engleski

Unexpected functions of the FNR-TROL bifurcation

The last step in light-reactions of photosynthesis is the transfer of electrons from reduced ferredoxin (Fd) to NADP+. This reaction is catalysed by the flavoenzyme ferredoxin: NADP+ oxidoreductase (FNR) (1). FNR utilises two Fdred to produce one molecule of NADPH. Apart from being electron donor in this synthesis, Fd can distribute photosystem I (PSI)-generated electrons to a number of other reactions and/or biosynthetic pathways. Therefore, to maintain efficient electron flow toward NADPH production and to prioritise this pathway, FNR has to be positioned in the vicinity of PSI. It has been shown that two proteins, TROL and Tic62, participate in the dynamic FNR sequestration and membrane recruitment (2). TROL (thylakoid rhodanase-like protein) is an integral component of vascular plant thylakoids and inner chloroplast envelope (3). Tic62 (translocon of the inner chloroplast envelope 62) is a soluble protein with multiple localisations in chloroplasts. Unlike TROL, Tic62 can not be found in all vascular plants (4). Tic62 participates in FNR storage in conditions when this flavoenzyme is not needed (5). For the TROL, three different models have been postulated. Jurić et al. (6) proposed the importance of TROL in the maintenance of efficient linear electron flow (LEF). Lintala et. al. (7) concluded that TROL is not important for the LEF, while Twachtmann et al. (8) connected TROL with the cyclic electron flow (CEF). We collected several lines of evidence that FNR-TROL interaction plays the key role in the so far overlooked electron bifurcation that can partition electrons downstream of PSI to either energy-conserving or energy-dissipating pathways. In the absence of TROL very efficient electron sink is open. This sink can outcompete methyl viologen-mediated superoxide anion propagation (9). Further, in conditions when CO2 is not limiting, TROL is indeed important for efficient LEF. Overexpression of TROL in the model plant Arabidopsis causes distinctive conditional phenotype, accumulating stress avoidance pigments even in low-light conditions. In TROL mutants, levels of antioxidants glutathione and cysteine are perturbed in all cellular compartments studied. Finally, in the absence of TROL there is no light, or pH-dependent dynamic membrane recruitment of FNR (10). We propose that FNR-TROL interaction has evolved into an elegant short-term adaptation mechanism that is important for efficient LEF and ROS management in vascular plants. References 1. Berg, JM. ; Tymoczko, JL. ; Stryer, L (2007). Biochemistry (6th ed.). New York: W.H. Freeman. ISBN 0-7167-8724-5. 2. Alte, F. ; et al. (2010). Ferredoxin: NADPH oxidoreductase is recruited to thylakoids by binding to a polyproline type II helix in a pH-dependent manner. Proc Natl Acad Sci U S A. 107 (45): 19260–5. 3. Vojta, L ; Čuletić, A ; Fulgosi, H (2018). Effects of TROL presequence mutagenesis on its import and dual localization in chloroplasts. Int J Mol Sci. 19 (2) 4. Balsera, M. ; et al. (2007). Tic62: a protein family from metabolism to protein translocation. BMC Evol Biol. 7, 43. 5. Benz, JP ; et al. (2009). Arabidopsis Tic62 and ferredoxin-NADP(H) oxidoreductase form light-regulated complexes that are integrated into the chloroplast redox poise. Plant Cell. 21: 3965–3983. 6. Jurić, S. ; et al. (2009). Tethering of ferredoxin:NADP+ oxidoreductase to thylakoid membranes is mediated by novel chloroplast protein TROL. Plant J. 60: 783-794. 7. Lintala, M ; et al. (2014). Arabidopsis tic62 trol mutant lacking thylakoid bound ferredoxin-NADP+ oxidoreductase shows distinct metabolic phenotype. Mol Plant. 7, 45–57. 8. Twachtmann, M ; et al. (2012). N-terminal structure of maize ferredoxin:NADP+ reductase determines recruitment into different thylakoid membrane complexes. Plant Cell. 24: 2979-2991. 9. Vojta, L ; et al. (2015). TROL-FNR interaction reveals alternative pathways of electron partitioning in photosynthesis. Sci Rep. 5: 10085. 10. Vojta, L. ; Fulgosi, H. (2016). Data supporting the absence of FNR dynamic photosynthetic membrane recruitment in trol mutants. Data Brief. 7: 393-396.

photosynthetic electron transport ; Photosystem I ; ROS ; electron paramagnetic resonance

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