Biology subjects

Hippler, M.

Publications and source records attributed to Hippler, M..

At least 19 recordsLinked to original sources

bioRxiv2026

Lateral organization of cytochrome b6f in thylakoid membranes controls photosynthetic electron transfer efficiency

Photosynthetic electron transfer relies on the coordinated function and spatial organization of large protein complexes within the thylakoid membrane. The cytochrome b6f complex (b6f) functionally interconnects photosystem (PS) II and PSI in photosynthetic electron transfer and is equally distributed between appressed and non-appressed thylakoid membranes. Here, we investigate the functional link between the lateral distribution of b6f and efficient photosynthetic electron flow in Chlamydomonas reinhardtii. We engineered strains with stromal fusions between PetA of b6f and two fluorescent proteins (FPs) of different molecular mass: Clover and ATeam. Under oxic conditions, these strains exhibited significantly slower electron transfer rates (ETR), lower PSII quantum yields, and increased donor-side limitation of PSI. State transitions were diminished in the fusion strains, accompanied by a strong impairment of STT7-dependent function, suggesting that the presence of fused FPs at b6f interfere with STT7 function. Yet, ETR phenotypes were STT7-independent and FP fusion did not impact intrinsic b6f function. In situ cryogenic electron tomography revealed a significant depletion of b6f from appressed thylakoid membranes in the ATeam strains, while the overall membrane protein concentration remained unchanged. Overall, our data indicate that a balanced distribution of b6f between appressed and non-appressed thylakoid membranes is essential for regulating photosynthetic electron transfer, highlighting the functional importance of thylakoid molecular architecture in vivo.

plant biology↗

bioRxiv2025

Cryo-EM structure of Chlamydomonas Photosystem I complexedwith the alternative electron donor cytochrome c6

Photosynthetic electron transfer relies on small soluble carriers that shuttle electrons between the cytochrome bf complex and Photosystem I (PSI). While copper-containing plastocyanin (Pc) serves this role in plants, algae and cyanobacteria employ the heme protein cytochrome c (Cyt c) as well. Here we present a cryo-electron microscopy structure of a Cyt c:PSI complex from Chlamydomonas reinhardtii. The Cyt c heme is positioned [~]11 [A] from P700, stabilized by extensive contacts involving the N-terminal domain of PSAF. Importantly, R66 in Cyt c, a key residue in ancestral donors, forms a putative electrostatic contact with PsaB-D623 and participates in a tri-planar {pi}-stacking interaction with nearby aromatic residues. Our findings provide a structural framework for ancestral PSI interactions and illuminate the evolutionary diversification of electron transfer pathways.

biochemistry↗

bioRxiv2025

Advanced illumination-imaging reveals photosynthesis-triggered pH, ATP and NAD redox signatures across plant cell compartments

Photosynthesis provides energy and organic substrates to most life. In plants, photosynthesis dominates chloroplast physiology but represents only a fraction of the tightly interconnected metabolic network that spans the entire cell. Here, we explore how photosynthetic activity affects energy physiology within and beyond the chloroplast. We developed a new standard for the live-monitoring of subcellular energy physiology by combining confocal imaging of genetically encoded fluorescent protein biosensors with advanced on-stage illumination technology to investigate pH, MgATP2- and NADH/NAD+ dynamics at dark-light transitions in Arabidopsis mesophyll cells. Our findings reveal a stromal alkalinization signature induced by photosynthetic proton pumping, extending to the cytosol and mitochondria as an alkalinization wave. Photosynthesis leads to increased MgATP2- levels in both the stroma and cytosol. Additionally, we observed reduction of the NAD pool driven by photosynthesis-derived electron export. Arabidopsis lines defective in chloroplast NADP- and mitochondrial NAD-dependent malate dehydrogenases show more reduced cytosolic NAD redox status even in darkness, highlighting the involvement of chloroplasts and mitochondria in shaping cytosolic redox metabolism via malate metabolism. Our study sets a novel methodological standard for precision live-monitoring of photosynthetic cell physiology. Applying this technology reveals signatures of photosynthetic physiology within and beyond the chloroplast with unprecedented resolution. Those signatures link photosynthetic activity and the fundamental biochemical functions of phototrophic cells. Significance statementBy applying novel live microscopy monitoring using fluorescent protein biosensors in plant cells, we reveal that dark-light transitions trigger profound re-orchestration of subcellular pH, ATP and NAD redox physiology not limited to chloroplasts but extending into the cytosol and the mitochondria.

