Saitama University discovers novel enzyme in plant chloroplasts involved in dephosphorylation of intracellular electron transport molecules

A research group led by Professor Maki Kawai-Yamada from the Graduate School of Science and Engineering at Saitama University, in collaboration with Professor Yutaka Kodama from the Center for Bioscience Research and Education at Utsunomiya University, Senior Researcher Shin-Nosuke Hashida from the Sustainable Systems Research Laboratory at the Central Research Institute of Electric Power Industry, and Associate Professor Atsuko Miyagi from the Faculty of Agriculture at Yamagata University, has revealed that a protein called CCR4C in chloroplasts functions as an enzyme (NADP phosphatase) that dephosphorylates NADP(H), a molecule that transports electrons within cells. This achievement is expected to contribute to understanding the regulatory mechanisms of photosynthesis and stress response in plants. The results were published in the Proceedings of the National Academy of Sciences (PNAS) on October 15.

The coenzyme NAD(P)(H) plays a crucial role in plant photosynthesis, and its quantity fluctuates and is strictly regulated in accordance with conditions such as light environment. NADP⁺ and NADPH are used in photosynthesis, lipid synthesis, and biological defense against reactive oxygen species. NAD kinase (NADK2), which is localized in chloroplasts, is an enzyme that supplies NADP⁺ to the photosynthetic electron transport chain, where electrons excited by light energy are transferred to NADP⁺ to generate NADPH. However, the mechanism for reducing NAD(P)(H) levels remained unclear for a long time. Therefore, in this study, mutant strains that restore the phenotype of an Arabidopsis mutant (nadk2) in which NADK2 does not function were screened, and the novel enzyme CCR4C localized in chloroplasts was identified.

It was found that CCR4C functions as a dephosphorylation enzyme (NADP phosphatase) that converts NADP⁺ and NADPH to NAD⁺ and NADH respectively and plays a role in regulating NAD(P)(H) levels within chloroplasts.

Furthermore, it was found that CCR4C-deficient strains showed tolerance to oxidative stress and restored the abnormalities in leaf color and growth of the nadk2 mutant.

These findings demonstrate that CCR4C is a novel factor that regulates NAD(P)(H) levels within chloroplasts.

Going forward, detailed analysis of the molecular mechanisms of NAD(P)(H) regulation via CCR4C and its relationship with oxidative stress response will be applied to crop improvement to enhance photosynthetic efficiency and stress tolerance.

Kawai-Yamada commented: "I was very excited to finally identify the chloroplast-localized NADP(H) dephosphorylase whose identity had remained unknown for many years. This discovery provides an important clue for understanding the metabolic mechanisms unique to plants, namely photosynthesis. Additionally, the CCR4 protein we identified exists in multiple forms not only in plants but also in animals and may be involved in NAD(P)(H) regulation at various locations within cells, so I am even more excited about future research developments."

Journal Information
Publication: PNAS
Title: Identification of CCR4C as a chloroplast-localized NADP(H) phosphatase regulating NAD(P)(H) balance in Arabidopsis
DOI: 10.1073/pnas.2504605122