Characterization of low molecular mass proteins of photosystem II by N-terminal sequencing

The low molecular mass subunits of the photosynthetic supracomplex, photosystem II

The photosystem II (PSII) complex is located in the thylakoid membrane of higher plants, algae and cyanobacteria and drives the water oxidation process of photosynthesis, which splits water into reducing equivalents and molecular oxygen by solar energy. Electron and X-ray crystallography analyses have revealed that the PSII core complex contains between 34 and 36 transmembrane alpha-helices, depending on the organism. Of these helices at least 12-14 are attributed to low molecular mass proteins. However, to date, at least 18 low molecular mass (<10 kDa) subunits are putatively associated with the PSII complex. Most of them contain a single transmembrane span and their protein sequences are conserved among photosynthetic organisms. In addition, these proteins do not have any similarity to any known functional proteins in any type of organism, and only two of them bind a cofactor. These findings raise intriguing questions about why there are so many small protein subunits with single-transmembrane spans in the PSII complex, and their possible functions. This article reviews our current knowledge of this group of proteins. Deletion mutations of the low molecular mass subunits from both prokaryotic and eukaryotic model systems are compared in an attempt to understand the function of these proteins. From these comparisons it seems that the majority of them are involved in stabilization, assembly or dimerization of the PSII complex. The small proteins may facilitate fast dynamic conformational changes that the PSII complex needs to perform an optimal photosynthetic activity.

Subunit positioning and transmembrane helix organisation in the core dimer of photosystem II

Recently 3D structural models of the photosystem II (PSII) core dimer complexes of higher plants (spinach) and cyanobacteria (Synechococcus elongatus) have been derived by electron [Rhee et al. (1998) Nature 396, 283-286; Hankamer et al. (2001) J. Struct. Biol., in press] and X-ray [Zouni et al. (2001) Nature 409, 739-743] crystallography respectively. The intermediate resolutions of these structures do not allow direct identification of side chains and therefore many of the individual subunits within the structure are unassigned. Here we review the structure of the higher plant PSII core dimer and provide evidence for the tentative assignment of the low molecular weight subunits. In so doing we highlight the similarities and differences between the higher plant and cyanobacterial structures.

Chromatographic separation of a small subunit (PsbW/PsaY) and its assignment to Photosystem I reaction center

By using a hydroxyapatite column, the five major Photosystem I (PSI) subunits (PsaA,-B,-C,-D,-E) solubilized by sodium dodecyl sulfate (SDS) were fractionated from a spinach PSI reaction center preparation. Another small (5-6 kDa) polypeptide was also separated, and purified to homogeneity. Mass spectroscopy yielded its molecular weight to be 5942 +/- 10. This polypeptide had an N-terminal sequence homologous to those of previously reported 5-kDa subunits from spinach and wheat and a 6.1-kDa subunit of Chlamydomonas, which had all been assigned to Photosystem II (PSII) and designated as PsbW. However, we found similar 5-kDa polypeptides with highly conserved N-terminal sequences ubiquitously in PSI particles from other plants including Daikon (Raphanus sativus, Japanese radish), Chingensai (Brassica parachinensis, Chinese cabbage), parsley and Shungiku (Chrysanthemum coronarium, Garland chrysanthemum) as well. Preparations of spinach PSI particles prepared by using a mild detergent (digitonin) had this 5-kDa subunit, while PSII particles did not. Moreover, a bare-bone PSI reaction center preparation consisting of PsaA/B alone had a more than stoichiometric amount of this 5-kDa polypeptide. A mechanically (without detergent) fractionated stroma thylakoid preparation from Phytolacca americana, which lacked other PSII subunits, also contained this 5-kDa subunit. Thus, we propose that this 5-kDa polypeptide, previously designated as a PSII subunit (PsbW), is an integral subunit of PSI as well.

A nuclear-encoded subunit of the photosystem II reaction center

A nuclear-encoded polypeptide of 6.1 kDa was identified in isolated photosystem II (PSII) reaction center from Spinacia oleracea. The hydrophobic membrane protein easily escapes staining procedures such as Coomassie R-250 or silver staining, but it is clearly detected by immunodecoration with peptide-directed IgG. This additional subunit was found to be present in PSII reaction centers previously known to contain only the D1/D2/cytb559 proteins and the psbI gene product. Furthermore, cross-linking experiments using 1-(3-dimethylaminopropyl-) 3-ethylcarbodiimide showed that the nearest neighbors were the D1 and D2 proteins and the cytb559. The 6.1-kDa protein was purified by immune affinity chromatography. N-terminal sequence analysis of the isolated protein confirmed the identity of the 6.1-kDa protein and enabled finding of strong similarities with a randomly obtained cDNA from Arabidopsis thaliana. Using enzyme-linked immunosorbent assay in combination with thylakoid membrane preparations of different orientation, the N terminus of the protein, predicted to span the membrane once, is suggested to be exposed at the lumen side of the membrane. Consequently the 6.1-kDa protein seems to be the only subunit in the PSII reaction center that is nuclear encoded and has its N terminus on the lumen side of the membrane. These findings open for new interesting suggestions concerning the properties of photosystem II reaction center with respect to the photosynthetic activity, regulation and assembly in higher plants.