plant biology↗

bioRxiv2025

Chloroplast cpHsc70-1 interacts with VIPP1 C-terminal tail and controls VIPP1 oligomer disassembly in thylakoid membrane remodeling

Oxygenic photosynthetic organisms depend on the thylakoid membranes (TMs) for light-driven energy conversion. Recent studies on TM homeostasis (thylakostasis) have highlighted the essential role of the TM remodeling protein VIPP1 (Vesicle Inducing Protein in Plastids 1). As a member of the ESCRT-III/PspA/VIPP1 superfamily, VIPP1 forms large ring- and filament-like homo-oligomeric structures that exhibit a membrane remodeling activity. The oligomerization status was proposed to be modulated by the intrinsically disordered C-terminal tail (Vc), whereas its functional role remained unclear. Notably, this Vc region is conserved not only in photosynthetic VIPP1 but also in the PspA proteins of extremophilic species, implicating its role in membrane stress responses. To investigate the role of the Vc region in VIPP1 assembly, we performed co-immunoprecipitation assays in Arabidopsis chloroplasts and identified chloroplast-localized HSP70 proteins as major interactors. Among the two isoforms, cpHsc70-1 was found to be specifically required for modulating VIPP1 oligomeric assembly and dynamics in response to heat stress. Genetic analyses revealed that cpHsc70-1 facilitates the disassembly of VIPP1 oligomers, similarly to Vps4 ATPase in ESCRT-III; loss of either the Vc region or cpHsc70-1 impaired VIPP1 disassembly, resulting in more static oligomeric structures. Furthermore, cpHsc70-1 exhibited a broader role in chloroplast proteostasis, as the cphsc70-1 mutant showed impaired accumulation of GFP-fusion proteins. Together, our findings uncover a crucial crosstalk between proteostasis and thylakostasis in chloroplasts, coordinated by cpHsc70-1 and VIPP1 in response to membrane stress. Significance StatementOrganisms performing oxygen-evolving photosynthesis rely on the thylakoid membranes (TMs) for light-driven energy conversion. We reveal that VIPP1, a membrane remodeling protein forming dynamic oligomeric structures, is crucial for TM maintenance. Its intrinsically disordered C-terminal region (Vc) and the heat shock protein cpHsc70-1 coordinate VIPP1 disassembly during stress in Arabidopsis chloroplasts. Loss of either component causes defects in membrane remodeling and proteostasis. Our findings uncover a fundamental mechanism connecting membrane integrity and protein quality control, ensuring chloroplast resilience against environmental stress.

plant biology↗

bioRxiv2025

Submersion and oxidative stress triggers pyrenoid formation, carbon concentration-related protein remodeling and sub-plastidial rearrangements in hornworts

Plastids are important for controlling acclimation of plants to environmental changes. In hornworts, chloroplasts may contain a RuBisCO-enriched protein matrix, a pyrenoid-like structure, which enables them to perform a biophysical carbon concentration mechanism (CCM) at the single-cell level- a unique feature among land plants. However, much remains unknown about the function, formation, and regulation of hornwort pyrenoids, especially as they are unaffected by changes in atmospheric CO2. Here, we tested whether submersion and hyperoxia induce pyrenoid formation and CCM. By subjecting Anthoceros agrestis, a pyrenoid-forming hornwort species, and A. fusiformis, which develops no pyrenoids, to a series of submersion experiments and analyzing their molecular, physiological, and cell-morphological response patterns using label-free proteomics and transmission electron microscopy, with additional in silico analysis, we identified a core set of CCM candidate genes. Under submersion, both species expressed CCM-associated protein homologs, whereas hyperoxia induced or diminished the expression of CCM-like homologs in a species-specific manner. We discovered that a carbonic anhydrase, a CAH3 homolog, as well as thylakoid bicarbonate transporter (LCI11-like) are upregulated under both conditions in A. agrestis, but not in A. fusiformis, suggesting that an algae-like mechanism of bicarbonate pumping into the thylakoid lumen and CO2 conversion exists in A. agrestis. Corroborating these molecular findings, an ultrastructural analysis of plastids revealed increases in pyrenoid-like structures and rearrangements during submersion in A. agrestis, whereas A. fusiformis accumulated lipid droplets between thylakoid stacks. Together, our data highlight hornworts distinct acclimation strategies to adverse environmental conditions, highlighting the relevance of their pyrenoids and CCM. Significance statementCarbon concentration via pyrenoids, densely packed matrices of a CO2-fixing enzyme, holds significant biotechnological promise for enhancing crop resilience. Studying mechanisms underlying pyrenoid formation in hornworts--the only land plants to naturally develop pyrenoids--is of exceptional interest due to their evolutionary closeness to crops. This study closes a knowledge gap by showing that hornworts use pyrenoids and a carbon-concentrating mechanism to adapt to submersion and hyperoxia, allowing them to thrive under regular transitions between atmospheric and submerged environments, resembling conditions during plants water-to-land transition. Species lacking pyrenoids accumulate lipid droplets instead, in addition to altering developmental and physiological pathways. Together, this work highlights hornworts remarkable versatility and untapped potential in advancing our understanding of plant adaptation to terrestrial life.

