Data supporting the absence of FNR dynamic photosynthetic membrane recruitment in trol mutants

Thylakoid Rhodanese-like Protein-Ferredoxin:NADP+ Oxidoreductase Interaction Is Integrated into Plant Redox Homeostasis System

In vascular plants, the final photosynthetic electron transfer from ferredoxin (Fd) to NADP+ is catalyzed by the flavoenzyme ferredoxin:NADP+ oxidoreductase (FNR). FNR is recruited to thylakoid membranes via an integral membrane protein TROL (thylakoid rhodanese-like protein) and the membrane associated protein Tic62. We have previously demonstrated that the absence of TROL triggers a very efficient superoxide (O2•-) removal mechanism. The dynamic TROL-FNR interaction has been shown to be an apparently overlooked mechanism that maintains linear electron flow before alternative pathway(s) is(are) activated. In this work, we aimed to further test our hypothesis that the FNR-TROL pair could be the source element that triggers various downstream networks of chloroplast ROS scavenging. Tandem affinity purification followed by the MS analysis confirmed the TROL-FNR interaction and revealed possible interaction of TROL with the thylakoid form of the enzyme ascorbate peroxidase (tAPX), which catalyzes the H2O2-dependent oxidation of ascorbate and is, therefore, the crucial component of the redox homeostasis system in plants. Further, EPR analyses using superoxide spin trap DMPO showed that, in comparison with the wild type, plants overexpressing TROL (TROL OX) propagate more O2•- when exposed to high light stress. This indicates an increased sensitivity to oxidative stress in conditions when there is an excess of membrane-bound FNR and less free FNR is found in the stroma. Finally, immunohistochemical analyses of glutathione in different Arabidopsis leaf cell compartments showed highly elevated glutathione levels in TROL OX, indicating an increased demand for this ROS scavenger in these plants, likely needed to prevent the damage of important cellular components caused by reactive oxygen species.

A novel protein domain is important for photosystem II complex assembly and photoautotrophic growth in angiosperms

Photosystem II (PSII) is a multi-subunit protein complex of the photosynthetic electron transport chain that is vital to photosynthesis. Although the structure, composition, and function of PSII have been extensively studied, its biogenesis mechanism remains less understood. Thylakoid rhodanese-like (TROL) provides an anchor for leaf-type ferredoxin:NADP+ oxidoreductase. Here, we report the chacterizaton of a second type of TROL protein, TROL2, encoded by seed plant genomes whose function has not previously been reported. We show that TROL2 is a PSII assembly cofactor with essential roles in the establishment of photoautotrophy. TROL2 contains a 45-amino-acid domain, termed the chlorotic lethal seedling (CLS) domain, that is both necessary and sufficient for TROL2 function in PSII assembly and photoautotrophic growth. Phylogenetic analyses suggest that TROL2 may have arisen from ancestral TROL1 via gene duplication before the emergence of seed plants and acquired the CLS domain via evolution of the sequence encoding its N-terminal portion. We further reveal that TROL2 (or CLS) forms an assembly cofactor complex with the intrinsic thylakoid membrane protein LOW PSII ACCUMULATION2 and interacts with small PSII subunits to facilitate PSII complex assembly. Collectively, our study not only shows that TROL2 (CLS) is essential for photoautotrophy in angiosperms but also reveals its mechanistic role in PSII complex assembly, shedding light on the molecular and evolutionary mechanisms of photosynthetic complex assemblyin angiosperms.

