Isolation and characterization of the 10-kDa and 22-kDa polypeptides of higher plant photosystem 2

The 28 kDa apoprotein of CP 26 in PS II binds copper

Photosystem II (PS II) particles isolated from spinach in the presence of 10 μM CuSO4 contained 1.2 copper/300 Chl that was resistant to EDTA. When CuSO4 was not added during the isolation, PS II particles contained variable amounts of copper resistant to EDTA (0.1-1.1 copper/300 Chl). No correlation was found between copper content and oxygen evolving capacity of the PS II particles. To identify the copper binding protein, we developed a fractionation procedure which included solubilisation of PS II particles followed by precipitation with polyethylene glycol. A 22-fold purification of copper with respect to protein was achieved for a 28 kDa protein. Partial amino acid sequence of a 13 kDa fragment, obtained after V8 (endo Glu-C) protease treatment, showed identity with CP 26 over a 14 amino acid stretch. EPR measurements on the purified protein suggest oxygen and/or nitrogen as ligands for copper but tend to exclude sulfur. We conclude that the 28 kDa apoprotein of CP 26 from spinach binds one copper per molecule of CP 26. A possible function for this copper protein in the xanthophyll cycle is discussed.

Selective extraction of 22 kDa and 10 kDa polypeptides from Photosystem II without removal of 23 kDa and 17 kDa extrinsic proteins

Selective solubilization of Photosystem II membranes with the non-ionic detergent octyl thioglucopyranoside has allowed the isolation of a PS II system which has been depleted of the 22 and 10 kDa polypeptides but retains all three extrinsic proteins (33, 23 and 17 kDa). The PS II membranes which have been depleted of the 22 and 10 kDa species show high rates of oxygen evolution activity, external calcium is not required for activity and the manganese complex is not destroyed by exogenous reductants. When we compared this system to control PS II membranes, we observed a minor modification of the reducing side, and a conversion of the high-potential to the low-potential form of cytochrome b 559.

Characterization of a spinach psbS cDNA encoding the 22 kDa protein of photosystem II

An intrinsic 22 kDa polypeptide is found associated with the oxygen-evolving photosystem II (PSII) core complex in all green plants and cyanobacteria so far examined, although it does not appear to be required for oxygen evolution. Amino acid sequence information obtained from the purified 22 kDa protein was used to construct a probe that was employed to isolate a full-length cDNA clone encoding the 274-residue precursor of the 22 kDa protein. Hydropathy plot analysis predicts the existence of four membrane-spanning helices in the mature protein. The two halves of the approximately 200-residue mature protein show high sequence similarity to each other, suggesting that the psbS gene arose from an internal gene duplication. The 22 kDa protein has some sequence similarity to chlorophyll a/b-binding proteins.

Ultrastructural and biochemical characterization of a Synechocystis 6803 mutant with inactivated psbA genes

A constructed Synechocystis 6803 mutant with a deletion of the three psbA genes was subjected to ultrastructural and biochemical characterization. This D1-depleted mutant also lacks the D2 protein and the chlorophyll a-binding protein CP-47. A general ultrastructural comparison between the wild type and the mutant did not reveal any major changes in cell appearance. We found by freeze-fracture analysis that approximately 60% of the endoplasmic face particles found in the wild-type thylakoids were missing in the mutant. A corresponding increase in protoplasmic face particles in the mutant thylakoids may represent a subcomplex of those photosystem II (PS II) polypeptides which accumulate in the absence of the D1 protein. Correlation of the PS I:PS II ratio with freeze-fracture data indicates that there is only one reaction center in each PS II freeze-fracture particle. Fluorescence measurements show that the CP-43 polypeptide in the mutant binds chlorophyll and that it may be connected to the phycobilisomes. Excitation energy can be transferred from the phycobilisomes to photosystem I in the absence of the photosystem II reaction center heterodimer and CP-47. This suggests that exciton transfer to photosystem I is mediated either directly by a terminal phycobilisome transmitter or via CP-43.

