Assembly Of Light-harvesting Complexes Research Articles

The Oligomeric States of the Photosystems and the Light-Harvesting Complexes in the Chl b-Less Mutant

The reversible associations between the light-harvesting complexes (LHCs) and the core complexes of PSI and PSII are essential for the photoacclimation mechanisms in higher plants. Two types of Chls, Chl a and Chl b, both function in light harvesting and are required for the biogenesis of the photosystems. Chl b-less plants have been studied to determine the function of the LHCs because the Chl b deficiency has severe effects specific to the LHCs. Previous studies have shown that the amounts of the LHCs, especially the LHCII trimer, were decreased in the mutants; however, it is still unclear whether Chl b is required for the assembly of the LHCs and for the association of the LHCs with PSI and PSII. Here, to reveal the function of Chl b in the LHCs, we investigated the oligomeric states of the LHCs, PSI and PSII in the Arabidopsis Chl b-less mutant. A two-dimensional blue native-PAGE/SDS-PAGE demonstrated that the PSI-LHCI supercomplex was fully assembled in the absence of Chl b, whereas the trimeric LHCII and PSII-LHCII supercomplexes were not detected. The PSI-NAD(P)H dehydrogenase (NDH) supercomplexes were also assembled in the mutant. Furthermore, we detected two forms of monomeric LHC proteins. The faster migrating forms, which were detected primarily in the mutant, were probably apo-LHC proteins, whereas the slower migrating forms were probably the LHC proteins that contained Chl a. These findings increase our understanding of the Chl b function in the assembly of LHCs and the association of the LHCs with PSI, PSII and NDH.

Chlorophylls, ligands and assembly of light-harvesting complexes in chloroplasts.

Chlorophyll (Chl) b serves an essential function in accumulation of light-harvesting complexes (LHCs) in plants. In this article, this role of Chl b is explored by considering the properties of Chls and the ligands with which they interact in the complexes. The overall properties of the Chls, not only their spectral features, are altered as consequences of chemical modifications on the periphery of the molecules. Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains. These modifications influence formation of coordination bonds by which the central Mg atom, the Lewis acid, of Chl molecules interacts with amino acid sidechains, as the Lewis base, in proteins. Chl a is a versatile Lewis acid and interacts principally with imidazole groups but also with sidechain amides and water. The 7-formyl group on Chl b withdraws electron density toward the periphery of the molecule and consequently the positive Mg is less shielded by the molecular electron cloud than in Chl a. Chl b thus tends to form electrostatic bonds with Lewis bases with a fixed dipole, such as water and, in particular, peptide backbone carbonyl groups. The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group. These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

光合成でのアンテナ系タンパク質/色素複合体(LH2)の基板上への組織化

光合成でのアンテナ系タンパク質/色素複合体(LH2)の基板上への組織化

Ring-to-chain conformation may be a determining factor in the ability of xanthophylls to bind to the bulk light-harvesting complex of plants

The binding of xanthophylls to the main light-harvesting complex (LHC) of higher plants has been studied using the technique of in vitro reconstitution. This demonstrated that the carotenoid diol lactucaxanthin (native to many LHC) would not support the assembly of LHC whilst other diols, notably zeaxanthin and lutein would. Analysis of the most stable forms of the carotenoid end-groups found in xanthophylls native to higher plant LHC (as determined by theoretical calculations) revealed profound differences in the adiabatic potential energy curves for the C5–C6–C7–C8–torsion angle for the ε end-groups in lactucaxanthin (6-s- trans), in comparison to carotenoids possessing a 3-hydroxy β end-group (zeaxanthin; 6-s- cis), 3-hydroxy-4-keto β end-group (astaxanthin, 6-s- cis) or a 3-hydroxy-5,6-epoxy end-group (violaxanthin, distorted 6-s- cis). The (ε end-groups of other carotenoids studied were 6-s- trans. We examine the possible relationship between carotenoid ring-to-chain conformation and binding to LHC.

