Structural basis for assembly and function of a diatom photosystem I-light-harvesting supercomplex

Ryo Nagao,Yasuhiro Kashino,Jian-Ren Shen,Fusamichi Akita,Seiji Akimoto,Ikuo Uchiyama,Koji Kato,Takehiro Suzuki,Minoru Kumazawa,Naoyuki Miyazaki,Kentaro Ifuku,Naoshi Dohmae

doi:10.1038/s41467-020-16324-3

Abstract

Photosynthetic light-harvesting complexes (LHCs) play a pivotal role in collecting solar energy for photochemical reactions in photosynthesis. One of the major LHCs are fucoxanthin chlorophyll a/c-binding proteins (FCPs) present in diatoms, a group of organisms having important contribution to the global carbon cycle. Here, we report a 2.40-Å resolution structure of the diatom photosystem I (PSI)-FCPI supercomplex by cryo-electron microscopy. The supercomplex is composed of 16 different FCPI subunits surrounding a monomeric PSI core. Each FCPI subunit showed different protein structures with different pigment contents and binding sites, and they form a complicated pigment–protein network together with the PSI core to harvest and transfer the light energy efficiently. In addition, two unique, previously unidentified subunits were found in the PSI core. The structure provides numerous insights into not only the light-harvesting strategy in diatom PSI-FCPI but also evolutionary dynamics of light harvesters among oxyphototrophs.

Highlights

Photosynthetic light-harvesting complexes (LHCs) play a pivotal role in collecting solar energy for photochemical reactions in photosynthesis
The overall atomic model was built based on this density map, which is composed of a photosystem I (PSI) core and 16 FCPI subunits surrounding the core (Fig. 1b, c)
Four LHCI subunits are found in the plant PSI-LHCI3,4, whereas four-ten LHCI subunits are found in the green algal PSILHCI5–8

Summary

Introduction

Photosynthetic light-harvesting complexes (LHCs) play a pivotal role in collecting solar energy for photochemical reactions in photosynthesis. Each FCPI subunit showed different protein structures with different pigment contents and binding sites, and they form a complicated pigment–protein network together with the PSI core to harvest and transfer the light energy efficiently. To harvest enough energy for the photochemical reactions, photosynthetic organisms have developed various lightharvesting pigment–protein complexes (LHCs), which play crucial roles in supplying energy to the photosystems by capturing unique spectral components of the light. This is achieved by different types and numbers of pigments including chlorophylls (Chls) and carotenoids (Cars) that are bound to LHCs, resulting in variations in the color of the organisms that we see in our life[2]. The structure revealed the presence of 16 FCPI subunits associated with the PSI core, and identified most of the pigments they bind, including Chls a and c, Fx and diadinoxanthin (Ddx) clearly, thereby revealing a highly complicated pigment–protein network involved in EET and energy quenching

Methods

Results

Conclusion