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J. Biol. Chem., Vol. 280, Issue 15, 14865-14876, April 15, 2005

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Structural Mechanism Governing the Quaternary Organization of Monocot Mannose-binding Lectin Revealed by the Novel Monomeric Structure of an Orchid Lectin^*

Wei Liu, Na Yang¶, Jingjin Ding¶, Ren-huai Huang, Zhong Hu||, and Da-Cheng Wang**

From the Center for Structural and Molecular Biology, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, ^¶Graduate School of Chinese Academy of Sciences, Beijing 100039, and ^||Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650204, People's Republic of China

Two isoforms of an antifungal protein, gastrodianin, were isolated from two subspecies of the orchid Gastrodia elata, belonging to the protein superfamily of monocot mannose-specific lectins. In the context that all available structures in this superfamily are oligomers so far, the crystal structures of the orchid lectins, both at 2.0 Å, revealed a novel monomeric structure. It resulted from the rearrangement of the C-terminal peptide inclusive of the 12th -strand, which changes from the "C-terminal exchange" into a "C-terminal self-assembly" mode. Thus, the overall tertiary scaffold is stabilized with an intramolecular -sheet instead of the hybrid observed on subunit/subunit interface in all known homologous dimeric or tetrameric lectins. In contrast to the constrained extended conformation with a cis peptide bond between residues 98 and 99 commonly occurring in oligomers, a -hairpin forms from position 97 to 101 with a normal trans peptide bond at the corresponding site in gastrodianin, which determines the topology of the C-terminal peptide and thereby its unique fold pattern. Sequence and structure comparison shows that residue replacement and insertion at the position where the -hairpin occurs in association with cis-trans inter-conversion of the specific peptide bond (97–98) are possibly responsible for such a radical structure switch between monomers and oligomers. Moreover, this seems to be a common melody controlling the quaternary states among bulb lectins through studies on sequence alignment. The observations revealed a structural mechanism by which the quaternary organization of monocot mannose binding lectins could be governed. The mutation experiment performed on maltose-binding protein-gastrodianin fusion protein followed by a few biochemical detections provides direct evidence to support this conclusion. Potential carbohydrate recognition sites and biological implications of the orchid lectin based on its monomeric state are also discussed in this paper.

Received for publication, October 13, 2004 , and in revised form, January 11, 2005.

^* The work was supported by National Natural Science Foundation of China (NSFC Grant 30070162) Ministry of Sciences and Technology of China Grants 863-2002BA711A13 and 973-G1999075064, and Chinese Academy of Sciences (Grants KSCX1-SW-17 and KSCX2-SW-322). 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 (codes 1XD5 (gastrodianin-1) and 1XD6 (gastrodianin-4)) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

Both authors contributed equally to this work.

^** To whom correspondence should be addressed. Tel.: 86-10-64888547; Fax: 86-10-64888560; E-mail: dcwang{at}sun5.ibp.ac.cn.

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