemical Quenching in the LHCSR3 Protein of Chlamydomonas reinhardtii.

ABSTRACT: Photosynthetic organisms possess photoprotection mechanisms from excess light conditions. The fastest response consists in the pH-triggered activation of a dissipation channel of the energy absorbed by the chlorophylls into heat, called nonphotochemical quenching. In green algae, the pigment binding complex LHCSR3 acts both as a chlorophyll quencher and as a pH detector. In this work, we study the quenching of the LHCSR3 protein in vitro considering two different protein aggregation states and two pH conditions using a combination of picosecond time-resolved photoluminescence and femtosecond transient absorption in the visible and NIR spectral regions. We find that the mechanisms at the basis of LHCSR3 quenching activity are always active, even at pH 7.5 and low aggregation. However, quenching efficiency is strongly enhanced by pH and by aggregation conditions. In particular, we find that electron transfer from carotenoids to chlorophylls is enhanced at low pH, while quenching mediated by protein-protein interactions is increased by going to a high aggregation state. We also observe a weak pH-dependent energy transfer from the chlorophylls to the S₁ state of carotenoids.

SUBMITTER: de la Cruz Valbuena G

PROVIDER: S-EPMC6613783 | biostudies-literature | 2019 May

REPOSITORIES: biostudies-literature

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Project description:In photosynthetic organisms, feedback dissipation of excess absorbed light energy balances harvesting of light with metabolic energy consumption. This mechanism prevents photodamage caused by reactive oxygen species produced by the reaction of chlorophyll (Chl) triplet states with O?. Plants have been found to perform the heat dissipation in specific proteins, binding Chls and carotenoids (Cars), that belong to the Lhc family, while triggering of the process is performed by the PsbS subunit, needed for lumenal pH detection. PsbS is not found in algae, suggesting important differences in energy-dependent quenching (qE) machinery. Consistent with this suggestion, a different Lhc-like gene product, called LhcSR3 (formerly known as LI818) has been found to be essential for qE in Chlamydomonas reinhardtii. In this work, we report the production of two recombinant LhcSR isoforms from C. reinhardtii and their biochemical and spectroscopic characterization. We found the following: (i) LhcSR isoforms are Chl a/b- and xanthophyll-binding proteins, contrary to higher plant PsbS; (ii) the LhcSR3 isoform, accumulating in high light, is a strong quencher of Chl excited states, exhibiting a very fast fluorescence decay, with lifetimes below 100 ps, capable of dissipating excitation energy from neighbor antenna proteins; (iii) the LhcSR3 isoform is highly active in the transient formation of Car radical cation, a species proposed to act as a quencher in the heat dissipation process. Remarkably, the radical cation signal is detected at wavelengths corresponding to the Car lutein, rather than to zeaxanthin, implying that the latter, predominant in plants, is not essential; (iv) LhcSR3 is responsive to low pH, the trigger of non-photochemical quenching, since it binds the non-photochemical quenching inhibitor dicyclohexylcarbodiimide, and increases its energy dissipation properties upon acidification. This is the first report of an isolated Lhc protein constitutively active in energy dissipation in its purified form, opening the way to detailed molecular analysis. Owing to its protonatable residues and constitutive excitation energy dissipation, this protein appears to merge both pH-sensing and energy-quenching functions, accomplished respectively by PsbS and monomeric Lhcb proteins in plants.

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Molecular Mechanisms of Nonphotochemical Quenching in the LHCSR3 Protein of Chlamydomonas reinhardtii.

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