Reconstitution of oxygen evolution in high salt washed photosystem II particles

Proteome map of the chloroplast lumen of Arabidopsis thaliana

The thylakoid membrane of the chloroplast is the center of oxygenic photosynthesis. To better understand the function of the luminal compartment within the thylakoid network, we have carried out a systematic characterization of the luminal thylakoid proteins from the model organism Arabidopsis thaliana. Our data show that the thylakoid lumen has its own specific proteome, of which 36 proteins were identified. Besides a large group of peptidyl-prolyl cis-trans isomerases and proteases, a family of novel PsbP domain proteins was found. An analysis of the luminal signal peptides showed that 19 of 36 luminal precursors were marked by a twin-arginine motif for import via the Tat pathway. To compare the model organism Arabidopsis with another typical higher plant, we investigated the proteome from the thylakoid lumen of spinach and found that the luminal proteins from both plants corresponded well. As a complement to our experimental investigation, we made a theoretical prediction of the luminal proteins from the whole Arabidopsis genome and estimated that the thylakoid lumen of the chloroplast contains approximately 80 proteins.

Importance of the N-terminal sequence of the extrinsic 23 kDa polypeptide in Photosystem II in ion retention in oxygen evolution

The function of the extrinsic 23 kDa polypeptide (OEC23) in Photosystem II (PS II) is to retain Ca(2+) and Cl(-) during the S-state turnover of the Mn cluster in photosynthetic oxygen evolution. Recombinant OEC23s from several plant species were produced in Escherichia coli to characterize the molecular mechanism of the OEC23 function then used in reconstitution experiments. One tobacco isoform, OEC23 (2AF), had much less oxygen-evolving activity than the spinach and cucumber OEC23s when PS II activities were reconstituted in salt-washed spinach PS II particles. The fact that the OEC23s had similar binding affinities for PS II particles suggests different ion-retention capacities for the individual OEC23s: The chimeric OEC23s produced between spinach and 2AF and those produced between cucumber and 2AF show that 58 N-terminal amino acid residues are important for PS II activity. Further dissection of the sequence and site-directed mutagenesis indicated the importance of 20 N-terminal amino acid residues for activity, in particular the asparagine at the 15th position. In spinach the N15D mutation decreased PS II activity, whereas in 2AF the D15N mutation increased it. This shows the importance of the N-terminal sequence of OEC23 in ion retention during the water-splitting process.

The high affinity binding site for calcium on the oxidizing side of photosystem II

Preparations of photosystem II complex from spinach chloroplasts with Triton X-100 were treated with 1 M KCl to release 17 KDa and 23 KDa polypeptides. The inhibited oxygen evolution activity could be reactivated by adding high concentration (mM) of Ca++ or by reconstituting 17 KDa and 23 KDa polypeptides which were found to promote high affinity binding of Ca++ to the reconstituted membranes (Ghanotakis et al. FEBS (1984) 170, 169-173). Inclusion of 50 mM Ca++ during KCl treatment did not prevent the release of 17 KDa and 23 KDa polypeptides but protected oxygen evolution from being inactivated. It is explained by preservation of the high affinity binding site for Ca++ if, Ca++ is present during the salt treatment even though depletion of 17 KDa and 23 KDa polypeptides usually results in replacement by a low affinity (mM) binding site for Ca++. It also implies that the high affinity binding site is not located on 17 KDa and 23 KDa polypeptides.

Intracellular coding sites of polypeptides associated with photosynthetic oxygen evolution of photosystem II

Three hydrophilic polypeptides of approximately 34, 23, and 16 kd located on the inner thylakoid surface are associated with the water-splitting activity of photosystem II. Stable transcripts for the three proteins were found only in cytosolic (polyadenylated) RNA, suggesting that they are encoded in nuclear genes. The immunologically reacting products synthesized in a rabbit reticulocyte cell-free translation system are larger in size than the authentic mature proteins by about 6-10 kd. These larger precursors are imported post-translationally into isolated, intact chloroplasts, and are processed to their mature forms during or after translocation. The imported proteins can be extracted from thylakoids by procedures used to isolate the three native proteins of the water-splitting complex, suggesting that they have assembled properly into their final destination, the inner thylakoid surface.

The effects of mono- and divalent salts on the O2-evolution activity and low temperature multiline EPR spectrum of Photosystem II preparations from spinach

The ability of salts to inhibit the O2-evolution activity of PS II preparations is shown to parallel closely the Hofmeister series, suggesting that inhibition is related to the solubility of the 16, 24 and 33 kDa proteins in these salt solutions. An examination of the effect of salt inactivation on the low temperature multiline EPR signal indicates that the release of either the 16 and 24 kDa proteins, or additionally the 33 kDa protein blocks or greatly reduces the efficiency of the advancement of the water-splitting complex to the S2-state; under some conditions, this inhibition is reversible.

EPR studies on the photosystem II donor side in salt-washed and reconstituted inside-out thylakoids

EPR measurements on inside-out thylakoids revealed that salt-washing, known to inhibit oxygen evolution and release a 23 and a 16 kDa protein, induced a Signal IIf and decreased the EPR signal from state S2. Readdition of the released 23 kDa protein restored the oxygen evolution and decreased the Signal IIf, but did not relieve the decrease in the state S2 signal. It is suggested that salt-washing inhibits the electron transfer from the oxygen-evolving site to Z, the physiological donor to P680. In inhibited photosystem II units lacking Signal IIf, Z+ is rapidly reduced, possibly by a modified S-cycle unable to evolve oxygen.