Localization and characterization of the hyaluronan-binding site on the link module from human TSG-6

The interactions of hyaluronan (HA) with proteins are important in extracellular matrix integrity and leukocyte migration and are usually mediated by a domain termed a Link module. Although the tertiary structure of a Link module has been determined, the molecular basis of HA-protein interactions remains poorly understood.

Isothermal titration calorimetry was used to characterize the interaction of the Link module from human TSG-6 (Link_TSG6) with HA oligosaccharides of defined length (HA(4)-HA(16)). All oligomers bound (except HA(4)) with K(d) values ranging from 0.2-0.5 microM at 25 degrees C. The reaction is exothermic with a favourable entropy and the thermodynamic profile is similar to those of other glycosaminoglycan-protein interactions. The HA(8) recognition site on Link_TSG6 was localized by comparing nuclear magnetic resonance (NMR) spectra from a 1:1 complex with free protein. Residues perturbed on HA binding include both amino acids that are likely to be directly involved in the interaction (i.e., Lys11, Tyr59, Asn67, Phe70, Lys72 and Tyr78) and those affected by a ligand-induced conformational change in the beta4/beta5 loop. The sidechain of Asn67 becomes more rigid in the complex suggesting that it is in close proximity to the binding site.

In TSG-6 a single Link module is sufficient for a high-affinity interaction with HA. The HA-binding surface on Link_TSG6 is found in a similar position to that suggested previously for CD44, indicating that its location might be conserved across the Link module superfamily. Here we find no evidence for the involvement of linear sequence motifs in HA binding.