Photosynthetic water oxidation: The protein framework

Approximately 20 protein subunits are associated with the PS II complex, not counting subunits of peripheral light-harvesting antenna complexes. However, it is not yet established which proteins specifically are involved in the water-oxidation process. Much evidence supports the concept that the D1/D2 reaction center heterodimer not only plays a central role in the primary photochemistry of Photosystem II, but also is involved in electron donation to P680 and in ligation of the manganese cluster. This evidence includes (a) the primary donor to P680 has been shown to be a redox-active tyrosyl residue (Tyr161) in the D1 protein, and (b) site-directed mutagenesis and computer-assisted modeling of the reaction center heterodimer have suggested several sites with a possible function in manganese ligation. These include Asp170, Gln165 and Gln189 of the D1 protein and Glu69 of the D2 protein as well as the C-terminal portion of the mature D1 protein. Also, hydrophilic loops of the chlorophyll-binding protein CP43 that are exposed at the inner thylakoid surface could be essential for the water-splitting process.In photosynthetic eukaryotes, three lumenal extrinsic proteins, PS II-O (33 kDa), PS II-P (23 kDa) and PS II-Q (16 kDa), influence the properties of the manganese cluster without being involved in the actual catalysis of water oxidation. The extrinsic proteins together may have multiple binding sites to the integral portion of PS II, which could be provided by the D1/D2 heterodimer and CP47. A major role for the PS II-O protein is to stabilize the manganese cluster. Most experimental evidence favors a connection of the PS II-P protein with binding of the Cl(-) and Ca(2+) ions required for the water oxidation, while the PS II-Q protein seems to be associated only with the Cl(-) requirement. The two latter proteins are not present in PS II of prokaryotic organisms, where their functions may be replaced by a 10-12 kDa subunit and a newly discovered low-potential cytochrome c-550.

Nucleotide sequence and expression of the two genes encoding D2 protein and the single gene encoding the CP43 protein of Photosystem II in the cyanobacterium synechococcus sp. PCC 7002

The unicellular photoheterotrophic cyanobacterium Synechococcus sp. PCC 7002 was shown to encode two genes for the Photosystem II reaction center core protein D2 and one gene for the reaction center chlorophyhll-binding protein CP43. These three genes were cloned and their DNA sequences determined along with their flanking DNA sequences. Northern hybridization experiments show that both genes which encode D2, psbD1 and psbD2, are expressed at roughly equivalent levels. For each of the two psbD genes, there are 18 nucleotide differences among the 1059 nucleotides which are translated. The DNA sequences surrounding the coding sequences are nearly 70% divergent. Despite the DNA sequence differences in the genes, the proteins encoded by the two genes are predicted to be identical. The proteins encoded by psbD1 and psbD2 are ∼92% homologous to other sequenced cyanobacterial psbD genes and ∼86% homologous to sequenced chloroplast-encoded psbD genes.The single gene for CP43, psbC, overlaps the 3' end of psbD1 and is co-transcribed with it. Results from previous sequencing of psbC genes encoded by chloroplasts suggest that the 5' end of the psbC gene overlaps the 3' end of the coding sequence of psbD by ∼50 nucleotides. In Synechococcus sp. PCC 7002, the methionine codon previously proposed to be the start codon for psbC is replaced by an ACG (threonine) codon. We propose an alternative start for the psbC gene at a GTG codon 36 nucleotides downstream from the threonine codon. This GTG codon is preceded by a consensus E. coli-like ribosome binding sequence. Both the GTG start codon and its preceding ribosome binding sequence are conserved in all psbC genes sequenced from cyanobacteria and chloroplasts. This suggests that all psbC genes start at this alternative GTG codon. Based on this alternative start codon, the gene product is ∼85% identical to other cyanobacterial psbC gene products and ∼77% identical to eucaryotic chloroplast-encoded psbC gene products.

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.

N-terminal sequencing of photosystem II low-molecular-mass proteins. 5 and 4.1 kDa components of the O2-evolving core complex from higher plants

High resolution gel electrophoresis in the low-molecular-mass region combined with electroblotting using polyvinylidene difluoride membranes enabled us to sequence the low-molecular-mass proteins of photosystem II membrane fragments from spinach and wheat. The determined N-terminal sequences, all showing considerable homology between the two plants, involved two newly determined sequences for the 4.1 kDa protein and one for the 5 kDa proteins. The sequence of the 4.1 kDa protein did not match any part of the chloroplast DNA sequence from tobacco or liverwort, suggesting that it is encoded by the nuclear genome. In contrast, the sequence of the 5 kDa protein matched ORF38, which is located just downstream of psbE and psbF in the chloroplast DNA and is assumed to be co-transcribed with them. These two components were associated with the O2-evolving core complex. Sequences of other low-molecular-mass proteins confirmed the previous identification as photosystem II components.