Abstract
In plants, photosystem II (PSII) associates with light-harvesting complexes II (LHCII) to form PSII–LHCII supercomplexes. They are multi-subunit supramolecular systems embedded in the thylakoid membrane of chloroplast, functioning as energy-converting and water-splitting machinery powered by light energy. The high-resolution structure of a PSII–LHCII supercomplex, previously solved through cryo-electron microscopy, revealed 34 well-defined lipid molecules per monomer of the homodimeric system. Here we characterize the distribution of lipid-binding sites in plant PSII–LHCII supercomplex and summarize their arrangement pattern within and across the membrane. These lipid molecules have crucial roles in stabilizing the oligomerization interfaces of plant PSII dimer and LHCII trimer. Moreover, they also mediate the interactions among PSII core subunits and contribute to the assembly between peripheral antenna complexes and PSII core. The detailed information of lipid-binding sites within PSII–LHCII supercomplex may serve as a framework for future researches on the functional roles of lipids in plant photosynthesis.
Highlights
In plants, algae and cyanobacteria, photosystem II (PSII) cooperates with photosystem I (PSI) and cytochrome b6f (Cyt b6f) to carry out the light-driven electron transport process during the energy conversion process in oxygenic photosynthesis (Nelson and Ben-Shem 2004)
We focus on discussing the structural roles of lipid molecules in the C2S2M2-type PSII–light-harvesting complexes II (LHCII) supercomplex basing on the 2.7-Å resolution structure and compare them with those found in cyanobacterial and red algal PSII
Cluster 1 is located at the dimerization interface of PSII core, and nine lipid molecules per monomer including three SQDG, three PG, one DGDG and two MGDG are located at this region (Fig. 3A)
Summary
Algae and cyanobacteria, photosystem II (PSII) cooperates with photosystem I (PSI) and cytochrome b6f (Cyt b6f) to carry out the light-driven electron transport process during the energy conversion process in oxygenic photosynthesis (Nelson and Ben-Shem 2004). Crystal structure of a cyanobacterial PSII from Thermosynechococcus elongatus (TePSII) was solved at 2.9 Å (Guskov et al 2009) and revealed the presence of 25 lipid molecules per monomer of the PSII homodimer, including 11 MGDG, seven DGDG, five SQDG, and two PG molecules (Kern and Guskov 2011) These lipid molecules serve as multifunctional cofactors involved in the assembly and functional regulation of PSII (Mizusawa and Wada 2012). Bcarotene mainly exists in the PSII core region and there might be one located in CP24 complex (Fig. 1D) (Su et al 2017) These carotenoid molecules are mostly distributed in regions enriched with chlorophylls and form close interactions with them so as to fulfill the photoprotective function during photosynthesis. They are found in the peripheral antenna complexes (LHCII, CP29, CP26 and CP24), at the interfaces between LHCII/CP26/CP29 and CP43/CP47, and within the core complex
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