Abstract

The biochemical properties of PsbS protein, a nuclear-encoded Photosystem II subunit involved in the high energy quenching of chlorophyll fluorescence, have been studied using preparations purified from chloroplasts or obtained by overexpression in bacteria. Despite the homology with chlorophyll a/b/xanthophyll-binding proteins of the Lhc family, native PsbS protein does not show any detectable ability to bind chlorophylls or carotenoids in conditions in which Lhc proteins maintain full pigment binding. The recombinant protein, when refolded in vitro in the presence of purified pigments, neither binds chlorophylls nor xanthophylls, differently from the homologous proteins LHCII, CP26, and CP29 that refold into stable pigment-binding complexes. Thus, it is concluded that if PsbS is a pigment-binding protein in vivo, the binding mechanism must be different from that present in other Lhc proteins. Primary sequence analysis provides evidence for homology of PsbS helices I and III with the central 2-fold symmetric core of chlorophyll a/b-binding proteins. Moreover, a structural homology owed to the presence of acidic residues in each of the two lumen-exposed loops is found with the dicyclohexylcarbodiimide/Ca(2+)-binding domain of CP29. Consistently, both native and recombinant PsbS proteins showed [(14)C]dicyclohexylcarbodiimide binding, thus supporting a functional basis for its homology with CP29 on the lumen-exposed loops. This domain is suggested to be involved in sensing low luminal pH.

Highlights

  • Photosystem II (PSII)1 of higher plants is a multisubunit membrane complex composed of many polypeptides that are encoded by the chloroplast or nuclear genes

  • A structural homology owed to the presence of acidic residues in each of the two lumen-exposed loops is found with the dicyclohexylcarbodiimide/Ca2؉binding domain of CP29

  • Chloroplast gene products are located in the core complex where electron transport reactions are catalyzed, whereas the surrounding lightharvesting system is composed of nuclear-encoded chlorophyll a/b/xanthophyll proteins belonging to the Lhc family [1]

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Summary

Introduction

Photosystem II (PSII)1 of higher plants is a multisubunit membrane complex composed of many polypeptides that are encoded by the chloroplast or nuclear genes. PsbS was found in fractions 7–10 without contamination from other proteins as assessed by SDS-PAGE (data not shown), showing that it was possible to purify PsbS in a non-aggregated form while pale-green material migrated in fractions 2– 4 and 19 –20.

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