Narcissus and Daffodil

Narcissus lectins 389
All monocot mannose-binding lectins are composed of mature lectin polypeptides
of 11–14 kDa. Like the narcissus lectin, many other monocot mannose-binding
lectins are complex mixtures of isolectins resulting from the expression of a family
of closely related genes (Van Damme et al., 1998a). All DNA sequences encoding
these lectins show a high degree of homology, resulting in a highly conserved
three-dimensional structure for the different lectins (Barre et al., 1996; Hester et al.,
1995; Chantalat et al., 1996; Wright et al., 1997).
Hapten-inhibition assays with simple sugars have demonstrated that all currently
known monocot mannose-binding lectins are exclusively inhibited by mannose
(Van Damme et al., 1995). However, the mannose concentrations required for an
efficient inhibition are high (IC50 =20–200 mM), suggesting that these lectins have
a low affinity for the monosaccharide. The ability of the monocot mannose-binding
lectins to distinguish D-mannose from D-glucose units distinguishes this family
of lectins from the mannose/glucose-binding lectins belonging to the family of legume
lectins and from the mannose/maltose-binding lectins of the family of jacalinrelated
lectins (Van Damme et al., 1998a).
Despite the fact that all monocot mannose-binding lectins will react with mannose,
they differ in their fine carbohydrate-binding specificity and interaction with
oligo- and polysaccharides containing D-mannosyl groups. In their immobilised
form the lectins will also show different chromatographic behaviour towards
glycosyl-asparagine glycopeptides (Kaku et al., 1990, 1991).
GENERAL CONCLUSIONS
At present the lectin is one of the very few proteins from narcissus that has been
studied in detail. The lectin represents an important fraction of the total protein
content, at least in resting bulbs. Developmental regulation of the lectin concentration
and toxicity of the lectin towards insects indicates a possible involvement of
the lectin in storage metabolism and/or plant defence.
ACKNOWLEDGEMENTS
This work was supported in part by grants from the Katholieke Universiteit
Leuven and the Fund for Scientific Research Flanders. E.J.M.V.D. is Postdoctoral
Fellow and W.J.P. a Research Director of this fund. We would like to thank Prof.
P. Rougé and Dr. A. Barre for their help with the figures.
REFERENCES
Balzarini, J., Schols, D., Neyts, J., Van Damme, E., Peumans, W. and De Clercq, E. (1991)
α-(1–3)- and α-(1–6)-D-mannose-specific plant lectins are markedly inhibitory to human
immunodeficiency virus and cytomegalovirus infections in vitro. Antimicrobial Agents and
Chemotherapy, 35, 410–416.
Barre, A., Van Damme, E.J.M., Peumans, W.J. and Rougé, P. (1996) Structure-function
relationship of monocot mannose-binding lectins. Plant Physiology, 112, 1531–1540.