Fucoxanthin-Chlorophyll-Proteins in Photosystem II-Supercomplexes of diatoms

• Applicant: Professorin Dr. Claudia Büchel
• Subject Area: Plant Biochemistry and Biophysics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 544967429

Diatoms are ecologically extremely important eukaryotic photoautotrophs. The photosynthetic light reactions resemble those of vascular plants in many respects, but their light harvesting proteins (Lhc) show differences, although the Lhc belong to the same protein super family as the Lhc of vascular plants. Pigmentation differs, since they non-covalently bind chlorophyll (Chl) a, Chl c, fucoxanthin and the xanthophyll cycle pigments diadinoxanthin and diatoxanthin that are involved in photoprotection. In contrast to vascular plants, diatoms express many more Lhc proteins that display a huge variety in pigment stoichiometries. Due to the generally high fucoxanthin content, Lhc proteins of diatoms are also called fucoxanthin-chlorophyll proteins (FCP). Cyclotella meneghiniana is one of the best studied diatom species concerning excitation energy transfer in isolated FCPs and whole cells, under light harvesting as well as under photoprotective conditions. However, spectroscopic studies on isolated photosystem II (PSII) complexes including the associated FCPs are still missing, hampering our understanding of the excitation energy transfer pathways in this most important complex in oxygenic photosynthesis. The PSII-supercomplexes of C. meneghiniana contain an FCP protein that is putatively involved in photoprotection, rendering such studies even more important. Although a high resolution structure of the PSII-supercomplex of C. meneghiniana became available recently, the resolution is still not high enough to precisely attribute pigments and pigment orientations in the peripheral FCPs. This knowledge, however, is obligatory to elucidate pigment interactions in excitation energy transfer and photoprotection. Recent advances in cryo-electron-microscopy coupled to single-particle analysis allow structural determination at the resolution needed, but the quality of the biochemical preparations remains the limiting factor. Our recent advances in PSII-supercomplex preparations from the diatom Cyclotella meneghiniana now pave the way for a high resolution structure. We will improve these preparations further and elucidate the structure of PSII-supercomplexes using cryo-electron-microscopy. This will provide the basis to determine the excitation energy pathways into the PSII-core and to elucidate the mechanisms of photoprotection taking place close to the cores.

• DFG Programme: Research Grants