Light-induced D1 protein degradation is catalyzed by a serine-type protease

Light-induced degradation of the D1 protein in isolated spinach photosystem II core preparations was studied after addition of various protease inhibitors. The degradation was selectively inhibited by several serine protease inhibitors in particular diisopropylfluorophosphate. The results demonstrate that the D1 protein is degraded by a serine-type of proteolytic activity that is an integral part of photosystem II.

Chlorophyll a/b binding (CAB) polypeptides of CP29, the internal chlorophyll a/b complex of PSII: characterization of the tomato gene encoding the 26 kDa (type I) polypeptide, and evidence for a second CP29 polypeptide

CP29, the core chlorophyll a/b (CAB) antenna complex of Photosystem II (PSII), has two nuclear-encoded polypeptides of approximately 26 and 28 kDa in tomato (Lycopersicon esculentum). Cab9, the gene for the Type I (26 kDa) CP29 polypeptide was cloned by immunoscreening a tomato leaf cDNA library. Its identity was confirmed by sequencing tryptic peptides from the mature protein. Cab9 is a single-copy gene with five introns, the highest number found in a CAB protein. In vitro transcription-translation gave a 31 kDa precursor which was cleaved to about 26 kDa after import into isolated tomato chloroplasts. The Cab9 polypeptide has the two highly conserved regions common to all CAB polypeptides, which define the members of this extended gene family. Outside of the conserved regions, it is only slightly more closely related to other PSII CABs than to PSI CABs. Sequence analysis of tryptic peptides from the Type II (28 kDa) CP29 polypeptide showed that it is also a member of the CAB family and is very similar or identical to the CP29 polypeptide previously isolated from spinach. All members of the CAB family have absolutely conserved His, Gln and Asn residues which could ligate the Mg atoms of the chlorophylls, and a number of conserved Asp. Glu, Lys and Arg residues which could form H-bonds to the polar groups on the porphyrin rings. The two conserved regions comprise the first and third predicted trans-membrane helices and the stroma-exposed segments preceding them.

A plastome mutation affects processing of both chloroplast and nuclear DNA-encoded plastid proteins

Immunoblotting of a chloroplast mutant (pm7) of Oenothera showed that three proteins, cytochrome f and the 23 kDa and 16 kDa subunits of the oxygen-evolving subcomplex of photosystem II, were larger than the corresponding mature proteins of the wild type and, thus, appear to be improperly processed in pm7. The mutant is also chlorotic and has little or no internal membrane development in the plastids. The improperly processed proteins, and other proteins that are completely missing, represent products of both the plastid and nuclear genomes. To test for linkage of these defects, a green revertant of pm7 was isolated from cultures in which the mutant plastids were maintained in a nuclear background homozygous for the plastome mutator (pm) gene. In this revertant, all proteins analyzed co-reverted to the wild-type condition, indicating that a single mutation in a plastome gene is responsible for the complex phenotype of pm7. These results suggest that the defect in pm7 lies in a gene that affects a processing protease encoded in the chloroplast genome.

Quantitative correlation of peripheral and intrinsic core polypeptides of photosystem II with photosynthetic electron-transport activity ofAcetabularia mediterranea in red and blue light

The high photosynthetic activity (O2 production and CO2 consumption) ofAcetabularia mediterranea Lamour. (=A. acetabulum (L.) Silva) characteristic of cells cultured in white light decreases slowly when cells are kept in continuous red light, and is less than 20% of the original activity after three weeks. Subsequent blue irradiation restores the original activity completely within 3-5 d. The polypeptide composition of the thylakoids from cells grown in either red or blue light and after transfer from red to blue light was analyzed mainly with regards to photosystem II (PSII). The P700-containing reaction-centre complex of photosystem I, CPI, showed only minor quantitative alterations as a consequence of the growth-light quality, which correlated well with the activity of photosystem I under these conditions. In PSII, no drastic changes occurred in the quantity of the reaction-centre components D1 (herbicide-binding polypeptide) and D2, as determined by immunoblots. Likewise, the proteins associated with the water-splitting apparatus did not change detectably in thylakoids from red- or blue-light-treated cells (the 16-kDa component could not be found inAcetabularia thylakoids). The level of the major light-harvesting complex was completely unaffected by the light quality. In contrast, the quantities of the chlorophyll a-protein complexes of the core antenna, CP43 and CP47 (and probably CP29), changed, with kinetics similar to those of total photosynthetic activity. We postulate that the function of the PSII antenna became increasingly impaired in the absence of blue light (i.e. in red light), while blue light had a restoring effect. The peripheral antenna, comprising the light-harvesting complexes, is probably functionally connected with the reaction-centre chlorophylls via the core antenna chlorophyll-protein complexes (CP43, CP47 and probably CP29). A deficiency of these complexes would lead to uncoupling of antenna and reaction centre in the majority of PSII complexes after long periods of red-light treatment.

