Stable Accumulation of Photosystem II Requires ONE-HELIX PROTEIN1 (OHP1) of the Light Harvesting-Like Family.

The cellular functions of two Arabidopsis (Arabidopsis thaliana) one-helix proteins, OHP1 and OHP2 (also named LIGHT-HARVESTING-LIKE2 [LIL2] and LIL6, respectively, because they have sequence similarity to light-harvesting chlorophyll a/b-binding proteins), remain unclear. Tagged null mutants of OHP1 and OHP2 (ohp1 and ohp2) showed stunted growth with pale-green leaves on agar plates, and these mutants were unable to grow on soil. Leaf chlorophyll fluorescence and the composition of thylakoid membrane proteins revealed that ohp1 deletion substantially affected photosystem II (PSII) core protein function and led to reduced levels of photosystem I core proteins; however, it did not affect LHC accumulation. Transgenic ohp1 plants rescued with OHP1-HA or OHP1-Myc proteins developed a normal phenotype. Using these tagged OHP1 proteins in transgenic plants, we localized OHP1 to thylakoid membranes, where it formed protein complexes with both OHP2 and High Chlorophyll Fluorescence244 (HCF244). We also found PSII core proteins D1/D2, HCF136, and HCF173 and a few other plant-specific proteins associated with the OHP1/OHP2-HCF244 complex, suggesting that these complexes are early intermediates in PSII assembly. OHP1 interacted directly with HCF244 in the complex. Therefore, OHP1 and HCF244 play important roles in the stable accumulation of PSII.

Very rapid phosphorylation kinetics suggest a unique role for Lhcb2 during state transitions in Arabidopsis.

Light-harvesting complex II (LHCII) contains three highly homologous chlorophyll-a/b-binding proteins (Lhcb1, Lhcb2 and Lhcb3), which can be assembled into both homo- and heterotrimers. Lhcb1 and Lhcb2 are reversibly phosphorylated by the action of STN7 kinase and PPH1/TAP38 phosphatase in the so-called state-transition process. We have developed antibodies that are specific for the phosphorylated forms of Lhcb1 and Lhcb2. We found that Lhcb2 is more rapidly phosphorylated than Lhcb1: 10 sec of 'state 2 light' results in Lhcb2 phosphorylation to 30% of the maximum level. Phosphorylated and non-phosphorylated forms of the proteins showed no difference in electrophoretic mobility and dephosphorylation kinetics did not differ between the two proteins. In state 2, most of the phosphorylated forms of Lhcb1 and Lhcb2 were present in super- and mega-complexes that comprised both photosystem (PS)I and PSII, and the state 2-specific PSI-LHCII complex was highly enriched in the phosphorylated forms of Lhcb2. Our results imply distinct and specific roles for Lhcb1 and Lhcb2 in the regulation of photosynthetic light harvesting.

Arabidopsis plants grown in the field and climate chambers significantly differ in leaf morphology and photosystem components.

BACKGROUND: Plants exhibit phenotypic plasticity and respond to differences in environmental conditions by acclimation. We have systematically compared leaves of Arabidopsis thaliana plants grown in the field and under controlled low, normal and high light conditions in the laboratory to determine their most prominent phenotypic differences. RESULTS: Compared to plants grown under field conditions, the "indoor plants" had larger leaves, modified leaf shapes and longer petioles. Their pigment composition also significantly differed; indoor plants had reduced levels of xanthophyll pigments. In addition, Lhcb1 and Lhcb2 levels were up to three times higher in the indoor plants, but differences in the PSI antenna were much smaller, with only the low-abundance Lhca5 protein showing altered levels. Both isoforms of early-light-induced protein (ELIP) were absent in the indoor plants, and they had less non-photochemical quenching (NPQ). The field-grown plants had a high capacity to perform state transitions. Plants lacking ELIPs did not have reduced growth or seed set rates, but their mortality rates were sometimes higher. NPQ levels between natural accessions grown under different conditions were not correlated. CONCLUSION: Our results indicate that comparative analysis of field-grown plants with those grown under artificial conditions is important for a full understanding of plant plasticity and adaptation.

