PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

A dissipative conformation of the antenna protein Lhcb5 reveals a PsbS independent, zeaxanthin dependent, mechanism of photoprotection. Luca Dall'Osto¹, Caffarri Stefano ^{*,1, 2}, Roberto Bassi^{*,1, 2}, ¹ Dipartimento Scientifico e Tecnologico, Università di Verona., Verona, Italy² Laboratoire de Génétique et Biophysique des Plantes (LBTE), Marseille, France

ABSTRACT- The regulation of light harvesting in higher plant photosynthesis, defined as stress dependent modulation of the ratio of energy transfer to the reaction centres vs heat dissipation, was studied by means of carotenoid biosynthesis mutants and recombinant light harvesting complexes (Lhcs) with modified chromophore binding. The npq2 mutant of Arabidopsis thaliana, accumulating zeaxanthin, had a lower fluorescence yield of chlorophyll in vivo, and correspondingly larger energy dissipation, with respect to the WT strain and npq1 mutant, lacking zeaxanthin. Analysis of purified thylakoid membranes showed that the major source of the different fluorescence yield of npq2 vs WT preparations was pigment-protein interactions. Analysis of the xanthophyll binding Lhc proteins showed that the Lhcb5 PSII subunit undergoes a change in its isoelectric point (pI) upon binding of zeaxanthin, which is possibly related to differences in the conformation of the protein in the sample which binds zeaxanthin, npq2, and the samples which bind violaxanthin (WT and npq1). This hypothesis was confirmed by analysis of two recombinant proteins obtained by overexpression of the Lhcb5 apoprotein in E. coli and reconstitution in vitro with either violaxanthin or zeaxanthin. The pigment-protein complexes with either violaxanthin or zeaxanthin have different pIs and show respectively high and low fluorescence yield. These results confirm that Lhc proteins exist in multiple conformations, an idea suggested by previous spectroscopic measurements (Moya et al. 2001), and imply that the switch between the different Lhc protein conformations is activated by the binding of zeaxanthin to the allosteric site L2. The relationship between this photoprotective mechanism and the process of PsbS dependent light induced quenching were studied by using the npq1 npq4 and npq2 npq4 double mutants. The analysis of WT leaves and thylakoid proteins upon stress-induced zeaxanthin accumulation confirms that the quenching mechanism identified in npq2 is at work in WT plants. Growth of WT and mutant plants, including the npq4 genotype lacking PsbS and qE, in control and stressing conditions, show that the zeaxanthin dependent, PsbS independent, quenching mechanism is effective in photoprotection.

KEY WORDS: photoprotection, zeaxanthin, fluorescence queching, CP26