Light-harvesting complex II binds to several small subunits of photosystem I

Mobile light-harvesting complex II (LHCII) is implicated in the regulation of excitation energy distribution between Photosystem I (PSI) and Photosystem II (PSII) during state transitions. To investigate how LHCII interacts with PSI during state transitions, PSI was isolated from Arabidopsis thaliana plants treated with PSII or PSI light. The PSI preparations were made using digitonin. Chemical cross-linking using dithio-bis(succinimidylpropionate) followed by diagonal electrophoresis and immunoblotting showed that the docking site of LHCII (Lhcb1) on PSI is comprised of the PSI-H, -L, and -I subunits. This was confirmed by the lack of energy transfer from LHCII to PSI in the digitonin-PSI isolated from plants lacking PSI-H and -L. Digitonin-PSI was purified further to obtain an LHCII.PSI complex, and two to three times more LHCII was associated with PSI in the wild type in State 2 than in State 1. Lhcb1 was also associated with PSI from plants lacking PSI-K, but PSI from PSI-H, -L, or -O mutants contained only about 30% of Lhcb1 compared with the wild type. Surprisingly, a significant fraction of the LHCII bound to PSI in State 2 was not phosphorylated. Cross-linking prior to sucrose gradient purification resulted in copurification of phosphorylated LHCII in the wild type, but not with PSI from the PSI-H, -L, and -O mutants. The data suggest that migration of LHCII during state transitions cannot be explained sufficiently by different affinity of phosphorylated and unphosphorylated LHCII for PSI but is likely to involve structural changes in thylakoid organization.     

Plants impaired in state transitions can to a large degree compensate for their defect

Arabidopsis thaliana plants lacking the PSI-H or PSI-L subunit of photosystem I have been shown to be severely affected in their ability to perform state transitions, but no visual phenotype was observed when these plants were grown under different light quantities and qualities. However, the chloroplasts in the PSI-H- and PSI-L-less plants contained fewer and more extended grana stacks. The plants lacking PSI-H or PSI-L were characterised with respect to their photosynthetic performance. Wild-type plants adjusted the non-photochemical fluorescence quenching to maintain constant levels of PSII quantum yield and reduction of the plastoquinone pool. In contrast, the plants deficient in state transitions had a more reduced plastoquinone pool and consequently, a less efficient PSII-photochemistry under growth-light conditions and in state 2. The maximal photosynthetic capacity and the quantum efficiency of oxygen evolution were diminished by 8-14% in the PSI-H-less plants. Under growth-light conditions, the stroma was similarly reduced in the PSI-H-less plants and the rate of cyclic electron transport was unchanged. Pigment analysis showed that the xanthophyll cycle was not upregulated in order to compensate for the lack of state transitions. In general, the plants lacking PSI-H and PSI-L showed a decreased ability to optimise photosynthesis according to the light conditions.     

Cosuppression of photosystem I subunit PSI-H in Arabidopsis thaliana. Efficient electron transfer and stability of photosystem I is dependent upon the PSI-H subunit

PSI-H is an intrinsic membrane protein of 10 kDa that is a subunit of photosystem I (PSI). PSI-H is one of the three PSI subunits found only in eukaryotes. The function of PSI-H was characterized in Arabidopsis plants transformed with a psaH cDNA in sense orientation. Cosuppressed plants containing less than 3% PSI-H are smaller than wild type when grown on sterile media but are similar to wild type under optimal conditions. PSI complexes lacking PSI-H contain 50% PSI-L, whereas other PSI subunits accumulate in wild type amounts. PSI devoid of PSI-H has only 61% NADP+ photoreduction activity compared with wild type and is highly unstable in the presence of urea as determined from flash-induced absorbance changes at 834 nm. Our data show that PSI-H is required for stable accumulation of PSI and efficient electron transfer in the complex. The plants lacking PSI-H compensate for the less efficient PSI with a 15% increase in the P700/chlorophyll ratio, and this compensation is sufficient to prevent overreduction of the plastoquinone pool as evidenced by normal photochemical quenching of fluorescence. Nonphotochemical quenching is approximately 60% of the wild type value, suggesting that the proton gradient across the thylakoid membrane is decreased in the absence of PSI-H.     