The PsbS protein controls the macro-organisation of photosystem II complexes in the grana membranes of higher plant chloroplasts.

The PsbS protein is a critical component in the regulation of non-photochemical quenching (NPQ) in higher plant photosynthesis. Electron microscopy and image analysis of grana membrane fragments from wild type and mutant Arabidopsis plants showed that the semi-crystalline domains of photosystem II supercomplexes were identical in the presence and absence of PsbS. However, the frequency of the domains containing crystalline arrays was increased in the absence of PsbS. Conversely, there was a complete absence of such arrays in the membranes of plants containing elevated amounts of this protein. It is proposed that PsbS controls the macro-organisation of the grana membrane, providing an explanation of its role in NPQ.

The photosystem II light-harvesting protein Lhcb3 affects the macrostructure of photosystem II and the rate of state transitions in Arabidopsis.

The main trimeric light-harvesting complex of higher plants (LHCII) consists of three different Lhcb proteins (Lhcb1-3). We show that Arabidopsis thaliana T-DNA knockout plants lacking Lhcb3 (koLhcb3) compensate for the lack of Lhcb3 by producing increased amounts of Lhcb1 and Lhcb2. As in wild-type plants, LHCII-photosystem II (PSII) supercomplexes were present in Lhcb3 knockout plants (koLhcb3), and preservation of the LHCII trimers (M trimers) indicates that the Lhcb3 in M trimers has been replaced by Lhcb1 and/or Lhcb2. However, the rotational position of the M LHCII trimer was altered, suggesting that the Lhcb3 subunit affects the macrostructural arrangement of the LHCII antenna. The absence of Lhcb3 did not result in any significant alteration in PSII efficiency or qE type of nonphotochemical quenching, but the rate of transition from State 1 to State 2 was increased in koLhcb3, although the final extent of state transition was unchanged. The level of phosphorylation of LHCII was increased in the koLhcb3 plants compared with wild-type plants in both State 1 and State 2. The relative increase in phosphorylation upon transition from State 1 to State 2 was also significantly higher in koLhcb3. It is suggested that the main function of Lhcb3 is to modulate the rate of state transitions.

Effect of phosphorylation on the thermal and light stability of the thylakoid membranes.

Higher plant thylakoid membranes contain a protein kinase that phosphorylates certain threonine residues of light-harvesting complex II (LHCII), the main light-harvesting antenna complexes of photosystem II (PSII) and some other phosphoproteins (Allen, Biochim Biophys Acta 1098:275, 1992). While it has been established that phosphorylation induces a conformational change of LHCII and also brings about changes in the lateral organization of the thylakoid membrane, it is not clear how phosphorylation affects the dynamic architecture of the thylakoid membranes. In order to contribute to the elucidation of this complex question, we have investigated the effect of duroquinol-induced phosphorylation on the membrane ultrastructure and the thermal and light stability of the chiral macrodomains and of the trimeric organization of LHCII. As shown by small angle neutron scattering on thylakoid membranes, duroquinol treatment induced a moderate (~10%) increase in the repeat distance of stroma membranes, and phosphorylation caused an additional loss of the scattering intensity, which is probably associated with the partial unstacking of the granum membranes. Circular dichroism (CD) measurements also revealed only minor changes in the chiral macro-organization of the complexes and in the oligomerization state of LHCII. However, temperature dependences of characteristic CD bands showed that phosphorylation significantly decreased the thermal stability of the chiral macrodomains in phosphorylated compared to the non-phosphorylated samples (in leaves and isolated thylakoid membranes, from 48.3 degrees C to 42.6 degrees C and from 47.5 degrees C to 44.3 degrees C, respectively). As shown by non-denaturing PAGE of thylakoid membranes and CD spectroscopy on EDTA washed membranes, phosphorylation decreased by about 5 degrees C, the trimer-to-monomer transition temperature of LHCII. It also enhanced the light-induced disassembly of the chiral macrodomains and the monomerization of the LHCII trimers at 25 degrees C. These data strongly suggest that phosphorylation of the membranes considerably facilitates the heat- and light-inducible reorganizations in the thylakoid membranes and thus enhances the structural flexibility of the membrane architecture.

