Abstract
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.
Highlights
A nuclear-encoded polypeptide of 6.1 kDa was identified in isolated photosystem II (PSII) reaction center from Spinacia oleracea
In this paper we report on the identification and isolation of a nuclear-encoded 6.1-kDa polypeptide
The low molecular mass polypeptide was purified by first stepwise isocratically detaching other PSII polypeptides from the column by increasing the NaCI concentration from 10 to 500 mx and adding Triton X-I00 to a final concentration of 0.5% (v/v)
Summary
A nuclear-encoded polypeptide of 6.1 kDa was identified in isolated photosystem II (PSII) reaction center from Spinacia oleracea. The hydrophobic membrane protein 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 DI1D2/cytb559 proteins and the psbI gene product. 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. Light-induced photosynthetic water oxidation and plastoquinone reduction takes place in the thylakoids of cyanobacteria, algae, and plants These redox mediated reactions are catalyzed by a multisubunit membrane complex designated as photo system II [1,2,3,4]. Solubilized complex contains 10-13 different polypeptides, four manganese ions, at least one calcium ion, and about 50 chlorophyll a molecules
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