Structural and functional analysis of the reducing side of photosystem I

Structural analysis of the reducing side of photosystem I (PSI) has been carried out using chemical cross-linking and monospecific antibodies. Incubation of PSI isolated from barley (Hordeum vulgare L.) with the hydrophilic cross-linking agent N-ethyl-3-[3-(dimethylamino) propyl]-carbodiimide leads to cross-linking of the PSI-D subunit with the PSI-E and PSI-H subunits. In the presence of ferredoxin, cross-linking results in the formation of cross-linked products composed of PSI-D, PSI-E and ferredoxin and in a block in steady state NADP⁺ photoreduction. No cross-linking of ferredoxin occurs at elevated ionic strength or using heat-denatured ferredoxin. Cross-linking of ferredoxin does not inhibit electron transfer from plastocyanin to methyl viologen. Steady state NADP⁺ photoreduction was analyzed in PSI or thyla-koids incubated with antibodies against individual PSI subunits. Incubation with antibodies against PSI-C, -H, -I, or -L had no effect on PSI activity, whereas antibodies against PSI-D or PSI-E had similar effects and caused a large decrease in activity. The results provide evidence that the PSI-D and PSI-E subunits are localized on the reducing side of PSI, forming a barrier between PSI-C and the stroma as well as a docking site for ferredoxin. The PSI-H subunit has an exposed, stromal domain but this does not appear to contribute to the ferredoxin docking.     

Single and double knockouts of the genes for photosystem I subunits G,K, and H of Arabidopsis. Effects on photosystem I composition,photosynthetic electron flow,and state transitions

Photosystem I (PSI) of higher plants contains 18 subunits. Using Arabidopsis En insertion lines, we have isolated knockout alleles of the genes psaG, psaH2, and psaK, which code for PSI-G, -H, and -K. In the mutants psak-1 and psag-1.4, complete loss of PSI-K and -G, respectively, was confirmed, whereas the residual H level in psah2-1.4 is due to a second gene encoding PSI-H, psaH1. Double mutants, lacking PSI-G, and also -K, or a fraction of -H, together with the three single mutants were characterized for their growth phenotypes and PSI polypeptide composition. In general, the loss of each subunit has secondary, in some cases additive, effects on the abundance of other PSI polypeptides, such as D, E, H, L, N, and the light-harvesting complex I proteins Lhca2 and 3. In the G-less mutant psag-1.4, the variation in PSI composition suggests that PSI-G stabilizes the PSI-core. Levels of light-harvesting complex I proteins in plants, which lack simultaneously PSI-G and -K, indicate that PSI subunits other than G and K can also bind Lhca2 and 3. In the same single and double mutants, psag-1.4, psak-1, psah2-1.4, psag-1.4/psah2-1.4, and psag-1.4/psak-1 photosynthetic electron flow and excitation energy quenching were analyzed to address the roles of the various subunits in P700 reduction (mediated by PSI-F and -N) and oxidation (PSI-E), and state transitions (PSI-H). Based on the results, we also suggest for PSI-K a role in state transitions.     

Protein-protein and protein-function relationships in Arabidopsis photosystem I: cluster analysis of PSI polypeptide levels and photosynthetic parameters in PSI mutants

In flowering plants, photosystem I (PSI) mediates electron transport across the thylakoid membrane and contains at least 14 proteins. The availability of co-suppression and/or mutant lines deficient for individual PSI polypeptides in Arabidopsis thaliana allows one to assign functions to PSI subunits. We have performed cluster analysis on an extensive set of data on PSI polypeptide levels in ten different PSI mutants. This type of analysis serves to group proteins that exhibit similar changes in amount in different genotypes, and also identifies genotypes which show similar PSI compositions. The interdependence of levels of PSI-C, -D and -E, of -H and -L, and of Lhca2 and 3, which was previously proposed based on the study of single genotypes or on cross-linking experiments, was confirmed by our analyses. In addition, the levels of the lumenal subunits F and N are found to be interdependent. The incorporation of photosynthetic parameters into the cluster analysis revealed that the level of photosynthetic state transitions correlates with the abundance of PSI-H in all 8 genotypes tested, supporting the hypothesis that PSI-H serves as a docking site for LHCII during state transitions.     

