Kaori Kohzuma

Michigan State University, East Lansing, MI, United States

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Publications (8)30.04 Total impact

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    ABSTRACT: In plants, modulation of photosynthetic energy conversion in varying environments is often accompanied by adjustment of the abundance of photosynthetic components. In wild watermelon (Citrullus lanatus L.), proteome analysis revealed that the ε subunit of chloroplast ATP synthase occurs as two distinct isoforms with largely-different isoelectric points, although encoded by a single gene. Mass spectrometry (MS) analysis of the ε isoforms indicated that the structural difference between the ε isoforms lies in the presence or absence of an acetyl group at the N-terminus. The protein level of the non-acetylated ε isoform preferentially decreased in drought, whereas the abundance of the acetylated ε isoform was unchanged. Moreover, metalloprotease activity that decomposed the ε subunit was detected in a leaf extract from drought-stressed plants. Furthermore, in vitro assay suggested that the non-acetylated ε subunit was more susceptible to degradation by metalloaminopeptidase. We propose a model in which quantitative regulation of the ε subunit involves N-terminal acetylation and stress-induced proteases.
    Bioscience Biotechnology and Biochemistry 05/2013; · 1.27 Impact Factor
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    ABSTRACT: The chloroplast CFO-CF1-ATP synthase (ATP synthase) is activated in the light and inactivated in the dark by thioredoxin-mediated redox modulation of a disulfide bridge on its γ-subunit. The activity of the ATP synthase is also fine-tuned during steady-state photosynthesis in response to metabolic changes, e.g., altering CO2 levels to adjust the thylakoid proton gradient and thus the regulation of light harvesting and electron transfer. The mechanism of this fine-tuning is unknown. We test here the possibility that it also involves redox modulation. We found that modifying the Arabidopsis thaliana γ -subunit by mutating three highly conserved acidic amino acids from D211, E212 and E226 to V, L and L resulted in a mutant, termed mothra, in which ATP synthase lacked light-dark regulation, but with relatively small effects on maximal activity in vivo. In situ equilibrium redox titrations and thiol redox-sensitive labeling studies showed that the γ-subunit disulfide/sulfhydryl couple in the modified ATP synthase has a more reducing redox potential and thus remains predominantly oxidized under physiological conditions, implying that the highly conserved acidic residues in γ subunit structurally adjust thiol modulations in photosynthetic ATP synthase. In contrast to its altered light-dark regulation, mothra retained wild-type fine-tuning of ATP synthase activity in response to changes in ambient CO2 concentrations, indicating that the light-dark- and metabolic-related regulation occur through different mechanisms, possibly via small molecule allosteric effectors or covalent modification.
    Journal of Biological Chemistry 03/2013; · 4.65 Impact Factor
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    ABSTRACT: The chloroplast ATP synthase catalyzes the light-driven synthesis of ATP and acts as a key feedback regulatory component of photosynthesis. Arabidopsis possesses two homologues of the regulatory γ subunit of the ATP synthase, encoded by the ATPC1 and ATPC2 genes. Using a series of mutants, we show that both these subunits can support photosynthetic ATP synthesis in vivo with similar specific activities, but that in wild-type plants, only γ(1) is involved in ATP synthesis in photosynthesis. The γ(1)-containing ATP synthase shows classical light-induced redox regulation, whereas the mutant expressing only γ(2)-ATP synthase (gamma exchange-revised ATP synthase, gamera) shows equally high ATP synthase activity in the light and dark. In situ redox titrations demonstrate that the regulatory thiol groups on γ(2)-ATP synthase remain reduced under physiological conditions but can be oxidized by the strong oxidant diamide, implying that the redox potential for the thiol/disulphide transition in γ(2) is substantially higher than that for γ(1). This regulatory difference may be attributed to alterations in the residues near the redox-active thiols. We propose that γ(2)-ATP synthase functions to catalyze ATP hydrolysis-driven proton translocation in nonphotosynthetic plastids, maintaining a sufficient transthylakoid proton gradient to drive protein translocation or other processes. Consistent with this interpretation, ATPC2 is predominantly expressed in the root, whereas modifying its expression results in alteration of root hair development. Phylogenetic analysis suggests that γ(2) originated from ancient gene duplication, resulting in divergent evolution of functionally distinct ATP synthase complexes in dicots and mosses.
    Proceedings of the National Academy of Sciences 02/2012; 109(9):3293-8. · 9.74 Impact Factor
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    ABSTRACT: Higher plants possess two, distinct genes for the ATP synthase Ȗ subunit, atpC1 and atpC2. In Arabidopsis, atpC1 is the predominant form, and atpC2 is only weakly expressed in photosynthetic tissues. There is no evidence that it plays any role in energy transduction. Indeed, mutants lacking atpC1 are incapable of photoautotrophic growth, while those lacking atpC2 have no noticeable phenotype. To elucidate the possible function of these orthologs, we analyzed mutants expressing exclusively atpC1 or atpC2 in Arabidopsis thaliana. In vivo chlorophyll fluorescence and electrochromic shift (ECS) analyses demonstrated that both atpC1 and atpC2 can function in ATP synthesis, though even under a strong promoter, the activity of atpC2-containing ATP synthase was low. However, we observed a striking difference in the regulation of ATP synthase containing the two orthologs. With atpC1, the ATP synthase was inactivated in the dark, likely via oxidation of the regulatory Ȗ subunit thiols. ATP synthase containing exclusively atpC2 showed no decrease in activity even after extensive dark adaptation. We propose that atpC2 may function to catalyze low levels of ATP-driven proton translocation in the dark, when the bulk of ATP synthase is inactivated, maintaining sufficient transthylakoid proton gradient to drive protein translocation or other processes.
