Photosynthesis is the largest energy and materials conversion process. To maintain a maximal efficiency of photosynthetic electron transport and avoid photodamage, plants redistribute the excitation energy between the Photosystem I (PSI) and Photosystem II (PSII) by moving part of the PSII light-harvesting complex (LHCII) through state transition.

In state 2, a part of LHCII moves to PSI, forming a PSI-LHCI-LHCII supercomplex. 

A research team from the Institute of Botany of the Chinese Academy of Sciences (IBCAS) and their collaborator from Zhejiang University revealed the structure of PSI-LHCI-LHCII supercomplex from green alga Chlamydomonas reinhardtii (C. reinhardtii) in state 2 at resolution of 3.42 Å.

The study was published in Nature communications on Feb. 17. 

The researchers found that the PSI-LHCI-LHCII of C. reinhardtii binded two LHCII trimers in addition to ten LHCI subunits. Two LHCII trimers located close to each other at the PsaO-PsaL-PsaH-Lhca2 side. One LHCII trimer (LHCII-1) attached to the PSI core by PsaO, PsaH and PsaL subunits, whereas the other LHCII trimer (LHCII-2) attached to the Lhca2 and LHCII-1 subunits.  

The phosphorylation of the N-terminal Thr residue of the LHCII-1 trimer in the stromal side upon state 1 to state 2 transition induced extensive contacts of LHCII-1 with the PsaO/PsaH/PsaL subunits, which stabilized its binding to the PSI core.  

Meanwhile, the present results also revealed the distribution of pigments as well as the plausible excitation energy transfer (EET) pathways in the Cr-PSI-LHCI-LHCII and demonstrated a highly efficient network of EETs involved in the state transition of the green alga C. reinhardtii. 

"This is the first high-resolution structure of the PSI-LHCI-LHCII supercomplex from a green alga," said Dr. HAN Guangye, one of the corresponding author from IBCAS. "This structure is very important because green algae grow under water or in soil and suffer different light conditions, which results in unique organization of this supercomplex in state 2."