Pigment organization and energy transfer dynamics in isolated photosystem I (PSI) complexes from Arabidopsis thaliana depleted of the PSI-G, PSI-K, PSI-L, or PSI-N subunit

Green plant photosystem I (PSI) consists of at least 18 different protein subunits. The roles of some of these protein subunits are not well known, in particular those that do not occur in the well characterized PSI complexes from cyanobacteria. We investigated the spectroscopic properties and excited-state dynamics of isolated PSI-200 particles from wild-type and mutant Arabidopsis thaliana plants devoid of the PSI-G, PSI-K, PSI-L, or PSI-N subunit. Pigment analysis and a comparison of the 5 K absorption spectra of the various particles suggests that the PSI-L and PSI-H subunits together bind approximately five chlorophyll a molecules with absorption maxima near 688 and 667 nm, that the PSI-G subunit binds approximately two red-shifted beta-carotene molecules, that PSI-200 particles without PSI-K lack a part of the peripheral antenna, and that the PSI-N subunit does not bind pigments. Measurements of fluorescence decay kinetics at room temperature with picosecond time resolution revealed lifetimes of ~0.6, 5, 15, 50, 120, and 5000 ps in all particles. The 5- and 15-ps phases could, at least in part, be attributed to the excitation equilibration between bulk and red chlorophyll forms, though the 15-ps phase also contains a contribution from trapping by charge separation. The 50- and 120-ps phases predominantly reflect trapping by charge separation. We suggest that contributions from the core antenna dominate the 15-ps trapping phase, that those from the peripheral antenna proteins Lhca2 and Lhca3 dominate the 50-ps phase, and that those from Lhca1 and Lhca4 dominate the 120-ps phase. In the PSI-200 particles without PSI-K or PSI-G protein, more excitations are trapped in the 15-ps phase and less in 50- and 120-ps phases, which is in agreement with the notion that these subunits are involved in the interaction between the core and peripheral antenna proteins.     

Nearest-neighbor analysis of higher-plant photosystem I holocomplex.

Photosystem 1 (PSI) preparations from barley (Hordeum vulgare) and spinach (Spinacia oleracea) were subjected to chemical cross-linking using the cleavable homobifunctional cross-linkers dithiobis(succinimidylpropionate) and 3,3'-dithiobis(sulfosuccinimidyl-propionate). The overall pattern of cross-linked products was analyzed by the simple but powerful technique of diagonal electrophoresis, in which the disulfide bond in the cross-linker was cleaved between the first and second dimensions of the gel, and immunoblotting. A large number of cross-linked products were identified. Together with preexisting data on the structure of PSI, it was deduced that the subunits PSI-D, PSI-H, PSI-I, and PSI-L occupy one side of the complex, whereas PSI-E, PSI-F, and PSI-J occupy the other. PSI-K and PSI-G appear to be adjacent to Lhca3 and Lhca2, respectively, and not close to the other small subunits. Experiments with isolated light-harvesting complex I preparations indicate that the subunits are organized as dimers, which seem to associate to the PSI-A/PSI-B proteins independent of each other. We suggest which PSI subunit corresponds to each membrane-spanning helix in the cyanobacterial PSI structure, and present a model for higher-plant PSI.     

