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J. Biol. Chem., Vol. 282, Issue 27, 19282-19291, July 6, 2007

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The CHLI1 Subunit of Arabidopsis thaliana Magnesium Chelatase Is a Target Protein of the Chloroplast Thioredoxin^*

Akinori Ikegami¹, Naho Yoshimura¹, Ken Motohashi^¶||, Shigekazu Takahashi, Patrick G. N. Romano^||, Toru Hisabori^¶||, Ken-ichiro Takamiya, and Tatsuru Masuda²

From the Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Nagatsuta 4259, Yokohama 226-8501, Japan, the ^¶ATP System Project, Exploratory Research for Advanced Technology, Japan Science and Technology Corporation, Nagatsuta 5800-3, Yokohama 226-0026, Japan, the ^||Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Yokohama 226-8503, Japan, and the Graduate School of Arts and Sciences, University of Tokyo, Komaba 3-8-1, Tokyo 153-8902, Japan

Insertion of magnesium into protoporphyrin IX by magnesium chelatase is a key step in the chlorophyll biosynthetic pathway, which takes place in plant chloroplasts. ATP hydrolysis by the CHLI subunit of magnesium chelatase is an essential component of this reaction, and the activity of this enzyme is a primary determinant of the rate of magnesium insertion into the chlorophyll molecule (tetrapyrrole ring). Higher plant CHLI contains highly conserved cysteine residues and was recently identified as a candidate protein in a proteomic screen of thioredoxin target proteins (Balmer, Y., Koller, A., del Val, G., Manieri, W., Schurmann, P., and Buchanan, B. B. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 370–375). To study the thioredoxin-dependent regulation of magnesium chelatase, we first investigated the effect of thioredoxin on the ATPase activity of CHLI1, a major isoform of CHLI in Arabidopsis thaliana. The ATPase activity of recombinant CHLI1 was found to be fully inactivated by oxidation and easily recovered by thioredoxin-assisted reduction, suggesting that CHLI1 is a target protein of thioredoxin. Moreover, we identified one crucial disulfide bond located in the C-terminal helical domain of CHLI1 protein, which may regulate the binding of the nucleotide to the N-terminal catalytic domain. The redox state of CHLI was also found to alter in a light-dependent manner in vivo. Moreover, we successfully observed stimulation of the magnesium chelatase activity in isolated chloroplasts by reduction. Our findings strongly suggest that chlorophyll biosynthesis is subject to chloroplast biogenesis regulation networks to coordinate them with the photosynthetic pathways in chloroplasts.

Received for publication, April 20, 2007 , and in revised form, May 1, 2007.

^* This work was supported by Grants-in-Aid for Scientific Research 16570030 and 18570034 and by funds from the 21st Century Center of Excellence program from the Ministry of Education, Culture, Sports, Science and Technology of Japan. This work is also supported by a grant from Yamada Science Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1.

This work is dedicated to Prof. Ken-ichiro Takamiya of Tokyo Institute of Technology (Yokohama, Japan) who passed away as a result of a traffic accident in 2005.

¹ These authors contributed equally to this work.

² To whom correspondence should be addressed. Tel.: 81-3-5454-6627; Fax: 81-3-5454-4321; E-mail: ctmasuda{at}mail.ecc.u-tokyo.ac.jp.

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