Stt-photosynthesis regulation – PhotoRegul

Plant primary productivity is at the basis of our human societies: for food, for fuel and for renewable materials. Photosynthesis is currently proposed as the final frontier for increasing biomass production. For such an ambitious undertaking, an integrated approach, starting from the molecular level and analyzing the effects of changes at increasingly organized levels is essential. Both in Europe and internationally, a number of prospective meetings have been organized to address the problem of the limitations of photosynthesis, research projects have been undertaken to tackle these questions, and a few examples of photosynthetic yield improvements in the field have been proposed. We have chosen to target cytochrome b6f (cyt b6f), which represents one of the limiting steps for photosynthesis. The cyt b6f is one of the most complex chloroplast proteins. It is central in the regulation of both electron transfer and proton transfer, thus key in the regulation of electron fluxes through the photosynthetic electron transfer chain. It is also a pivotal regulatory point for photo-protective mechanisms such as state transitions.
In photosynthesis, O2 production and CO2 capture is possible because of electron transfer through two photosystems coupled in series by cyt b6f complex. This complex does not only transfer electrons, it also contributes to regulate the light distribution between Photosystem I (PSI) and Photosystem II (PSII) to optimize the quantum yield of photosynthesis. We are far from understanding exactly how it works, except that cyt b6f is a hub between the pool of quinones (binding to the Qo and Qi site of the complex) and the serine-threonine specific MAP kinase, named Stt7, that phosphorylates the light harvesting complexes 2 (LHCII) which migrate from the grana stacks rich in PSII, to PSI in the lamellae. The mechanism of activation of the kinase by cytochrome b6f was poorly understood as we were left with the idea that the kinase domain of Stt7 was located on the stromal side of the membrane (Qi site) when the activation signal was supposed to originate from the luminal side of the membrane (Qo site).
We have recently deciphered a new mechanism for the regulation of photosynthesis: the activation mechanism of the kinase involved in state transitions (Dumas, Zito et al. 2017). We disclosed that the triggering of the Stt7 kinase to phosphorylate LHCII proteins occurs via a direct interaction with the chloroplast cytochrome b6f complex in the stromal compartment. This discovery broadens ours horizons because we have now spotted key regulatory residues of cyt b6f on the stromal side of the membrane. We showed that cytochrome b6f subunit IV was directly involved in the activation of the Stt7 kinase (Dumas, Zito et al. 2017). We have now lifted the main scientific and technical barrier: arginine in position 125 (Arg125) from SuIV (Arg125SuIV) is involved in a direct and pivotal interaction with Stt7. We are now making the hypothesis that Stt7 is activated by autophosphorylation, through an unknown mechanism involving Arg125SuIV. We are planning to dissect this mechanism by a series of biochemical and biophysical methods, from purified complexes and recombinant proteins up to increasingly integrated systems, native membranes and whole cells. Our research project is a structure / function project, with scientific objectives focused on the mechanisms and on the dynamic interaction between cyt b6f complex and Stt7 kinase.