A previously found thylakoid membrane protein of 14kDa (TMP14) is a novel subunit of plant photosystem I and is designated PSI-P

We show that the thylakoid membrane phosphoprotein TMP14 is a novel subunit of plant photosystem I (PSI). Blue native/SDS-PAGE and sucrose gradient fractionation demonstrated the association of the protein exclusively with PSI. We designate the protein PSI-P. The presence of PSI-P subunit in Arabidopsis mutants lacking other PSI subunits was analyzed and suggested a location in the proximity of PSI-L, -H and -O subunits. The PSI-P protein was not differentially phosphorylated in state 1 and state 2.     

Reconstitution of barley photosystem I reveals that the N-terminus of the PSI-D subunit is essential for tight binding of PSI-C

Removal of the peripheral subunits PSI-C, -D and -E from the photosystem I (PSI) complex of barley requires a urea treatment much harsher than required to remove the similar subunits from cyanobacterial PSI. The resulting PSI barley core was reconstituted by addition of the E. coli expressed subunits PSI-C and -D, and PSI-E isolated from barley. Western blotting, flash photolysis and NADP⁺ photoreduction measurements demonstrated complete and specific removal of the three subunits from the core and efficient reconstitution of the complex after addition of PSI-C, -D and -E. Flash photolysis reveals that PSI-D is essential for binding of functional PSI-C to the PSI core. An N-terminally truncated barley PSI-D lacking 24 amino acid residues and thus being without the N-terminal extension characteristic for higher plant PSI-D proteins reconstitutes the PSI core to 50% of the level obtained with intact PSI-D as demonstrated by flash photolysis and NADP⁺ photoreduction measurements. Cyanobacterial PSI-D is functionally equivalent to truncated barley PSI-D with respect to its activity to reconstitute the PSI core. This shows that the N-terminal extension of plant PSI-D plays a key role in binding PSI-C to the core. The plant-specific N-terminus of PSI-D is hypothesized to execute its function through interaction with a plant-specific PSI subunit, possibly PSI-H. An anchoring function of the N-terminus of PSI-D would also explain the harsh treatment needed to obtain a plant PSI core. PSI-E is important for efficient NADP⁺ reduction but does not influence electron transfer to iron-sulphur centres A/B nor binding of PSI-C. The enhancing effect of PSI-E on NADP⁺ reduction is independent of the presence of the N-terminus of PSI-D.     

Knock-out of the chloroplast-encoded PSI-J subunit of photosystem I in Nicotiana tabacum

The plastid-encoded psaJ gene encodes a hydrophobic low-molecular-mass subunit of photosystem I (PSI) containing one transmembrane helix. Homoplastomic transformants with an inactivated psaJ gene were devoid of PSI-J protein. The mutant plants were slightly smaller and paler than wild-type because of a 13% reduction in chlorophyll content per leaf area caused by an approximately 20% reduction in PSI. The amount of the peripheral antenna proteins, Lhca2 and Lhca3, was decreased to the same level as the core subunits, but Lhca1 and Lhca4 were present in relative excess. The functional size of the PSI antenna was not affected, suggesting that PSI-J is not involved in binding of light-harvesting complex I. The specific PSI activity, measured as NADP(+) photoreduction in vitro, revealed a 55% reduction in electron transport through PSI in the mutant. No significant difference in the second-order rate constant for electron transfer from reduced plastocyanin to oxidized P700 was observed in the absence of PSI-J. Instead, a large fraction of PSI was found to be inactive. Immunoblotting analysis revealed a secondary loss of the luminal PSI-N subunit in PSI particles devoid of PSI-J. Presumably PSI-J affects the conformation of PSI-F, which in turn affects the binding of PSI-N. This together renders a fraction of the PSI particles inactive. Thus, PSI-J is an important subunit that, together with PSI-F and PSI-N, is required for formation of the plastocyanin-binding domain of PSI. PSI-J is furthermore important for stability or assembly of the PSI complex.     

