In higher plants and green algae, Photosystem II (PSII) usually combines with Light Harvesting Complex II (LHCII) to form the PSII-LHCII supercomplex. Under low-light conditions, the PSII-LHCII supercomplexes are organized laterally into higher-order PSII-LHCII megacomplexes and semi-crystalline arrays to optimize photosynthetic efficiency.

A recent collaborative study has deciphered the cryo-EM structure of the spinach type I PSII-LHCII megacomplex, providing insights into the principles of higher-order assembly and the potential regulatory mechanisms of plant PSII under low-light conditions.

The study, led by researchers from the Institute of Biophysics of the Chinese Academy of Sciences, Saint Louis University, and the University of Washington in St. Louis, was published in Science Advances on December 14.

By combining structural biology, crosslinking mass spectrometry, ultrafast time-resolved fluorescence spectroscopy, and other advanced techniques, the researchers elucidated the detailed architectural principles of a major type of spinach photosynthetic megacomplex (type I). This megacomplex consists of four PSII cores (C₄), four strongly associated LHCII trimers (S₄), and two moderately associated LHCII trimers (M₂)—collectively referred to as the type I C₄S₄M₂ megacomplex.

The study identified two small membrane proteins, PsbR (PSII 10 kDa polypeptide R) and PsbY (PSII reaction center protein Y), which are located in the central region of the megacomplex assembly interface, sandwiched between two adjacent C₂S₂M supercomplexes (Figure 1).

PsbR, a long-sought, small yet crucial subunit of PSII, plays a role in adjusting the central assembly interfaces of both type I and type II PSII-LHCII megacomplexes in native thylakoid membranes. Its absence disrupts the higher-order organization of PSII particles on the thylakoid membrane.

In addition to PsbR, PsbY is another small subunit located at the periphery of the PSII core, near Cyt b₅₅₉ (a protective subunit of PSII). Together, PsbR and PsbY form a shield around Cyt b₅₅₉, potentially modulating its redox potential.

Further structural and functional studies suggest that the two innermost PSII cores in the PSII-LHCII megacomplex are most likely trapped in an inhibited state, preventing them from transferring electrons outward. As a result, the oxygen-producing activity of the megacomplex is reduced by approximately 50% compared to the supercomplex sample.

“Our findings resolve a long-standing puzzle regarding the structure and function of the higher-order assembly of plant photosystem II,” said Prof. LIU. “This enhances our understanding of how plants organize PSII in the thylakoid membranes and adjust its function in response to constantly changing light conditions.”

Fig. 1 Overall architecture of a type I PSII-LHCII megacomplex from spinach. The key components of the PSII-LHCII megacomplex are shown in different colors. The black dashed lines indicate the dimer interface of the megacomplex, and the black solid ovals represent the two-fold (C2) rotational axis passing through the center. (Image by LIU Zhenfeng's group)