Structural features of the diatom photosystem II–light‐harvesting antenna complex

Abstract

In photosynthesis, light energy is captured by pigments bound to light-harvesting antenna proteins (LHC) that then transfer the energy to the photosystem (PS) cores to initiate photochemical reactions. The LHC proteins surround the PS cores to form PS-LHC supercomplexes. In order to adapt to a wide range of light environments, photosynthetic organisms have developed a large variety of pigments and antenna proteins to utilize the light energy efficiently under different environments. Diatoms are a group of important eukaryotic algae and possess fucoxanthin (Fx) chlorophyll a/c proteins (FCP) as antenna which have exceptional capabilities of harvesting blue-green light under water and dissipate excess energy under strong light conditions. We have solved the structure of a PSII-FCPII supercomplex from a centric diatom Chaetoceros gracilis by cryo-electron microscopy, and also the structure of an isolated FCP dimer from a pennate diatom Phaeodactylum tricornutum by X-ray crystallography at a high resolution.These results revealed the oligomerization states of FCPs distinctly different from those of LHCII found in the green lineage organisms, the detailed binding patterns of Chl c and Fxs, a huge pigment network, and extensive protein-protein, pigment-protein, and pigment-pigment interactions within the PSII-FCPII supercomplex. These results therefore provide a solid structural basis for examining the detailed mechanisms of the highly efficient energy transfer and quenching processes in diatoms.

Pre-purification of diatom pigment protein complexes provides insight into the heterogeneity of FCP complexes

“…The absence of Lhcf11 is most probably explained by the fact that in the present protein separations by SDS-PAGE a minor 18 kDa FCP band was not resolved. The presence of Lhcf4 in the 18 kDA band of Fraction 2, which seems to be enriched in PSII, is in line with the recent isolation of a PSII-FCP supercomplex of C. gracilis [35]. Based on the structural data it was proposed that the FCP-E monomer, which is rather tightly associated with the PSII core complex and mediates the interaction of one of the FCP-A tetramers with the core, represents an Lhcf4-like subunit.…”

“…With respect to the concept of the FCP trimer as the basic unit of diatom antenna proteins, this idea was recently challenged by the first detailed cryo-electron microscopic and X-ray crystallography studies on diatom pigmentprotein complexes. Using cryo-electron microscopy Wang et al [35] resolved a PSII-antenna supercomplex from the centric diatom Chaetoceros gracilis and reported a tetrameric organization of FCP proteins in the vicinity of PSII. With regard to the molecular structure and stoichiometric organization of individual FCP proteins, x-ray crystallography data of the Lhcf3 and Lhcf4 proteins from P. tricornutum at 1.8 Å reveal a dimeric organization [34].…”

Differential Membrane Lipid Profiles and Vibrational Spectra of Three Edaphic Algae and One Cyanobacterium

“…As is well known, the composition and structure of the thylakoid membrane of diatoms differ substantially from those of green and other algal classes [12,19,[59][60][61]. In diatoms, fucoxanthin plays a more active role than that played by the xanthophylls in the green algae [11], with fucoxanthin being the major contributor to the fucoxanthin protein complexes (FPC), along with chlorophyll a.…”

“…This is to say, different mechanistic architectures may accomplish similar functions using diverse routes, even within the same algal class [11]. An important role for lipid molecules has been acknowledged with regard to the stabilization of and interaction between the protein complexes of the photosystems, with apparently specific roles for each glycerolipid class [7,12]. Thus, it is acknowledged, at present, that the membrane composition influences the dynamic physicochemical properties of the lipid bilayer, which ultimately determines the optimal photosynthetic protein organization in different organisms, according to their particular requirements [8,13].…”

Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae

“…The efficiency of this energy transfer varies depending on the carotenoid and chlorophyll compositions in the LHC. In addition to LHC, fucoxanthinchlorophyll a/c binding proteins (FCP) in diatoms [50,51] and the peridinin-chlorophyll a protein complex (PCP) in dinoflagellates [52,53] also act as light-harvesting complexes binding specific carotenoids.…”