Photosystem I lacking the PSI-G subunit has a higher affinity for plastocyanin and is sensitive to photodamage

PSI-G is an 11 kDa subunit of PSI in photosynthetic eukaryotes. Arabidopsis thaliana plants devoid of PSI-G have a decreased PSI content and an increased activity of NADP(+) photoreduction in vitro but otherwise no obvious phenotype. To investigate the biochemical basis for the increased activity, the kinetic parameters of the reaction between PSI and plastocyanin were determined. PSI-G clearly plays a role in the affinity for plastocyanin since the dissociation constant (K(D)) is only 12 muM in the absence of PSI-G compared to 32 muM for the wild type. On the physiological level, plants devoid of PSI-G have a more reduced Q(A). This indicates that the decreased PSI content is due to unstable PSI rather than an adaptation to the increased activity. In agreement with this indication of decreased stability, plants devoid of PSI-G were found to be more photo-inhibited both at low temperature and after high light treatment. The decreased PSI stability was confirmed in vitro by measuring PSI activity after illumination of a thylakoid suspension which clearly showed a faster decrease in PSI activity in the thylakoids lacking PSI-G. Light response of the P700 redox state in vivo showed that in the absence of PSI-G, P700 is more reduced at low light intensities. We conclude that PSI-G is involved in the binding dynamics of plastocyanin to PSI and that PSI-G is important for the stability of the PSI complex.     

Disruption of thylakoid-associated kinase 1 leads to alteration of light harvesting in Arabidopsis.

To survive fluctuations in quality and intensity of light, plants and algae are able to preferentially direct the absorption of light energy to either one of the two photosystems, PSI or PSII. This rapid process is referred to as a state transition and has been correlated with the phosphorylation and migration of the light-harvesting complex protein (LHCP) between PSII and PSI. We show here that thylakoid protein kinases (TAKs) are required for state transitions in Arabidopsis. Antisense TAK1 expression leads to a loss of LHCP phosphorylation and a reduction in state transitions. Preferential activation of PSII causes LHCP to accumulate with PSI, and TAK1 mutants disrupt this process. Finally, TAKs also influence the phosphorylation of multiple thylakoid proteins.     

A stable LHCII-PSI aggregate and suppression of photosynthetic state transitions in the psae1-1 mutant of Arabidopsis thaliana

During photosynthetic state transitions, a fraction of the major light-harvesting complex (LHCII) shuttles between photosystems II (PSII) and I (PSI), depending on whether or not it is phosphorylated. Its phosphorylation state in turn depends on the relative activity of the two photosystems, which is a function of redox state and illumination parameters. In the psae1-1 mutant of Arabidopsis thaliana (L.) Heynh., amounts of the PSI subunits E, C, D, H and L are decreased. A fraction of LHCII is stably associated with PSI when plants are exposed to low light conditions, giving rise to a high-molecular-mass protein-pigment complex detectable in native protein gels. The formation of this abnormal LHCII-PSI complex is associated with an almost complete suppression of state transitions, a drastic increase in the levels of phosphorylated LHCII under all light regimes tested, and a permanent reduction in PSII antenna size. All these observations suggest that the altered polypeptide composition of PSI perturbs the docking of phosphorylated LHCII, making psae1-1 a unique mutant for the study of PSI-LHCII interactions and additional effects of the mutation, such as a decrease in grana stacking and increased adenylate kinase activity.     

The interaction between plastocyanin and photosystem I is inefficient in transgenic Arabidopsis plants lacking the PSI-N subunit of photosystem I

