Calredoxin regulates the chloroplast NADPH-dependent thioredoxin reductase in Chlamydomonas reinhardtii

Calredoxin (CRX) is a calcium (Ca2+)-dependent thioredoxin (TRX) in the chloroplast of Chlamydomonas (Chlamydomonas reinhardtii) with a largely unclear physiological role. We elucidated the CRX functionality by performing in-depth quantitative proteomics of wild-type cells compared with a crx insertional mutant (IMcrx), two CRISPR/Cas9 KO mutants, and CRX rescues. These analyses revealed that the chloroplast NADPH-dependent TRX reductase (NTRC) is co-regulated with CRX. Electron transfer measurements revealed that CRX inhibits NADPH-dependent reduction of oxidized chloroplast 2-Cys peroxiredoxin (PRX1) via NTRC and that the function of the NADPH-NTRC complex is under strict control of CRX. Via non-reducing SDS-PAGE assays and mass spectrometry, our data also demonstrated that PRX1 is more oxidized under high light (HL) conditions in the absence of CRX. The redox tuning of PRX1 and control of the NADPH-NTRC complex via CRX interconnect redox control with active photosynthetic electron transport and metabolism, as well as Ca2+ signaling. In this way, an economic use of NADPH for PRX1 reduction is ensured. The finding that the absence of CRX under HL conditions severely inhibited light-driven CO2 fixation underpins the importance of CRX for redox tuning, as well as for efficient photosynthesis.

Photosystem I light-harvesting proteins regulate photosynthetic electron transfer and hydrogen production

Linear electron flow (LEF) and cyclic electron flow (CEF) compete for light-driven electrons transferred from the acceptor side of photosystem I (PSI). Under anoxic conditions, such highly reducing electrons also could be used for hydrogen (H2) production via electron transfer between ferredoxin and hydrogenase in the green alga Chlamydomonas reinhardtii. Partitioning between LEF and CEF is regulated through PROTON-GRADIENT REGULATION5 (PGR5). There is evidence that partitioning of electrons also could be mediated via PSI remodeling processes. This plasticity is linked to the dynamics of PSI-associated light-harvesting proteins (LHCAs) LHCA2 and LHCA9. These two unique light-harvesting proteins are distinct from all other LHCAs because they are loosely bound at the PSAL pole. Here, we investigated photosynthetic electron transfer and H2 production in single, double, and triple mutants deficient in PGR5, LHCA2, and LHCA9. Our data indicate that lhca2 and lhca9 mutants are efficient in photosynthetic electron transfer, that LHCA2 impacts the pgr5 phenotype, and that pgr5/lhca2 is a potent H2 photo-producer. In addition, pgr5/lhca2 and pgr5/lhca9 mutants displayed substantially different H2 photo-production kinetics. This indicates that the absence of LHCA2 or LHCA9 impacts H2 photo-production independently, despite both being attached at the PSAL pole, pointing to distinct regulatory capacities.

Novel Insights Into N-Glycan Fucosylation and Core Xylosylation in C. reinhardtii

Chlamydomonas reinhardtii (C. reinhardtii) N-glycans carry plant typical β1,2-core xylose, α1,3-fucose residues, as well as plant atypical terminal β1,4-xylose and methylated mannoses. In a recent study, XylT1A was shown to act as core xylosyltransferase, whereby its action was of importance for an inhibition of excessive Man1A dependent trimming. N-Glycans found in a XylT1A/Man1A double mutant carried core xylose residues, suggesting the existence of a second core xylosyltransferase in C. reinhardtii. To further elucidate enzymes important for N-glycosylation, novel single knockdown mutants of candidate genes involved in the N-glycosylation pathway were characterized. In addition, double, triple, and quadruple mutants affecting already known N-glycosylation pathway genes were generated. By characterizing N-glycan compositions of intact N-glycopeptides from these mutant strains by mass spectrometry, a candidate gene encoding for a second putative core xylosyltransferase (XylT1B) was identified. Additionally, the role of a putative fucosyltransferase was revealed. Mutant strains with knockdown of both xylosyltransferases and the fucosyltransferase resulted in the formation of N-glycans with strongly diminished core modifications. Thus, the mutant strains generated will pave the way for further investigations on how single N-glycan core epitopes modulate protein function in C. reinhardtii.