Topology of TROL protein in thylakoid membranes of Arabidopsis thaliana

Thylakoid rhodanase-like protein (TROL) is a nuclear-encoded protein of thylakoid membranes required for tethering of ferredoxin:nicotinamide adenine dinucleotide phosphate (NADPH) oxydoreductase (FNR). It has been proposed that the dynamic interaction of TROL with flavoenzyme FNR, influenced by environmental light conditions, regulates the fate of photosynthetic electrons, directing them either to NADPH synthesis or to other acceptors, including reactive oxygen species detoxification pathways. Inside the chloroplasts, TROL has a dual localization: an inner membrane precursor form and a thylakoid membrane mature form, which has been confirmed by several large-scale chloroplast proteomics studies, as well as protein import experiments. Unlike the localization, the topology of TROL in the membranes, which is a prerequisite for further studies of its properties and function, has not been experimentally confirmed yet. Thermolysin was proven to be a valuable protease to probe the surface of chloroplasts and membranes in general. By treating the total chloroplast membranes using increasing protease concentration, sequential degradation of TROL was observed, indicating protected polypeptides of TROL and possible domain orientation. To further substantiate the obtained results, TROL-overexpressing Arabidopsis line (OX) and line in which the central rhodanase-like domain (RHO) has been partially deleted (ΔRHO), were used as well. While OX line showed the same degradation pattern of TROL as the wild-type, surprisingly, TROL from ΔRHO membranes was not detectable even at the lowest protease concentration applied, indicating the importance of this domain to the integrity of TROL. In conclusion, TROL is a polytopic protein with a stroma-exposed C-terminal FNR-binding region, and the thylakoid lumen-located RHO domain.

Effects of TROL Presequence Mutagenesis on Its Import and Dual Localization in Chloroplasts

Thylakoid rhodanase-like protein (TROL) is involved in the final step of photosynthetic electron transport from ferredoxin to ferredoxin: NADP⁺ oxidoreductase (FNR). TROL is located in two distinct chloroplast compartments—in the inner envelope of chloroplasts, in its precursor form; and in the thylakoid membranes, in its fully processed form. Its role in the inner envelope, as well as the determinants for its differential localization, have not been resolved yet. In this work we created six N-terminal amino acid substitutions surrounding the predicted processing site in the presequence of TROL in order to obtain a construct whose import is affected or localization limited to a single intrachloroplastic site. By using in vitro transcription and translation and subsequent protein import methods, we found that a single amino acid exchange in the presequence, Ala67 to Ile67 interferes with processing in the stroma and directs the whole pool of in vitro translated TROL to the inner envelope of chloroplasts. This result opens up the possibility of studying the role of TROL in the chloroplast inner envelope as well as possible consequence/s of its absence from the thylakoids.

Molecular and functional characterization of ferredoxin NADP(H) oxidoreductase from Gracilaria chilensis and its complex with ferredoxin

Backgroud

Ferredoxin NADP(H) oxidoreductases (EC 1.18.1.2) (FNR) are flavoenzymes present in photosynthetic organisms; they are relevant for the production of reduced donors to redox reactions, i.e. in photosynthesis, the reduction of NADP⁺ to NADPH using the electrons provided by Ferredoxin (Fd), a small FeS soluble protein acceptor of electrons from PSI in chloroplasts. In rhodophyta no information about this system has been reported, this work is a contribution to the molecular and functional characterization of FNR from Gracilaria chilensis, also providing a structural analysis of the complex FNR/Fd.

Methods

The biochemical and kinetic characterization of FNR was performed from the enzyme purified from phycobilisomes enriched fractions. The sequence of the gene that codifies for the enzyme, was obtained using primers designed by comparison with sequences of Synechocystis and EST from Gracilaria. 5′RACE was used to confirm the absence of a CpcD domain in FNRPBS of Gracilaria chilensis. A three dimensional model for FNR and Fd, was built by comparative modeling and a model for the complex FNR: Fd by docking.

Results

The kinetic analysis shows K_M^NADPH of 12.5 M and a k_cat of 86 s⁻¹, data consistent with the parameters determined for the enzyme purified from a soluble extract. The sequence for FNR was obtained and translated to a protein of 33646 Da. A FAD and a NADP+ binding domain were clearly identified by sequence analysis as well as a chloroplast signal sequence. Phycobilisome binding domain, present in some cyanobacteria was absent. Transcriptome analysis of Gch revealed the presence of two Fd; FdL and FdS , sharing the motif CX5CX2CX29X. The analysis indicated that the most probable partner for FNR is FdS.

Conclusion

The interaction model produced, was consistent with functional properties reported for FNR in plants leaves, and opens the possibilities for research in other rhodophyta of commercial interest.

Electronic supplementary material

The online version of this article (10.1186/s40659-017-0144-5) contains supplementary material, which is available to authorized users.