Identity of urinary trypsin inhibitor-binding protein to link protein

Urinary trypsin inhibitor (UTI), a Kunitz-type protease inhibitor, directly binds to some types of cells via cell-associated UTI-binding proteins (UTI-BPs). Here we report that the 49-kDa protein (UTI-BP₄₀) was purified from the cultured human chondrosarcoma cell line HCS-2/8 by UTI affinity chromatography. Purified UTI- BP₄₀ was digested with trypsin, and the amino acid sequences of the peptide fragments were determined. The sequences of six tryptic fragments of UTI-BP₄₀ were identical to subsequences present in human link protein (LP). Authentic bovine LP and UTI- BP40 displayed identical electrophoretic and chromatographic behavior. The UTI-binding properties of UTI-BP₄₀ and LP were indistinguishable. Direct binding and competition studies strongly demonstrated that the NH₂-terminal fragment is the UTI-binding part of the LP molecule, that the COOH-terminal UTI fragment (HI-8) failed to bind the NH₂-terminal subdomain of the LP molecule, and that LP and UTI-BP₄₀ exhibited significant hyaluronic acid binding. These results demonstrate that UTI-BP₄₀ is identical to LP and that the NH₂-terminal domain of UTI is involved in the interaction with the NH₂-terminal fragment of LP, which is bound to hyaluronic acid in the extracellular matrix.