The development of antenna complexes of barley (Hordeum vulgare cv. Akcent) under different light conditions as judged from the analysis of 77 K chlorophyll a fluorescence spectra.

Using 77 K chlorophyll a (Chl a) fluorescence spectra in vivo, the development was studied of Photosystems II (PS II) and I (PS I) during greening of barley under intermittent light followed by continuous light at low (LI, 50 mumol m(-2) s(-1)) and high (HI, 1000 mumol m(-2) s(-1)) irradiances. The greening at HI intermittent light was accompanied with significantly reduced fluorescence intensity from Chl b excitation for both PS II (F685) and PS I (F743), in comparison with LI plants, indicating that assembly of light-harvesting complexes (LHC) of both photosystems was affected to a similar degree. During greening at continuous HI, a slower increase of emission from Chl b excitation in PS II as compared with PS I was observed, indicating a preferred reduction in the accumulation of LHC II. The following characteristics of 77 K Chl a fluorescence spectra documented the photoprotective function of an elevated content of carotenoids in HI leaves: (1) a pronounced suppression of Soret region of excitation spectra (410-450 nm) in comparison with the red region (670-690 nm) during the early stage of greening indicated a strongly reduced excitation energy transfer from carotenoids to the Chl a fluorescing forms within PS I and PS II; (2) changes in the shape of the excitation band of Chl b and carotenoids (460-490 nm) during greening under continuous light confirmed that the energy transfer from carotenoids to Chl a within PS II remained lower as compared with the LI plants.

A potential role of chlorophylls b and c in assembly of light-harvesting complexes

Chlorophyll (Chl)-containing light-harvesting complexes (LHCs) in chloroplasts of plant and algal cells usually include an oxidized Chl (Chl b or c) in addition to Chl a. Oxidation of peripheral groups on the tetrapyrrole structure increases the Lewis acid strength of the central Mg atom. We propose that the resulting stronger coordination bonds between oxidized Chls and ligands in LHC apoproteins (LHCPs) stabilize the initial intermediates and thus promote assembly of LHCs within the chloroplast envelope.

Self-molecular assembly of light-harvesting complex isolated from photosynthetic bacteria on modified electrodes

Pigments play an important role on energy-transfer process in light-harvesting (LH) polypeptides complex of photosynthesis, where porphyrins such as bacteriochlorophylls (BChls) are assembled according to cooperative interactions between the LH polypeptides and BChls. It is interesting to note that the LH polypeptides organize the BChls so that an efficient energy transfer process between BChls may occur. In this paper, we examine the self-assemble of Zn-BChla using LH polypeptides separately isolated from photosynthetic bacteria to organize an artificial LH complex. The key to the molecular assembly is ready availability of the LH complex-formation on modified indium-tin-oxide (ITO) electrode. We selected the native LH polypeptides isolated from photosynthetic bacteria, because the native polypeptides formed a stable Zn-BChla complex in octylglucoside (OG) solution analogous to the LH 1-type complex. The ITO electrode was silanized to introduce amino groups and then, the LH polypeptides complex was assembled on the modified ITO by soaking the ITO electrode into the OG solution forming the LH polypeptides complex (phosphate buffer, pH = 7.0). The Qy band of Zn-BChla in the LH complex on the ITO was red-shifted analogous to the subunit-tipe complex of the LH 1 complex, indicating the self-molecular assembly of the LH 1-type complex on the electrode

Chlorophyll Binding to Peptide Maquettes Containing a Retention Motif

The motif Glu-X-X-His/Asn-X-Arg is conserved in the first and third membrane-spanning domains of all light-harvesting chlorophyll a/b- and a/c-binding proteins in chloroplasts. Molecular modeling of synthetic peptides containing the sequence Glu-Ile-Val-His-Ser-Arg, a motif found in the apoprotein of the major light-harvesting complex in plants, generated a loop structure formed by intrapeptide, electrostatic attraction between Glu and Arg. His, Asn, and charge-compensated Glu-Arg pairs are known ligands of the magnesium atom in chlorophyll. The prediction that this structure should bind two molecules of chlorophyll was confirmed experimentally with an assay based on fluorescence resonance energy transfer between peptides and chlorophyll a. Motifs with both potential ligands bound approximately two times the amount of chlorophyll as one in which His was replaced by Ala. These results support the conclusion that formation of this intermediate, within membranes of the envelope, is a crucial step in assembly of light-harvesting complexes and a mechanism that regulates import of the apoproteins into the chloroplast.

