Narcissus and Daffodil

386 E.J.M. Van Damme and W.J. Peumans
(Asp and Asn), O3 (Gln) and O4 (Tyr) of mannose through a network of four
hydrogen bonds. Another hydrophobic residue, namely Val, interacts with C3 and
C4 of mannose through hydrophobic interactions.
Recently the narcissus lectin has been crystallised, and its crystal structure in
complex with α1,3 mannobiose has been determined by X-ray crystallography at
2 Å resolution (Sauerborn et al., 1999).
CARBOHYDRATE-BINDING PROPERTIES
The carbohydrate-binding specificity of the narcissus lectin was assessed by quantitative
inhibition, sugar hapten inhibition assays using a series of simple sugars,
and affinity chromatography of glycoconjugates on the immobilised lectin (Kaku
et al., 1990). Of all monosaccharides tested, only D-mannose was inhibitory in hapten
inhibition assays. A more detailed study of the carbohydrate-binding specificity
of narcissus lectin revealed that it had the highest affinity for both terminal and
internal α1,6-linked mannosyl residues. The narcissus lectin also strongly precipitates
several yeast mannans (e.g., Saccharomyces cerevisae and Pichea pastoris mannans
containing multiple D-mannosyl side chains attached to the α1,6-linked
mannose backbone) but does not precipitate α-D-glucans. Since oligosaccharides
are better inhibitors than the methyl-α-D-mannoside (e.g., α1,6-linked mannotriose
being twice as good an inhibitor as Manα1,6Manα-O-Me, and ten times better
than methyl-α-D-mannoside), it can be concluded that the lectin possesses an
extended binding site complementary to at least three 1,6-linked α-mannosyl units
(Kaku et al., 1990). Glycosylasparagine glycopeptides containing α1,6-Man units
were retarded on a column with the immobilised narcissus lectin. However, glycopeptides
with hybrid type glycans were not retarded. Therefore the lectin will
prove to be a useful tool for the detection and preliminary characterisation of
glycoconjugates.
It should be indicated that the analyses of the carbohydrate-binding specificity
of the narcissus lectin were performed with a total lectin preparation containing
several isolectins. Affinity chromatography experiments suggested differences in
affinity of different isolectins for a mannose-Sepharose 4B column. Indeed, affinity
chromatography of the lectin from Narcissus ‘Fortune’ on a column of immobilised
mannose and elution of the lectin with a linear gradient of increasing mannose
(0–0.2 M) revealed that some isolectins desorbed from the column at low concentrations
of mannose, whereas other isolectins were still retained on the column
after washing with 0.2 M mannose (Van Damme et al., 1990a). These results clearly
suggest differences in affinity for mannose among the different isoforms of the
narcissus lectin.
BIOLOGICAL ACTIVITIES
The narcissus lectins readily agglutinate rabbit erythrocytes but are completely
inactive towards human red blood cells, irrespective of the blood group. Agglutination
of the rabbit red blood cells is enhanced after treatment of the erythrocytes
with trypsin. The minimum concentrations required for agglutination of