The PSI-N subunit of photosystem I (PSI) is restricted to higher plants and is the only subunit located entirely in the thylakoid lumen. The role of the PSI-N subunit in the PSI complex was investigated in transgenic Arabidopsis plants which were generated using antisense and co-suppression strategies. Several lines without detectable levels of PSI-N were identified. The plants lacking PSI-N assembled a functional PSI complex and were capable of photoautotrophic growth. When grown on agar media for several weeks the plants became chlorotic and developed significantly more slowly. However, under optimal growth conditions, the plants without PSI-N were visually indistinguishable from the wild-type although several photosynthetic parameters were affected. In the transformants, the second-order rate constant for electron transfer from plastocyanin to P700+, the oxidized reaction centre of PSI, was only 55% of the wild-type value, and steady-state NADP+ reduction was decreased to a similar extent. Quantum yield of oxygen evolution and PSII photochemistry were about 10% lower than in the wild-type at leaf level. Photochemical fluorescence quenching was lowered to a similar extent. Thus, the 40-50% lower activity of PSI at the molecular level was much less significant at the whole-plant level. This was partly explained by a 17% increase in PSI content in the plants lacking PSI-N.     

Isolation and characterization of a cDNA clone encoding an 18-kDa hydrophobic photosystem I subunit (PSI-L) from barley (Hordeum vulgare L.)

Photosystem I in barley contains a polypeptide with an apparent molecular mass of 14 kDa. The polypeptide is N-terminally blocked to amino acid sequencing, but partial amino acid sequences have been determined from three fragments obtained by chemical and enzymatic cleavage. Using an oligonucleotide probe specifying this amino acid sequence, a full length cDNA clone was isolated. The deduced amino acid sequence does not correspond to any previously identified photosystem I subunit. We designate the novel photosystem I subunit PSI-L and the corresponding nuclear gene PsaL. The cDNA clone encodes a precursor polypeptide of 209 amino acid residues with a deduced molecular mass of 22,210 Da. The precursor has a transit peptide typical of proteins imported into chloroplasts. Based on a putative maturation site, the deduced molecular mass of the mature protein is 18 kDa. The PSI-L polypeptide is hydrophobic and predicted to have at least two membrane-spanning alpha-helices. Northern blot analysis shows that the expression of the PsaL gene is light-induced similar to other of the barley photosystem I genes. Southern blot analysis indicates that PsaL is a single copy gene. Partial amino acid sequences of an N-terminally blocked 9-kDa polypeptide show high sequence similarity to the PSI-G polypeptide of spinach and Chlamydomonas reinhardtii. The gene product of PsaG in spinach has previously been assigned as subunit V (Steppuhn, J., Hermans, J., Nechushtai, R., Ljungberg, U., Thümmler, F., Lottspeich, F., and Herrmann, R. G. (1988) FEBS Lett. 237, 218-224). The present study suggests that PSI-L is equivalent to subunit V and that PSI-G is a subunit migrating closely to PSI-H (subunit VI) and PSI-C (subunit VII).     

Consequences of state transitions on the structural and functional organization of Photosystem I in the green alga Chlamydomonas reinhardtii

State transitions represent a photoacclimation process that regulates the light‐driven photosynthetic reactions in response to changes in light quality/quantity. It balances the excitation between photosystem I (PSI) and II (PSII) by shuttling LHCII, the main light‐harvesting complex of green algae and plants, between them. This process is particularly important in Chlamydomonas reinhardtii in which it is suggested to induce a large reorganization in the thylakoid membrane. Phosphorylation has been shown to be necessary for state transitions and the LHCII kinase has been identified. However, the consequences of state transitions on the structural organization and the functionality of the photosystems have not yet been elucidated. This situation is mainly because the purification of the supercomplexes has proved to be particularly difficult, thus preventing structural and functional studies. Here, we have purified and analysed PSI and PSII supercomplexes of C. reinhardtii in states 1 and 2, and have studied them using biochemical, spectroscopic and structural methods. It is shown that PSI in state 2 is able to bind two LHCII trimers that contain all four LHCII types, and one monomer, most likely CP29, in addition to its nine Lhcas. This structure is the largest PSI complex ever observed, having an antenna size of 340 Chls/P700. Moreover, all PSI‐bound Lhcs are efficient in transferring energy to PSI. A projection map at 20 Å resolution reveals the structural organization of the complex. Surprisingly, only LHCII type I, II and IV are phosphorylated when associated with PSI, while LHCII type III and CP29 are not, but CP29 is phosphorylated when associated with PSII in state2.     

