Analysis of a two-domain binding site for the urokinase-type plasminogen activator-plasminogen activator inhibitor-1 complex in low-density-lipoprotein-receptor-related protein

The low-density-lipoprotein-receptor (LDLR)-related protein (LRP) is compos ed of several classes of domains, including complement-type repeats (CR), w hich occur in clusters that contain binding sites for a multitude of differ ent ligands. Each approximate to 40-residue CR domain contains three conser ved disulphide linkages and an octahedral Ca2+ cage. LRP is a scavenging re ceptor for ligands from extracellular fluids, e.g. alpha (2)-macroglobulin (alpha M-2)-proteinase complexes. lipoprotein-containing particles and seri ne proteinase-inhibitor complexes, like the complex between urokinase-type plasminogen activator (uPA) and the plasminogen activator inhibitor-1 (PAI- I). In the present study we analysed the interaction of the uPA-PAI-1 compl ex with an ensemble of fragments representing a complete overlapping set of two-domain fragments accounting for the ligand-binding cluster II (CR3-CR1 0) of LRP, By ligand blotting, solid-state competition analysis and surface -plasmon-resonance analysis, we demonstrate binding to multiple CR domains, but show a preferential interaction between the uPAPAI-1 complex and a two -domain fragment comprising CR domains 5 and 6 of LRP. We demonstrate that surface-exposed aspartic acid and tryptophan residues at identical position s in the two homologous domains, CR5 and CR6 (Asp(958.CR5), Asp(999.CR6), T rp(953.CR5) and Trp(994.CR6)), are critical for the binding of the complex as well as for the binding of the receptor-associated protein (RAP)- the fo lding chaperone/escort protein required for transport of LRP to the cell su rface. Accordingly, the present work provides (1) an identification of a pr eferred binding site within LRP CR cluster II: (2) evidence that the uPA-PA I-1 binding sire involves residues from two adjacent protein domains; and ( 3) direct evidence identifying specific residues as important for the bindi ng of uPa-PAI-1 as well as for the binding of RAP.