Heavy chain-1 of inter-α-inhibitor has an integrin-like structure with immune regulatory activities

Inter--inhibitor (II) is a proteoglycan essential for mammalian reproduction that also plays a less well-characterised role in inflammation. II is composed of 2 homologous heavy chains (HC1 and HC2) covalently attached to chondroitin sulphate on the bikunin core protein. Prior to ovulation HCs are transferred onto the polysaccharide hyaluronan (HA), thereby stabilising a matrix that is required for fertilisation. Here we show that human HC1 has a structure similar to integrin {beta}-chains and contains a functional MIDAS (metal ion-dependent adhesion site) motif that can mediate self-association of heavy chains, providing a mechanism for matrix crosslinking. Surprisingly, its interaction with RGD-containing integrin ligands, such as vitronectin and the latency-associated peptides of TGF{beta}, occurs in a MIDAS/cation-independent manner. However, HC1 utilises its MIDAS motif to bind to, and inhibit the cleavage of, complement C3, thus identifying it as a novel regulator of innate immunity through inhibition of the alternative pathway C3 convertase.\n\nAbbreviationsADPs, atomic displacement parameter; AUC, analytical ultracentrifugation; CMG2, capillary morphogenesis protein-2; COC, cumulus-oocyte complex; CS, chondroitin sulphate; FB, complement factor B; FnIII; fibronectin type III; HA, hyaluronan; HC, heavy chain; HC*HA, covalent complex of HC with HA; II, inter--inhibitor; ITGA, integrin -chain; ITGB, integrin {beta}-chain; LAP, latency associated peptide; LLC, large latent complex; LTBP, latent TGF{beta} binding protein; MIDAS, metal ion-dependent adhesion site; PI, pre--inhibitor; PTX3, pentraxin-3; rHC1, recombinant HC1; SAXS, small-angle X-ray scattering; SHAP, serum-derived HA binding protein; SLC, small latent complex; TEM8, tumour endothelial marker-8; TGF{beta}, transforming factor {beta}; TSG-6, tumour necrosis factor-stimulated gene-6; TSG-6*HC, covalent complex of TSG-6 and HC; vWFA domain, von Willebrand Factor A domain.