Photosystem I-less Synechocystis 6803 mutants carrying modified PsbH proteins, derived from different combinations of wild-type cyanobacterial and maize genes, were constructed. The mutants were analyzed in order to determine the relative importance of the intra- and extra-membrane domains of the PsbH subunit in the functioning of Photosystem (PS) II, by a combination of biochemical, biophysical and physiological approaches. The results show that besides D1, the whole PsbH protein is necessary to determine the correct structure of a QB/herbicide-binding site.Furthermore the data indicate that a functional PsbH protein is necessary i) for rapid degradation of photo-damaged D1 molecules, which is essential to avoid further oxidative damage to the PSII core, and ii) for the insertion of newly synthesized D1 molecules into the thylakoid membrane. PsbH is thus required for both initiation and completion of the repair cycle of the PSII complex in cyanobacteria. In higher plants, the PsbS subunit plays a crucial role in pH- and xanthophyll-dependent non-photochemical quenching (NPQ) of excess absorbed light energy, thus contributing to the defence mechanism against photoinhibition. By means of an antiserum produced against a recombinant PsbS, the thylakoid membranes of various higher plant species reveal the presence of a 42-kDa protein band, indicating the formation of a dimer of the 21- kDa PsbS protein. Cross-linking experiments and immunoblotting with other antisera have excluded the formation of a heterodimer with other PSII protein components. The PsbS monomer/dimer ratio varies reversibly with lumenal pH , the monomer being the prevalent form at acidic pH. In intact chloroplasts and whole plants, dimer-to-monomer conversion is reversibly induced by light. The finding of the existence of light-induced change in the quaternary structure of the PsbS subunit may contribute to understanding of the mechanism of PsbS action during non-photochemical quenching.

Role of the PsbH and PsbS subunits of Photosystem II

GIACOMETTI, GIORGIO;BERGANTINO, ELISABETTA;SZABO', ILDIKO';RIGONI, FERNANDA
2004

Abstract

Photosystem I-less Synechocystis 6803 mutants carrying modified PsbH proteins, derived from different combinations of wild-type cyanobacterial and maize genes, were constructed. The mutants were analyzed in order to determine the relative importance of the intra- and extra-membrane domains of the PsbH subunit in the functioning of Photosystem (PS) II, by a combination of biochemical, biophysical and physiological approaches. The results show that besides D1, the whole PsbH protein is necessary to determine the correct structure of a QB/herbicide-binding site.Furthermore the data indicate that a functional PsbH protein is necessary i) for rapid degradation of photo-damaged D1 molecules, which is essential to avoid further oxidative damage to the PSII core, and ii) for the insertion of newly synthesized D1 molecules into the thylakoid membrane. PsbH is thus required for both initiation and completion of the repair cycle of the PSII complex in cyanobacteria. In higher plants, the PsbS subunit plays a crucial role in pH- and xanthophyll-dependent non-photochemical quenching (NPQ) of excess absorbed light energy, thus contributing to the defence mechanism against photoinhibition. By means of an antiserum produced against a recombinant PsbS, the thylakoid membranes of various higher plant species reveal the presence of a 42-kDa protein band, indicating the formation of a dimer of the 21- kDa PsbS protein. Cross-linking experiments and immunoblotting with other antisera have excluded the formation of a heterodimer with other PSII protein components. The PsbS monomer/dimer ratio varies reversibly with lumenal pH , the monomer being the prevalent form at acidic pH. In intact chloroplasts and whole plants, dimer-to-monomer conversion is reversibly induced by light. The finding of the existence of light-induced change in the quaternary structure of the PsbS subunit may contribute to understanding of the mechanism of PsbS action during non-photochemical quenching.
BIOPHYSICAL JOURNAL
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11577/2479363
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