Structural and functional analysis of NDH-1 supercomplexes

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Membrane protein complexes play a crucial role in bioenergetics, particularly in photosynthetic organisms. They are responsible for the light-to-chemical energy conversion in the primary reactions of photosynthesis, thereby driving the energy gradient that promotes and sustains almost all life on our planet. In this fundamental process, they combine the light-driven transport of electrons with the vectorial transport of protons across a membrane. We are particularly interested in one type of these molecular machines, complex I (aka NDH-1), which is part of photosynthetic as well as respiratory electron transport chains. It combines electron transfer from ferredoxin (Fd) to plastoquinone with the transfer of protons across the thylakoid membrane in a yet unknown fashion and many questions concerning their precise molecular function remain unanswered due to the lack of structure-function based mutagenesis studies in vitro and in vivo. In principle, as highly efficient proton pumps, they could contribute to light-driven proton motive force (PMF) formation substantially by accepting electrons from photosystem I (PSI) via Fd in cyclic electron transport, but their role in vivo is rather unclear. It is known from land plants that PSI and NDH-1 form a supercomplex, however current structural models are incomplete and miss the Fd interacting subunits. Moreover, in cyanobacteria supercomplex formation of NDH-1 with PSI and CpcG2-phycobilisomes or with H2ase/Diaphorase as an alternative pathway is still elusive. The project will provide novel molecular insights into NDH-1 function and its integration into the dynamic electron transport network of the cell based on our recent advances in structural analysis of the cyanobacterial complex. Specifically, we will focus on supercomplex formation with PSI and with H2ase/Diaphorase to understand its role in PMF formation. Cyanobacterial supercomplexes will be targeted in the model organisms Synechocystis sp. PCC 6803 and the filamentous, diazotrophic cyanobacterium Anabaena sp. PCC 7120, which will be compared to the land plant supercomplex form the bryophyte P. patens, a genetically accessible system that allows comparatively easy upscaling of biomass production.