Structure and function of photosystem I related supercomplexes in Chlamydomonas reinhardtii

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Herein we aim to address the dynamic remodeling of photosystem I (PSI) supercomplexes in C. reinhardtii. These supercomplexes are formed between PSI and its associated light-harvesting proteins (LHCI), between PSI-LHCI and light-harvesting proteins of photosystem II (LHCII) during state transitions as well as between two PSI-LHCI monomers to constitute a PSI dimer. In addition, PSI-LHCI Cyt b6f supercomplexes can be formed. A focus of the current application will be on the role of LHCA2, LHCA9, PSAG, and PSAH in PSI-LHCI-LHCII, PSI-LHCI dimer as well as in PSI-LHCI Cyt b6f supercomplex formation and its consequences for regulation of electron partitioning at the PSI acceptor side, state transitions and the efficiency of photosynthetic electron flow. Another aim of the proposal is to isolate PSI-LHCI Cyt b6f supercomplexes and determine the structure by cryo-EM analyses. We will also employ cryo-ET with the aim to visualize PSI-LHCI dimer as well as in PSI-LHCI Cyt b6f supercomplexes in vivo. Moreover, we aim to determine the structure of PSI-LHCI-Cyt c6 complexes by cryo-EM. This is of particular interest, as eukaryotic Cyt c6 is more similar to cyanobacterial electron donors compared to eukaryotic plastocyanin (PC). Therefore, the structure of PSI-LHCI-Cyt c6 complexes would give us insights into the binding of an evolutionary intermediate, which could be readily compared with the known Chlamydomonas and vascular plant PSI-LHCI-PC structures.