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Glycobiology Advance Access published online on June 15, 2005
Glycobiology, doi:10.1093/glycob/cwi087
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© The Author 2005. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oupjournals.org
Received April 23, 2005
Revised May 29, 2005
Accepted June 1, 2005

Article

Unusual sugar specificity of banana lectin from Musa paradisiaca and its probable evolutionary origin. Crystallographic and modelling studies

D. D. Singha 1, K. Saikrishnana 1, Prashant Kumara 1, K. Sekarb 2, A. Suroliaa 1, and M. Vijayana 1*

1 Molecular Biophysics Unit, Indian Institute of Science, Bangalore-560 012, INDIA
2 Bioinformatics Centre and Supercomputer Education and Research Centre, Indian Institute of Science, Bangalore-560 012, INDIA.

* To whom correspondence should be addressed.
M. Vijayana, E-mail: mv{at}mbu.iisc.ernet.in


  Abstract

The crystal structure of a complex of methyl-{alpha}-D-mannoside with banana lectin from Musa paradisiaca reveals two primary binding sites in the lectin, unlike in other lectins with {beta}-prism I fold which essentially consists of three Greek key motifs. It has been suggested that the fold evolved through successive gene duplication and fusion of an ancestral Greek key motif. In other lectins, all from dicots, the primary binding site exists on one of the three motifs in the threefold symmetric molecule. Banana is a monocot and the three motifs have not diverged enough to obliterate sequence similarity among them. Two Greek key motifs in it carry one primary binding site each. A common secondary binding site exists on the third Greek key. Modelling shows that both the primary sites can support 1-2, 1-3 and 1-6 linked mannosides with the second residue interacting in each case primarily with the secondary binding site. Modelling also readily leads to a bound branched mannopentose with the non-reducing ends of the two branches anchored at the two primary binding sites, providing a structural explanation for the lectin’s specificity for branched {alpha}-mannans. A comparison of the dimeric banana lectin with other {beta}-prism I fold lectins, provide interesting insights into the variability in their quaternary structure.

Keywords: {beta}-prism I fold lectin/evolution of carbohydrate specificity/lectin - branched sugar interaction/quaternary association/oligosaccharide modelling.
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