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    ABSTRACT: Cyclic electron flow (CEFI) has been proposed to balance the chloroplast energy budget, but the pathway, mechanism, and physiological role remain unclear. We isolated a new class of mutant in Arabidopsis thaliana, hcef for high CEF1, which shows constitutively elevated CEF1. The first of these, hcef1, was mapped to chloroplast fructose-1,6-bisphosphatase. Crossing hcef1 with pgr5, which is deficient in the antimycin A-sensitive pathway for plastoquinone reduction, resulted in a double mutant that maintained the high CEF1 phenotype, implying that the PGR5-dependent pathway is not involved. By contrast, crossing hcef1 with crr2-2, deficient in thylakoid NADPH dehydrogenase (NDH) complex, results in a double mutant that is highly light sensitive and lacks elevated CEF1, suggesting that NDH plays a direct role in catalyzing or regulating CEF1. Additionally, the NdhI component of the NDH complex was highly expressed in hcef1, whereas other photosynthetic complexes, as well as PGR5, decreased. We propose that (1) NDH is specifically upregulated in hcef1, allowing for increased CEF1; (2) the hcef1 mutation imposes an elevated ATP demand that may trigger CEF1; and (3) alternative mechanisms for augmenting ATP cannot compensate for the loss of CEF1 through NDH.
    The Plant Cell 01/2010; 22(1):221-33. · 9.25 Impact Factor
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    ABSTRACT: Proton motive force (pmf) across thylakoid membranes is not only for harnessing solar energy for photosynthetic CO2 fixation, but also for triggering feedback regulation of photosystem II antenna. The mechanisms for balancing these two roles of the proton circuit under the long-term environmental stress, such as prolonged drought, have been poorly understood. In this study, we report on the response of wild watermelon thylakoid ‘proton circuit’ to drought stress using both in vivo spectroscopy and molecular analyses of the representative photosynthetic components. Although drought stress led to enhanced proton flux via a ∼34% increase in cyclic electron flow around photosystem I (PS I), an observed ∼fivefold decrease in proton conductivity, gH+, across thylakoid membranes suggested that decreased ATP synthase activity was the major factor for sustaining elevated qE. Western blotting analyses revealed that ATP synthase content decreased significantly, suggesting that quantitative control of the complex plays a pivotal role in down-regulation of gH+. The expression level of cytochrome b6f complex – another key control point in photosynthesis – also declined, probably to prevent excess-reduction of PS I electron acceptors. We conclude that plant acclimation to long-term environmental stress involves global changes in the photosynthetic proton circuit, in which ATP synthase represents the key control point for regulating the relationship between electron transfer and pmf.
    Plant Cell and Environment 02/2009; 32(3):209 - 219. · 5.14 Impact Factor
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    ABSTRACT: Photosynthetic energy conversion in plants involves the formation of a proton gradient across thylakoid membranes, but the mechanisms for balancing the membrane potential have been poorly elucidated. We found that drought stress induces selective decomposition of the ε subunit in the CFoCF1 ATP synthase. Thylakoid membranes from stressed leaves showed reduced efficiencies for proton gradient formation and energy coupling, but addition of the recombinant ε subunit significantly suppressed their “leaky” property. We conclude that the selective decomposition of the e subunit induces partial uncoupling of thylakoid membranes under drought, and hence contributes to the avoidance of over-acidification in the thylakoid lumen under excess light conditions.
    12/2007: pages 617-621;
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    ABSTRACT: The majority of higher plants are unable to survive extreme drought in the presence of strong solar radiation. However, a small group of vascular plants termed ‘‘xerophytes’’ have evolved drought and high light stress tolerance, and successfully thrives in the arid areas. This chapter will focus on the physiological, biochemical and molecular responses of wild watermelon (Citrullus lanatus), a xerophyte which is indigenous to the Kalahari Desert despite carrying out C3-type photosynthesis. The electrochromic shift of carotenoids in the thylakoid membranes was analyzed in vivo, which revealed that the proton efflux through chloroplast ATP synthase was strongly suppressed under drought and high light stresses. In addition, cyclic electron flow around photosystem I was significantly activated under the stress, suggesting the functional relevance of these processes to the build-up of large ΔpH across thylakoid membranes, for sustaining high qE quenching under excess light conditions. Biochemical analyses showed that key components for ROS metabolism, such as chloroplastic ascorbate peroxidase and monodehydroascorbate reductase, were markedly fortified in this plant. Moreover, unique responses of wild watermelon under the stress were described like metabolism and function of citrulline, a novel compatible solute with potent activity for scavenging hydroxyl radicals. Furthermore, characteristic gene expression patterns were observed in this plant under drought, which are exemplified by the induction of cytochrome b 561, a trans-plasma membrane protein for transferring reducing equivalents from cytosol to the apoplasts. Interestingly, unprecedentedly high activity of ascorbate oxidase was observed in the leaf apoplasts, suggesting the electron flux from cytosol to this terminal oxidase may be activated under drought. Taken together, these findings offer intriguing implications on how terrestrial plants can achieve effective adaptation to the harsh environmental conditions.

Publication Stats

76 Citations
30.04 Total Impact Points

Institutions

  • 2012–2013
    • Michigan State University
      • MSU-DOE Plant Research Laboratory
      East Lansing, MI, United States
  • 2010
    • Washington State University
      • Institute of Biological Chemistry
      Pullman, WA, United States
  • 2009
    • Nara Institute of Science and Technology
      • Graduate School of Biological Sciences
      Ikoma, Nara, Japan