Photosynthesis is one of the most important chemical reactions on Earth. Oxygenic phototrophs convert solar energy into chemical energy and release oxygen, thus sustaining nearly all life on our planet . Photosystem I (PSI) and photosystem II (PSII) are two membrane-embedded complexes critical for the light reactions of photosynthesis. Both photosystems are composed of a core complex and a peripheral antenna system. While the core complexes are well conserved, the light-harvesting antennae bound to the core vary greatly among different species, representing an adaptation mechanism to the specific ecological niches these phototrophs occupy.

Cryptophytes are a group of unicellular eukaryotic microalgae evolved from red algae via secondary endosymbiosis, and have peculiar antenna systems, which combine membrane-intrinsic chlorophyll a/c proteins (CACs) and membrane-extrinsic phycobiliproteins (PBPs). Analysis of the high-resolution structure of cryptophyte photosystem complexes will provide structural basis for the core-antenna assembly and the adaptation mechanisms of photosynthetic organisms .

On August 9, 2024, a research group led by Prof. LI Mei from the Institute of Biophysics, Chinese Academy of Sciences published a research paper entitled "Structural basis for the distinct core-antenna assembly of cryptophyte photosystem II" in Nature Communications.

The researchers solved a high-resolution structure of the PSII-CAC complex purified from cryptophyte cells in the stationary growth phase (Figure 1a). The structure shows that CACs are arranged as two arc-shaped belts that attach to the dimeric PSII core from one side in a non-parallel manner, which is distinct from the core-antenna organization of other photosynthetic organisms reported previously. A new core-antenna linker protein CAL-II was found to be crucial for the formation and stability of the PSII-CAC complex (Figure 1a). This study provides a structural basis for understanding the light energy utilization and adaptation mechanism of cryptophytes.

It is worth mentioning that on May 11, 2024, the same research group published a research paper entitled "Growth phase-dependent reorganization of cryptophyte photosystem I antennae" in Communications Biology, reporting the structural study of four PSI-CAC complexes purified from cryptophyte cells in different growth stages (logarithmic and stationary phases) (Figure 1b). This work reveals the presence of a novel PSI core subunit PsaQ, and the rearrangement mechanism of PsaQ and CACs at different growth phases of cryptophytes.

Figure 1. (a) Cryo-EM reconstructure of PSII-CAC complex. (b) Structures of four types of PSI-CAC complexes.

(Image by LI Mei's group)

Article link:

https://www.nature.com/articles/s41467-024-51206-y

https://www.nature.com/articles/s42003-024-06268-5

Contact: LI Mei

Institute of Biophysics, Chinese Academy of Sciences

Beijing 100101, China

Email: meili@ibp.ac.cn

(Report by Prof. LI Mei's group)