Structure - function analysis of photosystem II subunit S (PsbS) in vivo

Abstract

In land plants, photosystem II subunit S (PsbS) plays a key role in xanthophyll-and pH-dependent non-photochemical quenching (qE) of excess absorbed light energy. Arabidopsis thaliana (L.) Heynh. npq4 mutants are defective in the psbS gene and have impaired qE. Exactly how the PsbS protein is involved in qE is unclear, but it has been proposed that PsbS binds H + and/or de-epoxidized xanthophylls in excess light as part of the qE mechanism. To identify amino acid residues that are important for PsbS function, we sequenced the psbS gene from eight npq4 point mutant alleles isolated by forward genetics screening, including two new alleles. In the four transmembrane helices of PsbS, several amino acid residues were found to affect the stability and/or function of the protein. By comparing the predicted amino acid sequences of PsbS from several plant species and studying the proposed topological structure of PsbS, eight possible H + -binding amino acid residues on the lumenal side of the protein were identified and then altered by site-directed mutagenesis in vitro. The mutant psbS genes were transformed into npq4-1, a psbS deletion mutant, to test the stability and function of the mutant PsbS proteins in vivo. The results demonstrate that two conserved, protonatable amino acids, E122 and E226, are especially critical for the function of PsbS.

A Mechanism of Nonphotochemical Energy Dissipation, Independent from PsbS, Revealed by a Conformational Change in the Antenna Protein CP26

“…Zeaxanthin accumulation also has been involved in long term (overwinter) downregulation of photosynthesis (Verhoeven et al, 1999;Gilmore and Ball, 2000). Current understanding of energy dissipation suggests that PsbS catalyzes qE upon protonation of lumenal-exposed residues (Li et al, 2002b) and binding of zeaxanthin. An alternative mechanism of quenching has been proposed based on the observation that Viola, Anthera, and Zea bind to LHC proteins, particularly CP26 and CP24 to the L2 binding site that has been shown to have allosteric properties (Formaggio et al, 2001).…”

“…Low lumenal pH also results in protonation of specific light-harvesting complexes (LHCs) (Dominici et al, 2002;Li et al, 2002a;Li et al, 2002b;Pesaresi et al, 1997;Walters et al, 1996), which, synergistically with zeaxanthin binding, leads to a conformational change necessary for NPQ (Moya et al, 2001). The PSII subunit S (PsbS) was found to be essential for NPQ (Li et al, 2000) and may trigger the quenching in a series of steps involving protonation and, possibly, zeaxanthin binding (Li et al, 2002b;Holt et al, 2005). It was also shown that Lhcb (light-harvesting complexes of PSII) proteins replace violaxanthin for zeaxanthin in the allosteric L2 site that induces quenching of chlorophyll fluorescence in vitro (Formaggio et al, 2001).…”

Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

“…In higher plants the 'special' polypeptide which undergoes protonation is PsbS . The function of PsbS is essentially to sense the lumen pH, that is linked to several H + -binding amino acid residues present on the luminal loops of this protein (Dominici et al 2002;Li et al 2002). In green microalgae (Peers, G and Niyogi, KK, personal communication, 2008) and diatoms (Zhu and Green 2008), the Li-818 proteins which are up-regulated under high light, could play a similar role as PsbS in qE.…”

The Functional Significance of the Monomeric and Trimeric States of the Photosystem II Light Harvesting Complexes

“…Mutants lacking the Lhc-related protein PsbS almost completely lack qE (24), although the exact role of PsbS is unclear, partially due to uncertainties of its pigment binding capabilities (25)(26)(27)(28)(29)(30). It has been suggested that PsbS may be a regulatory subunit in qE rather than being the site of quenching per se (31), providing binding sites for protons (32) and zeaxanthin (33).…”