Light- and pH-dependent structural changes in the PsbS subunit of photosystem II

Abstract

In higher plants, the PsbS subunit of photosystem II (PSII) plays a crucial role in pH-and xanthophyll-dependent nonphotochemical quenching of excess absorbed light energy, thus contributing to the defense mechanism against photoinhibition. We determined the amino acid sequence of Zea mays PsbS and produced an antibody that recognizes with high specificity a region of the protein located in the stroma-exposed loop between the second and third putative helices. By means of this antiserum, the thylakoid membranes of various higher plant species revealed the presence of a 42-kDa protein band, indicating the formation of a dimer of the 21-kDa PsbS protein. Crosslinking experiments and immunoblotting with other antisera seem to exclude the formation of a heterodimer with other PSII protein components. The PsbS monomer͞dimer ratio in isolated thylakoid membranes was found to vary with luminal pH in a reversible manner, the monomer being the prevalent form at acidic and the dimer at alkaline pH. In intact chloroplasts and whole plants, dimer-to-monomer conversion is reversibly induced by light, known to cause luminal acidification. Sucrose-gradient centrifugation revealed a prevalent association of the PsbS monomer and dimer with light-harvesting complex and PSII core complexes, respectively. The finding of the existence of a light-induced change in the quaternary structure of the PsbS subunit may contribute to understanding the mechanism of PsbS action during nonphotochemical quenching.

A zeaxanthin-independent nonphotochemical quenching mechanism localized in the photosystem II core complex

“…It is able to bind N,NЈ-dicyclohexylcarbodiimide (28), a qE inhibitor (11), and likely able to associate with carotenoids (29). PsbS can associate with either the antenna or the PSII core complex, thanks to a pH-dependent shift between a monomeric and a homodimeric form (30). PsbS might therefore be involved in promoting or stabilizing a conformation of the PSII core suitable to promote fluorescence quenching, thus promoting generation of antenna qE.…”

Direct interaction of the major light-harvesting complex II and PsbS in nonphotochemical quenching

“…In a next step, LHC-II was added to the proteoliposome system containing Zea and/or PsbS to investigate whether PsbS forms a Chl fluorescence quenching unit with an LHC in a Car-dependent manner. Because the interaction of LHC-II with PsbS is well documented (12,29), we focused on LHC-II as a likely qE interaction partner.…”

“…Even though the qE process is obviously more than just the sum of such individual interactions, the characterization of these interactions in a well-defined membrane system is critical for a deeper understanding of the entire mechanism. The reconstituted PsbS was mostly monomeric, as also occurs after a pH-dependent dimer-to-monomer transition in high-light conditions (12). It formed a heterodimer with LHC-II on coreconstitution, resulting in substantial Chl fluorescence quenching and a concomitant increase in electronic interactions between Car S 1 and Chl,…”

“…Studies with native PsbS and PsbS refolded in vitro have been inconsistent with respect to its pigment-binding ability (4)(5)(6)(7)(8)(9)(10). One interesting feature of PsbS is its reported pH-dependent dimer-to-monomer transition under high-light conditions, thought to be due to the protonation of luminal glutamates (11,12). The dissociation of the PsbS dimer goes along with a change of location within the PSII supercomplex.…”

“…The dissociation of the PsbS dimer goes along with a change of location within the PSII supercomplex. Dimers appear to be mainly associated with the PSII core, whereas monomers associate with the major LHC of PSII (LHC-II) (12).…”