Limitation in Electron Transfer in Photosystem I Donor Side Mutants of Chlamydomonas reinhardtii: LETHAL PHOTO-OXIDATIVE DAMAGE IN HIGH LIGHT IS OVERCOME IN A SUPPRESSOR STRAIN DEFICIENT IN THE ASSEMBLY OF THE LIGHT HARVESTING COMPLEX

Strains of Chlamydomonas reinhardtii lacking the PsaF gene or containing the mutation K23Q within the N-terminal part of PsaF are sensitive to high light (>400 microE m(-2) s(-1)) under aerobic conditions. In vitro experiments indicate that the sensitivity to high light of the isolated photosystem I (PSI) complex from wild type and from PsaF mutants is similar. In vivo measurements of photochemical quenching and oxygen evolution show that impairment of the donor side of PSI in the PsaF mutants leads to a diminished linear electron transfer and/or a decrease of photosystem II (PSII) activity in high light. Thermoluminescence measurements indicate that the PSII reaction center is directly affected under photo-oxidative stress when the rate of electron transfer becomes limiting in the PsaF-deficient strain and in the PsaF mutant K23Q. We have isolated a high light-resistant PsaF-deficient suppressor strain that has a high chlorophyll a/b ratio and is affected in the assembly of light-harvesting complex. These results indicate that under high light a functionally intact donor side of PSI is essential for protection of C. reinhardtii against photo-oxidative damage when the photosystems are properly connected to their light-harvesting antennae.

Assembly of light harvesting complexes II (LHC-II) in the absence of lutein: A study on the α-carotenoid-free mutant C-2A′-34 of the green alga Scenedesmus obliquus

Stable assembly of the major LHC-II proteins (22, 26 kDa) is inhibited in the α-carotenoid-free mutant C-2A′-34 of the green alga Scenedesmus obliquus, whereas other LHC-II proteins (22, 23, 29 kDa) seem to be not affected. We conclude that lutein is essential for assembly of the major LHC-II proteins of S. obliquus, but not for assembly of the minor LHC-II proteins. It is proposed that in the minor LHC-II proteins lutein is replaced by violaxanthin, zeaxanthin and antheraxanthin.

Assembly of the Light-Harvesting Complexes (LHCs) of Photosystem II (Monomeric LHC IIb Complexes Are Intermediates in the Formation of Oligomeric LHC IIb Complexes).

The light-induced assembly of light-harvesting complex (LHC) II has been followed during the biogenesis of the plastid. Seedlings grown in intermittent light (IML) accumulate only small amounts of chlorophyll b. The minor LHC II apoproteins are present; however, the apoprotein levels of the major LHC II complex, LHC IIb, are severely depressed after exposure to IML. The levels of all LHC II apoproteins increase rapidly upon exposure to continuous illumination. The 25-kD, type 3 LHC IIb subunit appears to be more abundant during the early hours of greening in relation to its level in mature thylakoids. The LHC IIb apoproteins are initially associated with pigments to form monomeric pigment-protein complexes. The abundance of monomeric LHC IIb complexes gradually decreases during exposure to continuous light and a concomitant increase occurs in the amount of the trimeric and higher-order oligomeric forms. Pulse-chase experiments verify that labeled LHC IIb monomeric complexes are intermediates in the formation of trimeric and higher-order oligomeric LHC IIb-pigmented complexes. Therefore, the assembly of LHC II occurs via the initial pigmentation of the apoproteins to form monomeric complexes and proceeds in a sequential manner.