Light-induced D1-protein degradation in isolated photosystem II core complexes

Photoinhibitory illumination of isolated oxygen evolving photosystem II core complexes results in a substantial degradation of the D1-protein which is accompanied by the appearance of high amounts of at least 4 different degradation products. It is suggested that the degradation is due to a protease that is an integral part of the photosystem II complex.

Light-dependent degradation of the D1 protein in photosystem II is accelerated after inhibition of the water splitting reaction

Strong illumination of oxygen-evolving organisms inhibits the electron transport through photosystem II (photoinhibition). In addition the illumination leads to a rapid turnover of the D1 protein in the reaction center of photosystem II. In this study the light-dependent degradation of the D1 reaction center protein and the light-dependent inhibition of electron-transport reactions have been studied in thylakoid membranes in which the oxygen evolution has been reversibly inhibited by Cl- depletion. The results show that Cl(-)-depleted thylakoid membranes are very vulnerable to damage induced by illumination. Both the D1 protein and the inhibition of the oxygen evolution are 15-20 times more sensitive to illumination than in control thylakoid membranes. The presence, during the illumination, of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) prevented both the light-dependent degradation of the D1 protein and the inhibition of the electron transport. The protection exerted by DCMU is seen only in Cl(-)-depleted thylakoid membranes. These observations lead to the proposal that continuous illumination of Cl(-)-depleted thylakoid membranes generates anomalously long-lived, highly oxidizing radicals on the oxidizing side of photosystem II, which are responsible for the light-induced protein damage and inhibition. The presence of DCMU during the illumination prevents the formation of these radicals, which explains the protective effects of the herbicide. It is also observed that in Cl(-)-depleted thylakoid membranes, oxygen evolution (measured after the readdition of Cl-) is inhibited before electron transfer from diphenylcarbazide to dichlorophenolindophenol.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential biogenesis of photosystem-II in mesophyll and bundle-sheath cells of 'malic' enzyme NADP(+)-type C4 plants. A comparative protein and RNA analysis

We have investigated the photosystem-II organization in differentiating-bundle-sheath cells of the three malate dehydrogenase (oxaloacetate decarboxylating) (NADP+)-type C4 species maize, Sorghum and Pennisetum. Using a set of nine different antisera raised against individual subunits of photosystem-II, we demonstrate that photosystem-II components constitute a substantial part of the thylakoid membranes of young bundle-sheath chloroplasts. The abundance of subunits of the photosystem-II core, i.e. the 47-and 43-kDa chlorophyll-a-binding proteins, polypeptides D1 and D2, cytochrome b559, and the 34-kDa polypeptide, varies with the developmental state of the plant. However, the levels of the 23-kDa, 16-kDa and 10-kDa extrinsic polypeptides of the water-oxidation complex are drastically reduced in bundle-sheath chloroplasts of all three species analyzed, regardless of their state of differentiation. The reduction in protein abundance is also reflected at the transcript level: only traces of the nuclear-encoded mRNAs are found in differentiating bundle-sheath cells of Sorghum, suggesting that the transcription of these genes has been switched off. Our data are compatible with the idea that the water-oxidation complex is a prime site for initiating or maintaining the process leading to photosystem-II depletion during differentiation of bundle-sheath cells.

