Biology subjects

Ozawa, S.-I.

Publications and source records attributed to Ozawa, S.-I..

4 recordsLinked to original sources

bioRxiv2025

A nuclear CobW/WW-domain factor represses the CO2-concentrating mechanism in the green alga Chlamydomonas reinhardtii

Microalgae induce a CO2-concentrating mechanism (CCM) to maintain photosynthesis when dissolved CO2 is limited, but how this energy-intensive system is suppressed when CO2 levels rise has remained unclear. The CCM consumes 15-30% of photosynthetically-generated ATP, making its regulation critical for cellular energy balance. Here, we identify a nuclear repressor of the CCM in the green alga Chlamydomonas reinhardtii. A pull-down screen for interacting partners of the master activator CCM1/CIA5 revealed an uncharacterized protein that co-purifies with CCM1 even after high-salt washes. This protein, designated CCM1-binding protein 1 (CBP1), combines a CobW/CobW_C GTP-binding metallochaperone module with a WW domain characteristic of protein-protein interactions. CBP1 colocalizes with CCM1 in the nucleus regardless of CO2 conditions, and the two proteins interact in vivo. CRISPR/Cas9-based disruption of CBP1 does not affect growth or CCM induction under CO2 limitation but derepresses 27 of 41 CCM1-dependent low-CO2 inducible genes under high-CO2 conditions. These include the periplasmic and intracellular carbonic anhydrases (CAH1, LCIB) and inorganic carbon transporters/channels (LCIA, LCI1, BST1, BST3). Consistently, cbp1 mutants accumulate higher levels of CAH1 and LCIB proteins and exhibit a 40% increase in inorganic carbon affinity under high-CO2 conditions; this phenotype is rescued by CBP1 complementation or by acetazolamide treatment. These results demonstrate that CBP1 prevents unnecessary CCM activity when CO2 is abundant, acting upstream of both transporter/channel and carbonic anhydrase modules. Our findings suggest a regulatory mechanism potentially linking zinc-dependent protein chemistry to CCM gene repression, providing insights into energy-efficient CO2 sensing in aquatic photosynthetic organisms. Significance statementAlgae flourish by activating a CO2-concentrating mechanism (CCM) when dissolved CO2 is limited but must deactivate it when CO2 levels increase to conserve energy. We have identified the nuclear protein that functions as this long sought "off switch" in the model green alga. Deletion of this protein causes cells to overproduce CCM transporters and enzymes, maintaining CCM activity even under high CO2 conditions, demonstrating its essential role in suppressing the system when carbon is abundant. This finding illuminates how algae balance energy consumption with carbon capture and offers a new target for engineering strains that fix CO2 more efficiently for biofuel production or climate-mitigation technologies.

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↗

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↗

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↗