Abstract
In eukaryotic autotrophs, photosystems are composed of a core moiety, hosting charge separation and electron transport reactions, and an antenna system, enhancing light harvesting and photoprotection. In Chlamydomonas reinhardtii, the major antenna of PSII is a heterogeneous trimeric complex made up of LHCBM1-LHCBM9 subunits. Despite high similarity, specific functions have been reported for several members including LHCBM1, 2, 7, and 9. In this work, we analyzed the function of LHCBM4 and LHCBM6 gene products in vitro by synthesizing recombinant apoproteins from individual sequences and refolding them with pigments. Additionally, we characterized knock-down strains in vivo for LHCBM4/6/8 genes. We show that LHCBM4/6/8 subunits could be found as a component of PSII supercomplexes with different sizes, although the largest pool was free in the membranes and poorly connected to PSII. Impaired accumulation of LHCBM4/6/8 caused a decreased LHCII content per PSII and a reduction in the amplitude of state 1-state 2 transitions. In addition, the reduction of LHCBM4/6/8 subunits caused a significant reduction of the Non-photochemical quenching activity and in the level of photoprotection.
Highlights
Life on Earth is fueled by photon energy harvested by photosynthetic systems
Alignment of LHCBM4, LHCBM6, and LHCBM8 sequences with LHCBM1 and LHCBM2 suggested that all the residues involved in chlorophyll and neoxanthin binding at the N1 site were conserved (Liu et al, 2004; Caffarri et al, 2007)
The functional roles of LHCBM1, LHCBM2/7, and LHCBM9 have been previously described: LHCBM1 was reported to be involved in non-photochemical quenching of excitation energy (NPQ) induction, while LHCBM2/7 is involved in induction of state transitions (Elrad et al, 2002; Ferrante et al, 2012)
Summary
Life on Earth is fueled by photon energy harvested by photosynthetic systems. In green algae and land plants, photosynthesis occurs in chloroplasts, where two pigment-binding protein complexes, PSI and PSII, catalyze the light-dependent steps of electron transport from water to NADP+ which is coupled to proton transport to the thylakoid lumen for ATP synthesis. The outer antenna system of PSII is made up of pigmentbinding light-harvesting complexes called LHCII (light-harvesting complex II), a trimeric complex made by 22–26 kDa polypeptides with three transmembrane and two amphipatic α-helices exposed to the lumen (Kühlbrandt et al, 1994; Liu et al, 2004; Standfuss et al, 2005), each binding up to 14 chlorophylls and four xanthophylls. These chromophores are bound to multiple specific sites for xanthophylls (L1, L2, N1, and V1) as well as for chlorophylls (Chl601-614) (Croce et al, 1999a, b; Caffarri et al, 2001, 2004, 2007; Liu et al, 2004; Ballottari et al, 2012). LHC proteins harvest light energy and Abbreviations: amiRNA, artificial microRNA; CN, Clear Native; DCMU, [3-(3,4-dichlorophenyl)-1,1-dimethylurea]; DI, de-epoxidation index; LHC, light-harvesting complex; NPQ, non-photochemical quenching; 1O2, singlet oxygen; ROS, reactive oxygen species
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