Photoprotection in Plants Involves a Change in Lutein 1 Binding Domain in the Major Light-harvesting Complex of Photosystem II

Abstract

Nonphotochemical quenching (NPQ) is the fundamental process by which plants exposed to high light intensities dissipate the potentially harmful excess energy as heat. Recently, it has been shown that efficient energy dissipation can be induced in the major light-harvesting complexes of photosystem II (LHCII) in the absence of protein-protein interactions. Spectroscopic measurements on these samples (LHCII gels) in the quenched state revealed specific alterations in the absorption and circular dichroism bands assigned to neoxanthin and lutein 1 molecules. In this work, we investigate the changes in conformation of the pigments involved in NPQ using resonance Raman spectroscopy. By selective excitation we show that, as well as the twisting of neoxanthin that has been reported previously, the lutein 1 pigment also undergoes a significant change in conformation when LHCII switches to the energy dissipative state. Selective two-photon excitation of carotenoid (Car) dark states (Car S 1 ) performed on LHCII gels shows that the extent of electronic interactions between Car S 1 and chlorophyll states correlates linearly with chlorophyll fluorescence quenching, as observed previously for isolated LHCII (aggregated versus trimeric) and whole plants (with versus without NPQ).The photosynthetic process starts with the absorption of incoming solar photons by specialized pigment-protein complexes. In plants, light energy is absorbed by the two multisubunit protein-cofactor complexes, photosystem I (PSI) 3 and photosystem II (PSII), and the excitation energy is efficiently transferred to their reaction centers where photochemistry takes place. When these organisms are in low light environments, the process of light energy collection is extremely efficient. However, this process is regulated when the incoming light energy is above that which can be used in electron transport. In these latter conditions, the light-harvesting antenna is able to switch into a photoprotective mode, dissipating the excess energy as heat (1-3). This regulatory mechanism is measured as nonphotochemical quenching of chlorophyll (Chl) fluorescence (NPQ). In higher plants NPQ is a multicomponent process whose major component is called qE (energy-dependent quenching) (4), dependent upon the formation of the proton gradient (⌬pH) across the thylakoid membrane (which itself results from photosynthetic activity) (5). qE is facilitated by the deepoxidation of violaxanthin to zeaxanthin in the xanthophyll cycle (6). The PsbS protein also plays a crucial role in this process, possibly acting as a pH sensor (7,8). A key characteristic of the qE process is that it is induced in seconds and thus cannot involve de novo protein synthesis, but rather corresponds to a reorganization of the existing photosynthetic architecture.In the past decade a large amount of work has been performed to gain insight in the molecular mechanism(s) underlying this process, and several hypotheses have been put forward (9 -12). Among these, it was in particular proposed that excitation e...

Pigment Interactions in Light-harvesting Complex II in Different Molecular Environments

“…This CD band was previously attributed to lutein (24) but our results do not support this assignment. Evidently, the Nx site is sensitive to ␤-DM and the conformation of this carotenoid is altered when the detergent (or lipid) is removed from the medium, which is also evident from resonance Raman spectra (25). However, aggregation itself does not appear to be associated with a change in Nx conformation.…”

“…We set out to separate the CD changes appearing due to LHCII-LHCII interactions from those brought about by artificial surfactants or lipids. To this end, we compared the effects of different detergents, embedded the complexes in lipid vesicles, and employed the technique used by Ilioaia et al (24,25) for trapping solubilized LHCII trimers in polymer gels, allowing us to exchange the surfactant while preventing aggregation. As a reference for the CD of LHCII in its native environment we used thylakoid membranes isolated from lincomycin-treated plants lacking photosystem core proteins.…”

Mechanisms Underlying Carotenoid Absorption in Oxygenic Photosynthetic Proteins

“…3 arises from in-plane rocking vibrations of the methyl groups attached to the conjugated chain, coupled with in-plane bending modes of the adjacent C-Hs. In Raman spectra of fucoxanthin the 3 band is similarly composed of two components, which was attributed to differences in the methyl nearest neighbors in the chemical structure of the carotenoid (46). Along the same lines, the splitting of this band observed in the blue-absorbing Car may reflect the out-of-plane rotation of the end cycles, as this rotation is likely to perturb the rocking frequency of the neighboring methyl group.…”

“…Contributions of lut1 thus cannot be selectively observed in RR spectra at room temperature. Indeed, this was found to be difficult even at 77 K, where a "pure" lut1 spectrum was only obtained after removing the Neo contribution by differential analysis (46).…”

Nonphotochemical Chlorophyll Fluorescence Quenching: Mechanism and Effectiveness in Protecting Plants from Photodamage

“…The features of this quenching were similar to those of the aggregated low-pH-quenched LHCII. It has begun to emerge that the LHCII monomer/trimer undergoes some type of conformational change into the quenching state that involves specific changes in some of the xanthophyll (neoxanthin and lutein) and chlorophyll pigments, as was previously observed for LHCII aggregates (Robert et al, 2004;Ilioaia et al, 2011). However, to date, only the structure of the quenched conformation of trimeric LHCII has been solved (Liu et al, 2004;Pascal et al, 2005).…”