Glycobiology Advance Access originally published online on June 15, 2005
Glycobiology 2005 15(10):1025-1032; doi:10.1093/glycob/cwi087
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Unusual sugar specificity of banana lectin from Musa paradisiaca and its probable evolutionary origin. Crystallographic and modelling studies
2 Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560 012, Karnataka, India; and 3 Bioinformatics Centre and Supercomputer Education and Research Centre, Indian Institute of Science, Bangalore 560 012, Karnataka, India
1 To whom correspondence should be addressed; e-mail: mv{at}mbu.iisc.ernet.in
Received on April 23, 2005; revised on May 29, 2005; accepted on June 1, 2005
The crystal structure of a complex of methyl-
-D-mannoside with banana lectin from Musa paradisiaca reveals two primary binding sites in the lectin, unlike in other lectins with ß-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 three-fold 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 nonreducing ends of the two branches anchored at the two primary binding sites, providing a structural explanation for the lectin’s specificity for branched -mannans. A comparison of the dimeric banana lectin with other ß-prism I fold lectins, provides interesting insights into the variability in their quaternary structure.
Key words: ß-prism I fold lectin / evolution of carbohydrate specificity / lectin-branched sugar interaction / quaternary association / oligosaccharide modelling