plant biology↗

bioRxiv2025

New Insights into Plastocyanin-Cytochrome b6f Formation: the Role of Plastocyanin Phosphorylation

In this work we investigated the role of plastocyanin (PC) phosphorylation in photosynthetic electron transfer, focusing on interactions with both cytochrome-b6f (Cytb6f) and Photosystem-I (PSI) in Chlamydomonas reinhardtii. While the binding and electron transfer between PC and PSI are well characterized, the interaction between PC and Cytf remains less clear. Using chemical cross-linking combined with mass spectrometry, we identified two potential binding models for PC and Cytf: "Side-on" and "Head-on". To evaluate electron transfer, we developed an in vitro system that allowed oxidized PC, formed via light-driven electron transfer at PSI, to re-oxidize Cytf. Our data shows that a phosphomimetic variant of PC, where phosphorylated PC S49 residue interacts with PetA-K188, displays faster Cytf oxidation, likely optimizing binding and electron transfer between PC and Cytf. Additionally, PC phosphomimetic variants exhibited slower transfer rates than wild type, suggesting that phosphorylation modulates PCs interaction with PSI. This regulation likely optimizes Cytf oxidation and electron transfer under conditions of low PC availability, such as during high light stress. Overall, PC phosphorylation appears to play a role in fine-tuning electron transfer between PSI, Cytb6f, and PC, thereby ensuring efficient photosynthesis in dynamic environmental conditions.

plant biology↗

bioRxiv2025

N-terminal region of PetD is essential for cytochrome b6f function and controls STT7 kinase activity via STT7-dependent feedback loop phosphorylation

The cytochrome b6f complex (b6f) links photosystem I (PSI) and photosystem II (PSII) in the photosynthetic electron transport chain and is distributed across appressed and non-appressed thylakoid membranes. The b6f also activates the state-transition 7 kinase (STT7), which phosphorylates light-harvesting complex proteins, triggering their migration to enable energy redistribution between PSII and PSI. STT7-dependent phosphorylation has also been observed at T4 in the N-terminal domain of the b6f subunit-IV (PetD), though its functional significance has remained unclear. To investigate its role, we generated several chloroplast mutants. The phosphomimic mutation PetD T4E - but not T4A - inhibited STT7 kinase activity, as indicated by the absence of STT7-dependent phosphorylation and a State 1-locked phenotype. This reveals a novel feedback mechanism regulating STT7-dependent phosphorylation. Deletion of the first five N-terminal amino acids similarly inhibited STT7 activity and additionally disrupted electron transfer, underscoring a crucial role of the PetD N-terminus in b6f function.

plant biology↗

bioRxiv2024

Tethering ferredoxin-NADP+ reductase to photosystem I promotes photosynthetic cyclic electron transfer