Particles of photosystem 2 contain plantacyanin

PS2 particles prepared from chloroplasts of three plant species were shown to contain the basic blue copper protein, plantacyanin, which may be extracted from the particles by concentrated saline solutions containing Triton X-100. Antibodies to plantacyanin were found to inhibit the photosynthetic oxygen evolution performed by the particles. Thus, evidences were obtained for participation of this protein in the oxygen-evolving activity of PS2 particles.

Further investigations on structural and catalytic properties of O2 evolving preparations from tobacco and two chlorophyll deficient tobacco mutants

Previous investigations (Specht, S., Pistorius, E.K. and Schmid, G.H.: Photosynthesis Res. 13, 47-56, 1987) of Photosystem II membranes from tobacco (Nicotiana tabacum L. cv. John William's Broadleaf) which contain normally stacked thylakoid membranes and from two chlorophyll deficient tobacco mutants (Su/su and Su/su var. Aurea) which have low stacked or essentially unstacked thylakoids with occasional membrane doublings, have been extended by using monospecific antisera raised against the three extrinsic polypeptides of 33,21 and 16 kDa. The results show that all three peptides are synthesized as well in wild type tobacco as in the two mutants to about the same level and that they are present in thylakoid membranes of all three plants. However, in the mutants the 16 and 21 kDa peptides (but not the 33 kDa peptide) are easily lost during solubilization of Photosystem II membranes. In the absence of the 16 and 21 kDa peptide Photosystem II membranes from the mutants have a higher O2 evolving activity without addition of CaCl2 than the wild type Photosystem II membranes. On the other hand, after removal of the 33 kDa peptide no significant differences in the binding of Mn could be detected among the three plants. The results also show that reaction center complexes from wild type tobacco and the mutant Su/su are almost identical to the Triton-solubilized Photosystem II membranes from the mutant Su/su var. Aurea.

Current perceptions of Photosystem II

In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385-398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q400 component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field.

Mutation at the Chlamydomonas nuclear NAC2 locus specifically affects stability of the chloroplast psbD transcript encoding polypeptide D2 of PS II

The nuclear NAC2 locus of C. reinhardtii encodes a trans-acting factor that confers stability on the chloroplast message for the D2 protein of photosystem II (PS II). A mutant that carries a lesion in the NAC2 gene is unable to accumulate the psbD transcript encoding D2, but synthesizes all other PS II proteins normally. However, the PS II complex is completely unstable in the nac2-26 mutant, and all major PS II polypeptides, including the three oxygen-evolving enhancing proteins, are absent or greatly reduced in this strain because of posttranslational degradation. In C. reinhardtii the second exon of the trans-spliced psaA mRNA is cotranscribed with psbD (Choquet et al., 1988), but the nac2-26 mutation normally has no effect on psaA message maturation or stability. However, in double mutants carrying the nac2-26 mutation, as well as a mutation that prevents psaA splicing, splicing intermediates carrying psbD sequences are degraded. These results suggest that the NAC2 gene product acts in a very specific manner to control the half-life of psbD transcripts.

Posttranslational events leading to the assembly of photosystem II protein complex: a study using photosynthesis mutants from Chlamydomonas reinhardtii

We studied the assembly of photosystem II (PSII) in several mutants from Chlamydomonas reinhardtii which were unable to synthesize either one PSII core subunit (P6 [43 kD], D1, or D2) or one oxygen-evolving enhancer (OEE1 or OEE2) subunit. Synthesis of the PSII subunits was analyzed on electrophoretograms of cells pulse labeled with [14C]acetate. Their accumulation in thylakoid membranes was studied on immunoblots, their chlorophyll-binding ability on nondenaturating gels, their assembly by detergent fractionation, their stability by pulse-chase experiments and determination of in vitro protease sensitivity, and their localization by immunocytochemistry. In Chlamydomonas, the PSII core subunits P5 (47 kD), D1, and D2 are synthesized in a concerted manner while P6 synthesis is independent. P5 and P6 accumulate independently of each other in the stacked membranes. They bind chlorophyll soon after, or concomitantly with, their synthesis and independently of the presence of the other PSII subunits. Resistance to degradation increases step by step: beginning with assembly of P5, D1, and D2, then with binding of P6, and, finally, with binding of the OEE subunits on two independent high affinity sites (one for OEE1 and another for OEE2 to which OEE3 binds). In the absence of PSII cores, the OEE subunits accumulate independently in the thylakoid lumen and bind loosely to the membranes; OEE1 was found on stacked membranes, but OEE2 was found on either stacked or unstacked membranes depending on whether or not P6 was synthesized.

