The social amoeba Dictyostelium discoideum adopts a cohesive stage upon
starvation and then produces Discoidin I and II, two proteins able to bind
galactose and N-acetyl-galactosamine. The N-terminal domain or discoidin domain
(DS) is widely distributed in eukaryotes where it plays a role in extracellular
matrix binding while the C-terminal domain displays sequence similarities to
invertebrate lectins. We present the first X-ray structures of the wild-type and
recombinant Discoidin II in unliganded state and in complex with
monosaccharides. The protein forms a homotrimer which presents two binding
surfaces situated on the opposite boundaries of the structure. The binding sites
of the N-terminal domain contain PEG molecules that could mimics binding of
natural ligand. The C-terminal lectin domain interactions with
N-acetyl-D-galactosamine and methyl-beta-galactoside are described. The
carbohydrate binding sites are located at the interface between monomers.
Specificity for galacto configuration can be rationalized since the axial O4
hydroxyl group is involved in several hydrogen bonds with protein side chains.
Titration microcalorimetry allowed characterization of affinity and demonstrated
the enthalpy-driven character of the interaction. Those results highlight the
structural differentiation of the DS domain involved in many cell-adhesion
processes from the lectin activity of Dictyostelium discoidins.
