Narcissus and Daffodil

384 E.J.M. Van Damme and W.J. Peumans
Figure 16.1 Schematic representation of the biosynthesis, processing and topogenesis
of the narcissus lectin (NPA). The primary translation product Prepro-
NPA is processed in the endoplasmic reticulum (ER) by co-translational
removal of a signal peptide. Upon transport of Pro-NPA to the vacuole a
C-terminal propeptide is removed, resulting in mature NPA.
reading frame encoding a preprolectin. This precursor contained, besides the
coding sequence of the mature lectin of 109 amino acids, a signal peptide (24
amino acids) and a C-terminal peptide (30 or 38 amino acids) which are co- and
post-translationally removed, respectively (Figure 16.1; Van Damme and Peumans,
1988). The calculated molecular mass of the mature narcissus lectin polypeptides
varied between 11.6 and 11.9 kDa, which is in good agreement with the molecular
mass of 12.5 kDa determined by SDS-PAGE (Van Damme et al., 1988).
Interestingly, some differences in the deduced amino acid sequences of the
different cDNA clones resulted in different charges along the lectin polypeptides,
resulting in isoelectric points ranging from 3.66 to 4.24 for the mature polypeptides
encoding the narcissus lectin. These results indicated that the different cDNA
clones encoded isolectins with different isoelectric points. Hence the detailed
sequence analysis of different cDNA clones explained the occurrence of multiple
isolectins at the molecular level. Furthermore, since Southern blot analysis of
genomic DNA isolated from narcissus yielded numerous restriction fragments
hybridising with the lectin cDNA probe, it was evident that the expression of
the isolectins is under the control of a family of closely related lectin genes
(Van Damme et al., 1992).
Three-dimensional structure of the Narcissus lectin
A detailed analysis of the sequence encoding the mature narcissus lectin revealed
that the sequence shows strong homology with the sequence of snowdrop lectin,
and likewise is also composed of three very homologous domains (Figure 16.2),
each of which contains a carbohydrate-binding site (see below).
Molecular modelling of the deduced amino acid sequence of the mature
narcissus lectin (derived from the cDNA sequence) using the co-ordinates of the
related snowdrop lectin as a model, allowed the determination of the threedimensional
structure of the narcissus lectin (Barre et al., 1996). Like the snowdrop
lectin, the three-dimensional structure of the narcissus lectin (Figure 16.3) corresponds
to a β-barrel built up of three antiparallel four-stranded β-sheets (domains)
interconnected by loops (Hester et al., 1995). A detailed comparison of the
snowdrop and narcissus lectin sequences revealed that the residues forming the