Photosystem I lacking the PSI-G subunit has a higher affinity for plastocyanin and is sensitive to photodamage

PSI-G is an 11 kDa subunit of PSI in photosynthetic eukaryotes. Arabidopsis thaliana plants devoid of PSI-G have a decreased PSI content and an increased activity of NADP⁺ photoreduction in vitro but otherwise no obvious phenotype [P.E. Jensen, L. Rosgaard, J. Knoetzel, H.V. Scheller, Photosystem I activity is increased in the absence of the PSI-G subunit. J. Biol. Chem. 277, (2002) 2798-2803.]. To investigate the biochemical basis for the increased activity, the kinetic parameters of the reaction between PSI and plastocyanin were determined. PSI-G clearly plays a role in the affinity for plastocyanin since the dissociation constant (K_D) is only 12 μM in the absence of PSI-G compared to 32 μM for the wild type. On the physiological level, plants devoid of PSI-G have a more reduced Q_A. This indicates that the decreased PSI content is due to unstable PSI rather than an adaptation to the increased activity. In agreement with this indication of decreased stability, plants devoid of PSI-G were found to be more photoinhibited both at low temperature and after high light treatment. The decreased PSI stability was confirmed in vitro by measuring PSI activity after illumination of a thylakoid suspension which clearly showed a faster decrease in PSI activity in the thylakoids lacking PSI-G. Light response of the P700 redox state in vivo showed that in the absence of PSI-G, P700 is more reduced at low light intensities. We conclude that PSI-G is involved in the binding dynamics of plastocyanin to PSI and that PSI-G is important for the stability of the PSI complex.     

PSI-O, a new 10-kDa subunit of eukaryotic photosystem I

A novel polypeptide with an apparent molecular mass of 9 kDa was detected after sodium dodecyl sulphate–polyacrylamide gel electrophoresis of Arabidopsis photosystem I (PSI) and was N-terminally sequenced. Corresponding cDNA clones encode a precursor protein of 140 amino acid residues which was imported into isolated intact chloroplasts and processed to the mature protein, designated PSI-O. The mature protein has two transmembrane helices and a calculated mass of 10 104 Da. The PSI-O protein was also shown to be present in PSI isolated from barley and spinach, and was essentially absent in chloroplast grana. Expressed sequences encoding similar proteins are available from many species of plants and green algae.     

Functional Analyses of the Plant Photosystem I-Light-Harvesting Complex II Supercomplex Reveal That Light-Harvesting Complex II Loosely Bound to Photosystem II Is a Very Efficient Antenna for Photosystem I in State II

State transitions are an important photosynthetic short-term response that allows energy distribution balancing between photosystems I (PSI) and II (PSII). In plants when PSII is preferentially excited compared with PSI (State II), part of the major light-harvesting complex LHCII migrates to PSI to form a PSI-LHCII supercomplex. So far, little is known about this complex, mainly due to purification problems. Here, a stable PSI-LHCII supercomplex is purified from Arabidopsis thaliana and maize (Zea mays) plants. It is demonstrated that LHCIIs loosely bound to PSII in State I are the trimers mainly involved in state transitions and become strongly bound to PSI in State II. Specific Lhcb1-3 isoforms are differently represented in the mobile LHCII compared with S and M trimers. Fluorescence analyses indicate that excitation energy migration from mobile LHCII to PSI is rapid and efficient, and the quantum yield of photochemical conversion of PSI-LHCII is substantially unaffected with respect to PSI, despite a sizable increase of the antenna size. An updated PSI-LHCII structural model suggests that the low-energy chlorophylls 611 and 612 in LHCII interact with the chlorophyll 11145 at the interface of PSI. In contrast with the common opinion, we suggest that the mobile pool of LHCII may be considered an intimate part of the PSI antenna system that is displaced to PSII in State I.