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

莱茵衣藻的QE能力依赖于环境条件，并可通过强光下的生长诱导。QE能力随光依赖LHCSR3蛋白的增加而成比例增加。LHCSR3的缺失或耗尽会使细胞丧失耗散有害过剩能量的能力，这表明LHCSR3是有效量化宽松所必需的(Peers等人，2009年)。LHCSR3在功能上与PSII-LHCII超复合体相互作用(Tokutsu&Minagawa，2013)，并需要PSBR才能有效地与PSII-LHCII结合(薛等人，2015)。尽管对LHCSR3功能的了解已经取得了很大的进展，但主要的问题仍然没有解决，如：(I)LHCSR3蛋白磷酸化的功能后果是什么；(Ii)为什么LHCSR3与PSI以及更大的PSI-FNR和/或可能的CEF超复合体结合，LHCSR3在保护PSI方面起作用；(Iii)PSI超复合体中的蛋白质-蛋白质-相互作用伙伴(S)是什么，以及(Iv)LHCSR3与其他LHC多肽以及光合蛋白复合体PSII和PSI的化学计量比是什么。为了研究这些与LHCSR3功能相关的机制问题，我们设计了将反向遗传学、磷蛋白质组学和定量蛋白质组学与生理测量和时间分辨光学吸收光谱相结合的实验策略。

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.