The negatively charged amino acids in the lumenal loop influence the pigment binding and conformation of the major light-harvesting chlorophyll a/b complex of photosystem II.

The major chlorophyll (Chl) a/b complexes of photosystem II (LHCIIb), in addition to their primary light-harvesting function, play key roles in the organization of the granal ultrastructure of the thylakoid membranes and in various regulatory processes. These functions depend on the structural stability and flexibility of the complexes. The lumenal side of LHCIIb is exposed to broadly variable pH environments, due to the build-up and decay of the pH gradient during photosynthesis. Therefore, the negatively charged amino acids in the lumenal loop might be of paramount importance for adjusting the structure and functions of LHCIIb. In order to clarify the structural roles of these residues, we investigated the pigment stoichiometries, absorption, linear and circular dichroism spectra of the reconstituted LHCIIb complexes, in which the negatively charged amino acids in the lumenal loop were exchanged to neutral ones (E94G, E107V and D111V). The mutations influenced the pigment binding and the molecular architecture of the complexes. Exchanging E94 to G destabilized the 3(10) helix in the lumenal loop structure and led to an acquired pH sensitivity of the LHCIIb structure. We conclude that these amino acids are important not only for pigment binding in the complexes, but also in stabilizing the conformation of LHCIIb at different pHs.

Photosynthetic acclimation: does the dynamic structure and macro-organisation of photosystem II in higher plant grana membranes regulate light harvesting states?

The efficiency of light harvesting in higher plant photosynthesis is regulated in response to external environmental conditions. Under conditions of excess light, the normally highly efficient light-harvesting system of photosystem II is switched into a state in which unwanted, potentially harmful, energy is dissipated as heat. This process, known as nonphotochemical quenching, occurs by the creation of energy quenchers following conformational change in the light-harvesting complexes, which is initiated by the build up of the thylakoid pH gradient and controlled by the xanthophyll cycle. In the present study, the evidence to support the notion that this regulatory mechanism is dependent upon the organization of the different antenna subunits in the stacked grana membranes is reviewed. We postulate that nonphotochemical quenching occurs within a structural locus comprising the PsbS subunit and components of the light-harvesting antenna, CP26, CP24, CP29 and LHCIIb (the major trimeric light-harvesting complex), formed in response to protonation and controlled by the xanthophyll cycle carotenoids.

The PsbS protein controls the organization of the photosystem II antenna in higher plant thylakoid membranes.

The PsbS subunit of photosystem II (PSII) plays a key role in nonphotochemical quenching (NPQ), the major photoprotective regulatory mechanism in higher plant thylakoid membranes, but its mechanism of action is unknown. Here we describe direct evidence that PsbS controls the organization of PSII and its light harvesting system (LHCII). The changes in chlorophyll fluorescence amplitude associated with the Mg(2+)-dependent restacking of thylakoid membranes were measured in thylakoids prepared from wild-type plants, a PsbS-deficient mutant and a PsbS overexpresser. The Mg(2+) requirement and sigmoidicity of the titration curves for the fluorescence rise were negatively correlated with the level of PsbS. Using a range of PsbS mutants, this effect of PsbS was shown not to depend upon its efficacy in controlling NPQ, but to be related only to protein concentration. Electron microscopy and fluorescence spectroscopy showed that this effect was because of enhancement of the Mg(2+)-dependent re-association of PSII and LHCII by PsbS, rather than an effect on stacking per se. In the presence of PsbS the LHCII.PSII complex was also more readily removed from thylakoid membranes by detergent, and the level of PsbS protein correlated with the amplitude of the psi-type CD signal originating from features of LHCII.PSII organization. It is proposed that PsbS regulates the interaction between LHCII and PSII in the grana membranes, explaining how it acts as a pH-dependent trigger of the conformational changes within the PSII light harvesting system that result in NPQ.