Structure of photosystem I-LHCI-LHCII from the green alga Chlamydomonas reinhardtii in State 2

Zihui Huang,Wenda Wang,Guangye Han,Jian-Ren Shen,Liangliang Shen,Zhiyuan Mao,Xiaohan Yi,Tingyun Kuang,Xing Zhang

doi:10.1038/s41467-021-21362-6

Abstract

Photosystem I (PSI) and II (PSII) balance their light energy distribution absorbed by their light-harvesting complexes (LHCs) through state transition to maintain the maximum photosynthetic performance and to avoid photodamage. In state 2, a part of LHCII moves to PSI, forming a PSI-LHCI-LHCII supercomplex. The green alga Chlamydomonas reinhardtii exhibits state transition to a far larger extent than higher plants. Here we report the cryo-electron microscopy structure of a PSI-LHCI-LHCII supercomplex in state 2 from C. reinhardtii at 3.42 Å resolution. The result reveals that the PSI-LHCI-LHCII of C. reinhardtii binds two LHCII trimers in addition to ten LHCI subunits. The PSI core subunits PsaO and PsaH, which were missed or not well-resolved in previous Cr-PSI-LHCI structures, are observed. The present results reveal the organization and assembly of PSI core subunits, LHCI and LHCII, pigment arrangement, and possible pathways of energy transfer from peripheral antennae to the PSI core.

Highlights

Photosystem I (PSI) and II (PSII) balance their light energy distribution absorbed by their light-harvesting complexes (LHCs) through state transition to maintain the maximum photosynthetic performance and to avoid photodamage
The structure of a PSI-LHCI-LHCII supercomplex from maize were analyzed by single-particle cryo-electron microscopy at 3.3 Å resolution[30], and two dimensional projection map of PSI-LHCI-LHCII supercomplex from C. reinhardtii has been obtained at 20 Å resolution[36]
The PSI-LHCI-LHCII supercomplex was isolated from C. reinhardtii cells CC-137 in state 2 generated by the procedures described in Methods

Summary

Introduction

Photosystem I (PSI) and II (PSII) balance their light energy distribution absorbed by their light-harvesting complexes (LHCs) through state transition to maintain the maximum photosynthetic performance and to avoid photodamage. Recent reports show that both Lhcb[1] and Lhcb[2] are heavily phosphorylated in higher plants in state 2, but only the phosphorylated Lhcb[2] is present in the trimer bound to PSI28–30 This process is reversible, namely, the STN7/Stt[7] kinase is inactivated upon reoxidation of the PQ pool, and a thylakoid-bound phosphatase dephosphorylates the LHCIIs and the dephosphorylated LHCIIs detaches from PSI and recouples to PSII, regenerating state 125–27. The structure of a PSI-LHCI-LHCII supercomplex from maize were analyzed by single-particle cryo-electron microscopy (cryo-EM) at 3.3 Å resolution[30], and two dimensional projection map of PSI-LHCI-LHCII supercomplex from C. reinhardtii has been obtained at 20 Å resolution[36] These studies showed that while the plant PSI binds one LHCII trimer upon state transition, two LHCII trimers are moved to the green algal PSI upon state transition. Our structure revealed detailed organization and pigment arrangement of PSI-LHCI-LHCII from the green algae, enabling us to examine the excitation energy transfer (EET) pathways from the peripheral antennae to the core in a greater detail

Methods

Results

Conclusion