Calcium sensing via EF-hand 4 enables thioredoxin activity in the sensor-responder protein calredoxin in the green alga Chlamydomonas reinhardtii

Calcium (Ca2+) and redox signaling enable cells to quickly adapt to changing environments. The signaling protein calredoxin (CRX) from the green alga Chlamydomonas reinhardtii is a chloroplast-resident thioredoxin having Ca2+-dependent activity and harboring a unique combination of an EF-hand domain connected to a typical thioredoxin-fold. Using small-angle X-ray scattering (SAXS), FRET, and NMR techniques, we found that Ca2+-binding not only induces a conformational change in the EF-hand domain, but also in the thioredoxin domain, translating into the onset of thioredoxin redox activity. Functional analyses of CRX with genetically altered EF-hands revealed that EF-hand 4 is important for mediating the communication between the two domains. Moreover, we crystallized a variant (C174S) of the CRX target protein peroxiredoxin 1 (PRX1) at 2.4 Å resolution, modeled the interaction complex of the two proteins, and analyzed it by cross-linking and MS analyses, revealing that the interaction interface is located close to the active sites of both proteins. Our findings shed light on the Ca2+ binding-induced changes in CRX structure in solution at the level of the overall protein and individual domains and residues.

X-ray crystallographic and high-speed AFM studies of peroxiredoxin 1 from Chlamydomonas reinhardtii

Peroxiredoxins (PRXs) are a group of antioxidant enzymes that are found in all organisms, including plants and green algae. The 2-Cys PRX from Chlamydomonas reinhardtii (CrPRX1) is a chloroplast-localized protein that is critical for clearing reactive oxygen species in chloroplasts. CrPRX1 is reduced by thioredoxins or calredoxin (CrCRX), a recently identified calcium-dependent redox protein. The molecular interaction between PRXs and thioredoxin/CrCRX is functionally important, but discussion has been limited owing to a lack of structural information on CrPRX1, especially regarding its oligomeric state. In this study, high-speed atomic force microscopy (HS-AFM) images of CrPRX1 and an X-ray crystallographic analysis have enabled examination of the oligomeric state of CrPRX1. Diffraction data from a crystal of the Cys174Ser mutant of CrPRX1 indicate the existence of noncrystallographic fivefold symmetry. HS-AFM images of CrPRX1 further show that CrPRX1 particles form rings with pentagonal rotational symmetry. On the basis of these findings, the oligomeric state of CrPRX1 is discussed and it is concluded that this PRX exists in a ring-shaped decameric form comprising a pentamer of dimers.

Calredoxin represents a novel type of calcium-dependent sensor-responder connected to redox regulation in the chloroplast

Calcium (Ca(2+)) and redox signalling play important roles in acclimation processes from archaea to eukaryotic organisms. Herein we characterized a unique protein from Chlamydomonas reinhardtii that has the competence to integrate Ca(2+)- and redox-related signalling. This protein, designated as calredoxin (CRX), combines four Ca(2+)-binding EF-hands and a thioredoxin (TRX) domain. A crystal structure of CRX, at 1.6 Å resolution, revealed an unusual calmodulin-fold of the Ca(2+)-binding EF-hands, which is functionally linked via an inter-domain communication path with the enzymatically active TRX domain. CRX is chloroplast-localized and interacted with a chloroplast 2-Cys peroxiredoxin (PRX1). Ca(2+)-binding to CRX is critical for its TRX activity and for efficient binding and reduction of PRX1. Thereby, CRX represents a new class of Ca(2+)-dependent 'sensor-responder' proteins. Genetically engineered Chlamydomonas strains with strongly diminished amounts of CRX revealed altered photosynthetic electron transfer and were affected in oxidative stress response underpinning a function of CRX in stress acclimation.