The machinery positioning chlorophyll in Cytochrome b6f - Chlorophyll is the key molecule for photosynthesis on earth. In one m2 of a plant leaf area, about 10ˆ18 Chlorophylls are bound to the photosystem proteins and each Chlorophyll must find an exact position and orientation to harvest and extract energy from light. We do not know how this process is regulated. The key protein that regulates the light-dependent electron and proton flow in photosynthesis is the Cytochrome b6f complex. The protein binds only one Chlorophyll molecule. We characterize the protein machinery that positions this single Chlorophyll into the enzyme’s structure to learn how this key photosynthetic protein complex is assembled.
In oxygenic photosynthesis, the cytochrome b6f complex is the plastoquinol—plastocyanin reductase. It regulates electron and proton flow and binds a unique Chlorophyll molecule per complex monomer. This chlorophyll is evolutionarily conserved in oxygenic photosynthetic organisms, but its function is unknown. In angiosperms grown in darkness, chlorophyll can´t be synthesized. We found in this developmental stage that the complex still assembles; however, here protochlorophyll is bound. This finding confirmed the importance of the pigment. We showed that chlorophyll stabilizes the structure of cytochrome b6f and that it binds to cytochrome b6. We identified that the light-harvesting protein LIL3 is binding chlorophyll in parallel with the accumulation of chlorophyll in the cytochrome b6f dimer. This finding prompted us to isolate, characterize, and determine the atomic structure of the protein machinery that delivers and regulates the binding of chlorophyll to the cytochrome b6f complex. We differentiate between chlorophyll-binding to Cyt b6 and subunit IV and the assembly of the complex during or after the completion of translation and membrane integration. The cytochrome b6f complex is isolated in both the protochlorophyll and chlorophyll-bound state and the atomic structures are determined. For atomic structure determination and imaging, we continue to focus on cryo-electron microscopy and extend our efforts using the free-electron x-ray laser sources at the XFEL in Hamburg.