Fixing CO2 via photosynthesis requires ATP and NADPH. Linear electron transfer (LET) supplies both metabolites, yet depending on environmental conditions, additional ATP is required which can be generated by cyclic electron transfer (CET). How the balance between LET and CET is set remains largely unknown. Ferredoxin(FD)-NADP+ reductase (FNR) has been suggested to act as the switch, channelling photosynthetic electrons to LET when it is bound to photosystem I (PSI) or CET when bound to cytochrome b6f. The essential role of FNR in LET precludes the use of a direct gene knock-out to test this hypothesis. We circumvented this problem by using CRISPR-Cas9 gene editing in Chlamydomonas reinhardtii to create a chimeric form of FNR tethered to PSI via PSAF. Chimeric FNR mutants exhibited impaired photosynthetic growth relative to the wild type, and decreased LET and non-photochemical quenching (NPQ) under high CO2 conditions. However, higher rates of CET and increased NPQ were found in the mutants under low CO2 or low O2 conditions where CET is normally up-regulated. Thus, rather than PSI-FNR interactions promoting LET, tethering FNR to PSI promotes CET at the expense of LET and CO2 fixation.

biochemistry↗

bioRxiv2024

Unwrapping the ciliary coat: high-resolution structure and function of the ciliary glycocalyx

The glycocalyx, a highly heterogeneous glycoprotein layer of cilia regulates adhesion and force transduction, and is involved in signalling. The high-resolution molecular architecture of this layer is currently not understood. We describe the structure of the ciliary coat in the green alga Chlamydomonas reinhardtii by cryo-electron tomography and proteomic approaches and present the high-resolution cryoEM structure of the main component, FMG1B. We describe FMG1B as a mucin orthologue which lacks the major O-glycosylation of mammalian mucins, but is N-glycosylated. FMG1A, a previously undescribed isoform of FMG1B is expressed in C. reinhardtii. By microflow-based adhesion assays we observe increased surface adhesion in the glycocalyx deficient double-mutant fmg1a-fmg1b. We find this mutant to be capable of surface-gliding, with neither isoform required for extracellular force transduction by intraflagellar transport. Our results find FMG1 to form a protective layer with adhesion-regulative instead of adhesion-conferring properties and an example of an undescribed class of mucins.

cell biology↗

bioRxiv2024

Molecular underpinnings of hornwort carbon concentrating mechanisms: subcellular localization of putative key molecular components in the model hornwort Anthoceros agrestis.

O_LIBiophysical carbon concentrating mechanisms (CCMs) operating at the single-cell level have evolved independently in eukaryotic algae and a single land plant lineage, hornworts. An essential component for an efficient eukaryotic CCM is a pyrenoid whose biology is well-characterized in the unicellular green alga, Chlamydomonas reinhardtii. By contrast, pyrenoids and CCM are little understood in hornworts. C_LIO_LIHere, we investigate the molecular underpinnings and dynamics of hornwort pyrenoids. We do so by studying the subcellular localization of candidate proteins homologous to essential CCM genes in C. reinhardtii and assessing their mobility kinetics in the hornwort model Anthoceros agrestis. C_LIO_LIWe provide evidence that an EPYC1 analog and the RuBisCO co-localize in the pyrenoid but pyrenoids seem less dynamic in A. agrestis than in C. reinhardtii. We further found that a carbon anhydrase homolog (CAH3) localizes to the pyrenoid, while an LCIB-like homolog is less intimately linked to the pyrenoid than in C. reinhardtii. C_LIO_LIOur results imply that the pyrenoid-based CCM of hornworts is characterized by a mixture of Chlamydomonas-like as well as hornwort-specific features which is in line with its independent evolutionary origin. Using these observations, we provide a first mechanistic model of hornwort CCM. C_LI

plant biology↗

bioRxiv2023

Characterization of Tryptophan Oxidation Affecting D1 Degradation by FtsH in the Photosystem II Quality Control of Chloroplasts

