TSG-6, the secreted product of
tumor necrosis factor-stimulated gene-6, is not constitutively expressed but is up-regulated in various cell-types during inflammatory and
inflammation-like processes. The mature
protein is comprised largely of contiguous Link and CUB modules, the former binding several matrix components such as
hyaluronan (HA) and
aggrecan. Here we show that this domain can also associate with the
glycosaminoglycan heparin/
heparan sulfate. Docking predictions and site-directed mutagenesis demonstrate that this occurs at a site distinct from the HA binding surface and is likely to involve extensive electrostatic contacts. Despite these
glycosaminoglycans binding to non-overlapping sites on the Link module, the interaction of
heparin can inhibit subsequent binding to HA, and it is possible that this occurs via an allosteric mechanism. We also show that
heparin can modify another property of the Link module, i.e. its potentiation of the anti-
plasmin activity of
inter-alpha-inhibitor (IalphaI). Experiments using the purified components of IalphaI indicate that TSG-6 only binds to the bikunin chain and that this is at a site on the Link module that overlaps the HA binding surface. The association of
heparin with the Link module significantly increases the anti-
plasmin activity of the TSG-6.IalphaI complex. Changes in
plasmin activity have been observed previously at sites of TSG-6 expression, and the results presented here suggest that TSG-6 is likely to contribute to matrix remodeling, at least in part, through down-regulation of the
protease network, especially in locations containing
heparin/
heparan sulfate proteoglycans. The differential effects of HA and
heparin on TSG-6 function provide a mechanism for its regulation and functional partitioning in particular tissue microenvironments.