Mechanistic insights into the function of the ancient light-harvesting protein LHCSR3 in light energy dissipation in Chlamydomonas reinhardtii

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The qE capacity of Chlamydomonas reinhardtii is dependent on environmental conditions and is inducible by growth in high light. The qE capacity increases proportionally with the light-dependent increase of the LHCSR3 protein. Deletion or depletion of LHCSR3 abolishes the ability of the cells to dissipate harmful excess energy, demonstrating that LHCSR3 is required for efficient qE (Peers et al, 2009). LHCSR3 functionally interacts with PSII-LHCII supercomplexes (Tokutsu & Minagawa, 2013) and requires PSBR for efficient binding to PSII-LHCII (Xue et al, 2015). Despite the significant progress in the understanding of LHCSR3 function, major questions remain unsolved such as: (i) what are the functional consequences of LHCSR3 protein phosphorylation (ii) why does LHCSR3 bind to PSI and larger PSI-FNR and/or putative CEF supercomplexes, does LHCSR3 function in protecting PSI (iii) what are the protein-protein-interaction partner(s) in PSI supercomplexes, and (iv) what is the stoichiometry of LHCSR3 with other LHC polypeptides and the photosynthetic protein complexes PSII and PSI. To investigate these mechanistically related problems concerning LHCSR3 function in C. reinhardtii we designed experimental strategies combining reverse genetics, phosphoproteomics and quantitative proteomics with physiological measurements and time resolved optical absorption spectroscopy.