TSG-6, AN ARTHRITIS-ASSOCIATED HYALURONAN-BINDING PROTEIN, FORMS A STABLE COMPLEX WITH THE SERUM-PROTEIN INTER-ALPHA-INHIBITOR

TSG-6 is a secreted 35-kDa glycoprotein, inducible by TNF and IL-1. The N-terminal portion of TSG-6 shows sequence homology to members of the cartilage link protein family of hyaluronan binding proteins. The C-terminal half of TSG-6 contains a so-called CUB domain, characteristic for developmentally regulated proteins. High levels of TSG-6 protein are found in the synovial fluid of patients with rheumatoid arthritis and some other arthritic diseases. Here we show that TSG-6 readily formed a complex with a protein present in human, bovine, rabbit, and mouse serum. This complex was stable during SDS-PAGE under reducing conditions, and in the presence of 8 M urea. The protein that binds TSG-6 was purified from human serum and identified as inter-alpha-inhibitor (I alpha I) by N-terminal microsequencing. Microsequencing of the complex itself revealed the presence of TSG-6 and two of the three polypeptide chains of I alpha I (bikunin and HC2). Experiments with recombinant TSG-6 and I alpha I purified from human serum showed that the TSG-6/I alpha I complex is rapidly formed even in the apparent absence of other proteins at 37 degrees C, but not at 4 degrees C. The TSG-6/I alpha I complex was cleaved by chondroitin sulfate ABC lyase, suggesting that cross-linking by chondroitin sulfate is required for the stability of the complex. Although the functional consequences of TSG-6/I alpha I complex formation are presently unknown, the fact that TSG-6 forms similar complexes with I alpha I from several animal species indicates that the components involved in this interaction are conserved in evolution.