Photosynthesis is one of the most important reactions for sustaining our environment. Photosystem II (PSII) is the initial site of photosynthetic electron transfer by water oxidation. Light in excess, however, causes the simultaneous production of reactive oxygen species (ROS), leading to photo-oxidative damage in PSII. To maintain photosynthetic activity, the PSII reaction center protein D1, which is the primary target of unavoidable photo-oxidative damage, is efficiently degraded by FtsH protease. In PSII subunits, photo-oxidative modifications of several amino acids such as Trp have been indeed documented, whereas the linkage between such modifications and D1 degradation remains elusive. Here, we show that an oxidative post-translational modification of Trp residue at the N-terminal tail of D1 is correlated with D1 degradation by FtsH during high-light stress. We revealed that Arabidopsis mutant lacking FtsH2 had increased levels of oxidative Trp residues in D1, among which an N-terminal Trp-14 was distinctively localized in the stromal side. Further characterization of Trp-14 using chloroplast transformation in Chlamydomonas indicated that substitution of D1 Trp-14 to Phe, mimicking Trp oxidation enhanced FtsH-mediated D1 degradation under high light, although the substitution did not affect protein stability and PSII activity. Molecular dynamics simulation of PSII implies that both Trp-14 oxidation and Phe substitution cause fluctuation of D1 N-terminal tail. Furthermore, Trp-14 to Phe modification appeared to have an additive effect in the interaction between FtsH and PSII core in vivo. Together, our results suggest that the Trp oxidation at its N-terminus of D1 may be one of the key oxidations in the PSII repair, leading to processive degradation by FtsH. Competing Interest StatementThe authors have no conflict of interest, financial or otherwise, in relation to this study Impact StatementOxidative modification of Tryptophan residues in the reaction center protein D1 may be a key to drive the Photosystem II repair, likely enhancing accessibility of FtsH protease to D1.

plant biology↗

bioRxiv2022

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 reinhardtii with largely unclear physiological role. We elucidated the CRX functionality by performing in-depth quantitative proteomics of wild type cells in comparison with 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 interconnects 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. One-sentence summaryCalredoxin dependent redox regulation ensures efficient photosynthesis.

plant biology↗

bioRxiv2022

Hydrogen production in an aerobic environment by Escherichia coli K-12

Escherichia coli is a facultative anaerobe that can grow in a variety of environmental conditions. In the complete absence of O2, E. coli can perform a mixed-acid fermentation that contains within it an elaborate metabolism of formic acid. In this study, we use cavity-enhanced Raman spectroscopy (CERS), FTIR, liquid Raman spectroscopy, isotopic labelling, and molecular genetics to make advances in the understanding of bacterial formate and H2 metabolism. It is shown that, under anaerobic (anoxic) conditions, formic acid is generated endogenously, excreted briefly from the cell, and then taken up again to be disproportionated to H2 and CO2 by formate hydrogenlyase (FHL-1). However, exogenously added D-labelled formate behaves quite differently from the endogenous formate and is taken up immediately, independently, and possibly by a different mechanism, by the cell and converted to H2 and CO2. Our data support an anion-proton symport model for formic acid transport. In addition, when E. coli was grown in a micro-aerobic (micro-oxic) environment it was possible to analyse aspects of formate and O2 respiration occurring alongside anaerobic metabolism. While cells growing under micro-aerobic conditions generated endogenous formic acid, no H2 was produced. However, addition of exogenous formate at the outset of cell growth did induce FHL-1 biosynthesis and resulted in formate-dependent H2 production in the presence of O2.

microbiology↗

bioRxiv2021

The algal PETC-Pro171-Leu suppresses electron transfer in the cytochrome b6f under acidic lumenal condition

Linear photosynthetic electron flow (LEF) produces NADPH and generates a proton electrochemical potential gradient across the thylakoid membrane used to synthesize ATP, both of which are required for CO2 fixation. As cellular demand for ATP and NADPH are variable, cyclic electron flow (CEF) between PSI and cytochrome b6f complex (b6f) produces extra ATP. The b6f regulates LEF and CEF via photosynthetic control, which is a pH-dependent b6f slowdown of plastoquinol oxidation at the lumenal site. This protection mechanism is triggered at more alkaline lumen pH in the pgr1 mutant of the vascular plant Arabidopsis thaliana, carrying Pro194Leu in the b6f Rieske Iron-sulfur protein. In this work, we introduced pgr1 mutation in the green alga Chlamydomonas reinhardtii (PETC-P171L). Consistent with pgr1 phenotype, PETC-P171L displayed an impaired NPQ induction along with slower photoautotrophic growth under high light conditions. Our data provides evidence that the {Delta}pH component in PETC-P171L is dependent on oxygen availability. Only under low oxygen conditions the {Delta}pH component was sufficient to trigger a phenotype in algal PETC-P171L where the mutant b6f was more restricted to oxidize the PQ pool and showed a diminished electron flow through the b6f complex. One sentence summaryChange of PETC to P171L via site directed mutagenesis alters the pH dependency of the photosynthetic control mechanism

