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Crystal Structure of a Dimeric Mannose-specific Agglutinin from Garlic: Quaternary Association and Carbohydrate Specificity

Chandra, Nagasuma R and Ramachandraiah, Gosu and Bachhawat, Kiran and Dam, Tarun K and Surolia, A and Vijayan, M (1999) Crystal Structure of a Dimeric Mannose-specific Agglutinin from Garlic: Quaternary Association and Carbohydrate Specificity. In: Journal of Molecular Biology, 285 . pp. 1157-1168.

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A mannose-specific agglutinin, isolated from garlic bulbs, has been crystallized in the presence of a large excess of a-D-mannose, in space group C2 and cell dimensions, a=203.24, b=43.78, c=79.27 A, $\beta$ = 112.4 , with two dimers in the asymmetric unit. X-ray diffraction data were collected up to a nominal resolution of 2.4 A and the structure was solved by molecular replacement. The structure, refined to an R-factor of 22.6% and an $R_f_r_e_e$ of 27.8% reveals a $\beta$-prism II fold, similar to that in the snowdrop lectin, comprising three antiparallel four-stranded $\beta$-sheets arranged as a 12-stranded $\beta$-barrel, with an approximate internal 3-fold symmetry. This agglutinin is, however, a dimer unlike snowdrop lectin which exists as a tetramer, despite a high degree of sequence similarity between them. A comparison of the two structures reveals a few substitutions in the garlic lectin which stabilise it into a dimer and prevent tetramer formation. Three mannose molecules have been identified on each subunit. In addition, electron density is observed for another possible mannose molecule per dimer resulting in a total of seven mannose molecules in each dimer. Although the mannose binding sites and the overall structure are similar in the subunits of snowdrop and garlic lectin, their specificities to glycoproteins such as GP120 vary considerably. These differences appear, in part, to be a direct consequence of the differences in oligomerisation, implying that variation in quaternary association may be a mode of achieving oligosaccharide specificity in bulb lectins.

Item Type: Journal Article
Publication: Journal of Molecular Biology
Publisher: Academic Press
Additional Information: The copyright belongs to Academic Press
Keywords: Protein crystallography;Bulb lectin;Beta prism fold;Quaternary association;Mannose binding
Department/Centre: Division of Biological Sciences > Molecular Biophysics Unit
Date Deposited: 10 Jan 2007
Last Modified: 19 Sep 2010 04:14
URI: http://eprints.iisc.ac.in/id/eprint/1353

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