Analysis of the genes of the OEE1 and OEE3 proteins of the photosystem II complex from Chlamydomonas reinhardtii

The sequences of the nuclear genes of the 33 kDa (OEE1) and the 16 kDa (OEE3) polypeptides of the oxygen evolving complex of Chlamydomonas reinhardtii have been established. Comparison between the OEE1 protein sequences of C. reinhardtii and higher plants and cyanobacteria reveals 67 and 47% homology. In contrast, C. reinhardtii and higher plants have only 28% overall homology for OEE3 which is mostly limited to the central portion of the protein. The transit peptides of the C. reinhardtii proteins consist of 52 (OEE1) and, most likely, 51 (OEE1) amino acids. They have a basic amino terminal region and, at least in the case of OEE1, a hydrophobic segment at their carboxy terminal end typical of thylakoid lumen proteins. Comparison of the genomic and cDNA clones indicates that the OEE1 and OEE3 genes contain five and four introns, respectively, some of which are located within the coding sequences of the transit peptides.

A 10 kDa polypeptide associated with the oxygen-evolving complex of photosystem II has a putative C-terminal non-cleavable thylakoid transfer domain

The N-terminal amino acid sequence of the 10 kDa polypeptide associated with the oxygen-evolving complex of wheat photosystem II has been determined and shown to be homologous to the amino acid sequence of the product of the ST-LS1 gene from potato. The N-terminal sequence of the mature protein indicates that the polypeptide is synthesized with a 39 amino acid N-terminal presequence which is similar to chloroplast import sequences but which lacks a hydrophobic domain for transfer of the protein across the thylakoid membrane. The mature polypeptide has a C-terminal hydrophobic region which shows homology to the hydrophobic thylakoid transfer domain of other lumenal proteins and this hydrophobic region of the 10 kDa polypeptide is suggested to facilitate transfer of the protein across the thylakoid membrane.

Comparison of photosystem II complexes isolated from tobacco and two chlorophyll deficient tobacco mutants

A comparative study of photosystem II complexes isolated from tobacco (Nicotiana tabacum L. cv. John William's Broadleaf) which contains normal stacked thylakoid membranes, and from two chlorophyll deficient tobacco mutants (Su/su and Su/su var. Aurea) which have low stacked grana or essentially unstacked thylakoids with occasional membrane doublings, has been carried out. The corresponding photosystem II complexes had an O2 evolving activity ranging from 290 (for the wild type) to 1100 μmol O2 x mg chlorophyll(-1) x h(-1) (for the mutant Su/su var. Aurea). The reduced photosynthetic unit size was also obvious in the mangenese and cytochromeb559 content. The photosystem II complex from the wild type contained 4 Mn and 1 cytochromeb559 per 200 to 280 chlorophylls, while the corresponding value for the mutant Su/su var. Aurea was 4 Mn and 1 cytochromeb559 per 35 to 60 chlorophylls. We have also examined the polypeptide composition and show that the photosystem II complex from the wild type consisted of polypeptides of 48, 42, 33, 32, 30, 28, 23, 21, 18, 16 and 10 kDa, while the mutant complex mainly contained the polypeptides of 48, 42, 33, 32, 30, 28 and 10 kDa. In the mutant photosystem II complex the light-harvesting chlorophyll protein (peptide of 28 kDa) was reduced by a factor of 5 to 6 as compared to the wild type. With respect to the peptide composition and the photosynthetic unit size, the Triton-solubilized photosystem II complex from the mutant Su/su var. Aurea was very similar to O2 evolving photosystem II reaction center core complexes.