plant biology↗

bioRxiv2021

Photoprotection is regulated by light-independent CO2 availability

Photosynthetic algae have evolved mechanisms to cope with suboptimal light and CO2 conditions. When light energy exceeds CO2 fixation capacity, Chlamydomonas reinhardtii activates photoprotection, mediated by LHCSR1/3 and PSBS, and the CO2 Concentrating Mechanism (CCM). How light and CO2 signals converge to regulate these processes remains unclear. Here, we show that excess light activates photoprotection- and CCM-related genes by altering intracellular CO2 concentrations and that depletion of CO2 drives these responses, even in total darkness. High CO2 levels, derived from respiration or impaired photosynthetic fixation, repress LHCSR3/CCM genes while stabilizing the LHCSR1 protein. Finally, we show that the CCM regulator CIA5 also regulates photoprotection, controlling LHCSR3 and PSBS transcript accumulation while inhibiting LHCSR1 protein accumulation. This work has allowed us to dissect the effect of CO2 and light on CCM and photoprotection, demonstrating that light often indirectly affects these processes by impacting intracellular CO2 levels.

plant biology↗

bioRxiv2021

Chloroplast photosystem I dimer and high resolution model of the complex with plastocyanin

Photosystem I (PSI) enables photo-electron transfer and regulates photosynthesis in the bioenergetic membranes of cyanobacteria and chloroplasts. Being a multi-subunit complex, its macromolecular organization affects the dynamics of photosynthetic membranes. Here, we reveal a chloroplast PSI from the green alga Chlamydomonas reinhardtii that is organized as a homodimer, comprising 40 protein subunits with 118 transmembrane helices that provide scaffold for 568 pigments. Our cryo-EM structure identifies that the absence of PsaH and Lhca2 gives rise to a head-to-head relative orientation of the PSI-LHCI monomers in a way that is essentially different from the oligomer formation in cyanobacteria. The light-harvesting protein Lhca9 is the key element for mediating this dimerization. The interface between the monomers is lacking PsaH, and thus partially overlaps with the surface area that would bind one of the LHCII complexes in state transitions. We also define the most accurate available PSI-LHCI model at 2.3 [A] resolution, including a flexibly bound electron donor plastocyanin, and assign correct identities and orientations of all the pigments, as well as 621 water molecules that affect energy transfer pathways.

plant biology↗

bioRxiv2021

Functional basis of electron transport within photosynthetic complex I

Photosynthesis and respiration rely upon a proton gradient to produce ATP. In photosynthesis, the Respiratory Complex I homologue, Photosynthetic Complex I (PS-CI) is proposed to couple ferredoxin oxidation and plastoquinone reduction to proton pumping across thylakoid membranes, and is fundamental to bioenergetics in photosynthetic bacteria and some higher plant cell types. However, little is known about the PS-CI molecular mechanism and attempts to understand its function have previously been frustrated by its large size and high lability. Here, we overcome these challenges by pushing the limits in sample size and spectroscopic sensitivity, to determine arguably the most important property of any electron transport enzyme - the reduction potentials of its cofactors, in this case the iron-sulphur clusters of PS-CI, and unambiguously assign them to the structure using double electron-electron resonance (DEER). We have thus determined the bioenergetics of the electron transfer relay and provide insight into the mechanism of PS-CI, laying the foundations for understanding of how this important bioenergetic complex functions.

biophysics↗

bioRxiv2021

Structure of plant PSI-plastocyanin complex reveals strong hydrophobic interactions.

Photosystem I is defined as plastocyanin-ferredoxin oxidoreductase. Taking advantage of genetic engineering, kinetic analyses and cryo-EM, our data provide novel mechanistic insights into binding and electron transfer between PSI and Pc. Structural data at 2.74 [A] resolution reveals strong hydrophobic interactions in the plant PSI-Pc ternary complex, leading to exclusion of water molecules from PsaA-PsaB / Pc interface once the PSI-Pc complex forms. Upon oxidation of Pc, a slight tilt of bound oxidized Pc allows water molecules to accommodate the space between Pc and PSI to drive Pc dissociation. Such a scenario is consistent with the six times larger dissociation constant of oxidized as compared to reduced Pc and mechanistically explains how this molecular machine optimized electron transfer for fast turnover. One Sentence SummaryGenetic engineering, kinetics and cryo-EM structural data reveal a mechanism in a major step of oxygenic photosynthesis

biochemistry↗