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Structure of a Photosystem II Supercomplex from Diatom Unravels the Energy-harvesting, Transfer and Dissipation Mechanisms

Aug 05, 2019

Diatoms stem from red algae and account up to 40% of the net primary production in the ocean. The high photosynthesis capacity of diatoms are contributed largely by their light-harvesting antenna proteins, which are fucoxanthin (Fx) chlorophyll (Chl) a/c-binding proteins (FCPs) that can efficiently absorb blue-green light available underwater and also have strong nonphotochemical quenching (NPQ) abilities important for the highly fluctuating light environment in the surface of the ocean.
A large number of FCP subunits are associated with the PSII (Photosystem II) core, forming a PSII-FCPII supercomplex. However, the exact composition and organization of FCPs associated with the PSII core are still unknown, which restricts our understanding of the light-harvesting, energy transfer and dissipation processes in the PSII-FCPII supercomplex of diatoms.

Researchers from Institute of Botany of the Chinese Academy of Sciences and Tsinghua University cooperated to determine the structure of the PSII-FCP supercomplex from a centric diatom Chaetoceros gracilis by cryo-electron microscopy at a novel resolution.  

A large quantity of pigments is assigned in the supercomplex, which include 230 Chls a, 58 Chls c, 124 Fxs, and 2 Diadoxanthins (Ddx). Among these pigments, Chl c and Fxs are important for their ability to harvest blue-green light under water. One Ddx is confirmed in a monomeric FCP mediating the energy transfer towards the core, which render this Ddx as an ideal site for possible energy dissipation.  

The paper was accompanied by a perspective written by Professor Claudia Büchel, one of the pioneers in the studies of diatom photosynthesis: "How diatoms harvest light". In this perspective, Prof. Claudia Büchel said: "The FCP structures solved are milestones on the way to understanding the high flexibility of LHC/FCP proteins, where the same purpose is served by similar proteins in a very different manner."  

 

Fig.1. Model of a diatom PSII-FCPII supercomplex embedded in the thylakoid membrane (Image by IBCAS)

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KUANG Tingyun

Institute of Botany

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