Assembly mechanism of PSII-LHCII array from higher plants

Photosystem II (PSII) comprises reaction centers and light-harvesting complexes of the major and minor antennas, forming diverse supercomplexes with varying antenna sizes and are organized as PSII arrays in grana thylakoids to respond to fluctuating light. However, the assembly mechanism of PSII arrays, excitation energy transfer and its regulation mechanisms in vascular plants remain poorly understood. Here, we report the cryo-electron microscopy structures of a 1.4-MDa PSII-LHCII (light-harvesting complex II) dimer and a 2.8-MDa tetramer, and present an initial model of hexamer from Arabidopsis. Structural and genetic analyses reveals that the tetramer is formed by two C₂S₂M₂ dimers arranged side by side through interactions between CP26/PsbZ and moderate (M)-LHCII within PSII arrays in the grana thylakoid. Furthermore, conformational changes of M-LHCII and CP24 facilitate the assembly transition from dimer to tetramer/hexamer. Chlorophyll rearrangement, supported by computational calculations and spectral analysis, suggests enhanced energy transfer efficiency in the tetramer compared to the dimer. Therefore, our findings provide new insights into the dynamic assembly and excitation energy redistribution within PSII arrays in higher plants.