Functional and structural characterization of PSI-Cyt b6f supercomplexes

Basic data for this project

Description

The identification of a PSI-Cyt b6f-FNR supercomplex low-resolution structure via single particle transmission electron microscopy, plus the unpublished identification of a crosslinked peptide between LHCA1 and FNR, reveals that the complex (i) enables physical shortening of distances between soluble electron mediators and PSI-Cyt b6f redox centers, (ii) puts FNR via binding to LHCA1 in close distance to bridge electron transfer into the Cyt b6f complex, and indicates (iii) that remodeling of the LHCI protein complex configuration is required for its establishment. The decreased diffusion distances likely facilitate efficient electron transfer within the CEF complex allowing for effective proton gradient formation and ATP production. We propose a remodeling of the LHCI antenna, associated to dissociation of LHCA2 and LHCA9 from PSI-LHCI, which is linked to PSI-Cyt b6f complex formation. Notably, in the PSI-LHCI complex from the red alga C. merolae, these LHCI binding sites, equivalent to those of LHCA2 and LHCA9, appear to be also dynamically occupied by LHCr1 and LHCr2. Thus, the question arises how and why, i.e. for promotion of CEF, this process is dynamically regulated. This shall be investigated with a focus on C. reinhardtii. In line and for further in-depth mechanistic understanding, we aim to gather more functional data regarding FNR and LHCA protein interaction and its interconnection to CEF efficiency. Moreover, we aim to elucidate the structure of the PSI-Cyt b6f-FNR supercomplex by improving its purification employing new tags expressed in different genetic mutant backgrounds. An improved purification will aid chemical crosslinking and mass spectrometric identification of protein-protein interactions and likely allow obtaining a higher resolution structure of the complex via cryo electron microscopy. An improved purification will also permit functional analyses of electron transfer within the complex. In summary, the overall aim is to obtain new functional and structural insights into the PSI-Cyt b6f-FNR supercomplex.