Photosystem II (PSII), an important supramolecular photosynthetic machine, catalyzes the upstream reactions of the oxygenic photosynthesis process by splitting water molecules and producing oxygen. In plants and green algae, PSII is usually associated with the light-harvesting complexes II (LHCII) to form the PSII-LHCII supercomplexes, the basic units for harvesting and converting light energy.

The PSII-LHCII supercomplexes will further assemble into higher-order forms, such as the PSII-LHCII megacomplexes and semi-crystalline arrays, in the thylakoid membranes of chloroplasts. The formation and rearrangement of the high-order structures are related to the optimization of photosynthetic efficiency under constantly-changing natural light conditions.

On December 14, 2024, Dr. LIU, Zhenfeng's group from the Institute of Biophysics, Chinese Academy of Sciences, Dr. LIU, Haijun's group from the Department of Biology, Saint Louis University, and Dr. NIEDZWIEDZKI, Dariusz from Washington University in St. Louis jointly published a research paper entitled "Architecture and functional regulation of a plant PSII-LHCII megacomplex" in the《Science Advances》journal.

This work deciphered the detailed architectural principle for a major type of spinach photosynthetic megacomplex (type I) composed of four PSII core (C₄), four strongly-associated LHCII trimers (S₄) and two moderately-associated LHCII trimers (M₂). It is also termed as the type I C₄S₄M₂ megacomplex. Scientists discovered that two small membrane proteins named PsbR (PSII 10 kDa polypeptide R) and PsbY (PSII reaction center protein Y) subunits are located in the central region of the megacomplex assembly interface, sandwiched between two adjacent C₂S₂M supercomplexes (Figure 1).

PsbR is a long-saught small but important subunit of PSII. The results indicate that PsbR is involved in adjusting the central assembly interfaces of type I and type II PSII-LHCII megacomplexes in native thylakoid membranes, and its absence affects the high-order organization of PSII particles on the thylakoid membrane. Besides PsbR, PsbY is the other small subunit located at the peripherial region of PSII core nearby Cyt b₅₅₉ (a protective subunit of PSII). Both PsbR and PsbY form a shield around Cyt b₅₅₉ and may modulate its redox potential.

Further structural and functional studies indicated that the inner two of the four PSII cores in the PSII-LHCII megacomplex are most likely trapped in an inhibited state, unable to transfer electrons outward. Consequently, the oxygen-producing activity of the megacomplex is reduced by about half in comparison with the supercomplex sample.

The new results clarified a long-standing puzzle about the structure and function of the high-order assembly of plant photosystem II, and improve our understanding on how plants organize PSII in the thylakoid membranes and flexibly adjust the function of PSII in response to the constantly-changing light conditions.

Figure 1. Overall architecture of a type I PSII-LHCII megacomplex from spinach.

（Image by LIU Zhenfeng's group）

The key components of the PSII-LHCII megacomplex are in different colors. The black dash lines indicate the dimer interface of the megacomplex, and the black solid ovals indicate the two-fold (C2) rotational axis running through the center.

Article link: https://www.science.org/doi/10.1126/sciadv.adq9967

Contact: LIU Zhenfeng

Institute of Biophysics, Chinese Academy of Sciences

Beijing 100101, China

E-mail: liuzf@ibp.ac.cn

（Reported by Prof. LIU Zhenfeng's group）