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J. Biol. Chem., Vol. 282, Issue 46, 33252-33256, November 16, 2007

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Curculin Exhibits Sweet-tasting and Taste-modifying Activities through Its Distinct Molecular Surfaces^*

Eiji Kurimoto, Maiko Suzuki, Eiko Amemiya, Yoshiki Yamaguchi, Satoru Nirasawa, Nobuhisa Shimba^¶, Ningchun Xu^¶, Tatsuki Kashiwagi^¶, Misako Kawai^¶, Ei-ichiro Suzuki^¶, and Koichi Kato^||1

From the Graduate School of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe-dori, Mizuho-ku, Nagoya 467-8603, the Enzyme Laboratory, National Food Research Institute, 2-1-12 Kannondai, Tsukuba 305-8642, Ibaraki, the ^¶Institute of Life Sciences, Ajinomoto Co., Inc., 1-1 Suzuki-cho, Kawasaki-ku, Kawasaki 210-8681, and the ^||Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan

Curculin isolated from Curculigo latifolia, a plant grown in Malaysia, has an intriguing property of modifying sour taste into sweet taste. In addition to this taste-modifying activity, curculin itself elicits a sweet taste. Although these activities have been attributed to the heterodimeric isoform and not homodimers of curculin, the underlying mechanisms for the dual action of this protein have been largely unknown. To identify critical sites for these activities, we performed a mutational and structural study of recombinant curculin. Based on the comparison of crystal structures of curculin homo- and heterodimers, a series of mutants was designed and subjected to tasting assays. Mapping of amino acid residues on the three-dimensional structure according to their mutational effects revealed that the curculin heterodimer exhibits sweet-tasting and taste-modifying activities through its partially overlapping but distinct molecular surfaces. These findings suggest that the two activities of the curculin heterodimer are expressed through its two different modes of interactions with the T1R2-T1R3 heterodimeric sweet taste receptor.

Received for publication, August 29, 2007 , and in revised form, September 25, 2007.

^* This work was supported by Grants-in-aid for Scientific Research 17651126 and 18590101 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by a grant-in-aid for research in Nagoya City University. 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 atomic coordinates and structure factors (code 2DPF) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

The on-line version of this article (available at http://www.jbc.org) contains a supplemental table and three supplemental figures.

¹ To whom correspondence should be addressed. Tel./Fax: 81-52-836-3447; E-mail: kkato{at}phar.nagoya-cu.ac.jp.

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