Crystallography and Lectin Structure Database - CNRS

34 U. Krengel and A. Imberty
4.1.1. Fungal lectins – A case study
PVL, the lectin from P. velutina, represents an interesting case study for the molecular
basis of protein–carbohydrate interactions, because of its dual specificity. The
lectin was first described as a GlcNAc-binding protein [98], but later reported to also
bind sialic-acid containing glyconjugates, although with moderate affinity [99].
Three crystal structures have recently been solved for PVL: one concerning the unliganded
state (1.5Å resolution) and two complexes with GlcNAc (2.6Å) and
Neu5Ac (1.8Å) [91]. Electron density for six GlcNAc residues was clearly visible,
indicating that the sugar-binding sites are pockets located in the upper part of the
-propeller, in a space between two consecutive blades, accessible to the solvent.
The weaker binding to NeuAc resulted in lower occupancy, with only two to three
sites occupied per monomer. Comparison of the two binding modes demonstrated
that GlcNAc and NeuAc bind in the same pocket, albeit with different orientation.
In both cases, the N-acetyl group establishes the same hydrogen bonding network
and the same hydrophobic contacts to a histidine residue (Fig. 4).
Such dual specificity of a lectin-binding site for both GlcNAc and Neu5Ac has
already been observed for the isolectins of wheat germ agglutinin. Also in this case,
crystal structures (of WGA2) are available for both ligand complexes: one with
chitobiose [100] and one with Neu5Ac [101]. As observed for PVL, the N-acetyl
group of both ligands is buried in the same fashion in the protein-binding site, while
the orientation of the sugar ring is different. These two cases illustrate the fact that
the protein recognizes a scaffold of hydroxyl groups (and other prominent
functional groups like N-acetyl or methyl groups). Since the stereochemistry of carbohydrates
allows for different configuration at each position of the ring, it may
happen that different monosaccharides, viewed from different perspectives, present
similar arrangements of bioactive groups. Such an observation opens the route for
the design of non-carbohydrate mimetics that present the same active groups but
carried by different scaffolds, such as peptides.
4.2. Bacterial lectins
Bacteria use several strategies for targeting host sugars. Some bacteria use lectins
as virulence factors that enable the bacteria to recognize and bind to the glycoconjugate
receptors (glycolipids or glycoproteins) on the surface of their host
cells, in a first step to confer toxicity. Bacterial toxins that contain lectin domains
have been crystallized from several pathogenic organisms such as Vibrio cholera,
enterotoxigenic E. coli, and Bordella pertussis. They are referred to as AB 5 -type
proteins since they consist of one toxic ADP-ribosyltransferase subunit and five
lectin domains that bind to gangliosides of gut or lung epithelia [102].
Other lectin domains are located at the top of pili or flagella. Also these lectin
domains play a role in the attachment of the bacteria to epithelial cells. Such
lectins are part of complex multiprotein architectures and are therefore difficult to
express in soluble form and to crystallize. Only a limited number of structures of