Identification of an endothelial cell binding site on kininogen domain D3.

High and low molecular mass kininogen, two multidomain plasma proteins, bind to endothelial cells, platelets, and neutrophils in the intravascular compartment. The specific cell attachment site on their common heavy chain is mediated by domain-3, a cystatin-like structure with inhibitory capacity for papain-like proteinases (Jiang, Y., Müller-Esterl, W., and Schmaier, A. H. (1992) J. Biol. Chem. 267, 3712-3717). In this report, the domain-3 cell binding site is determined by an antibody-directed strategy. The epitope of monoclonal antibody HKH15, which binds to domain-3 and blocks the binding of kininogens to platelets and endothelial cells, was mapped using seven synthetic peptides, which span the entire domain-3 sequence. One peptide, LDC27, specifically bound to HKH15. Fine mapping of the epitope of HKH15 revealed that a minimal 13-residue segment in LDC27, named CNA13, is the antibody binding site. LDC27 and CNA13 inhibited biotinylated high molecular mass kininogen binding to endothelial cells with apparent IC50 values of 60.3 +/- 12 and 113.3 +/- 63.7 microM, respectively. Carboxymethylated papain and affinity-purified anti-LDC27 polyclonal antibodies also inhibited the binding of biotinylated high molecular mass kininogen to endothelial cells with an apparent IC50 of 1.04 microM and 59 nM, respectively. Biotinylated LDC27 itself directly bound to endothelial cells, and domain-3 inhibited biotinylated LDC27 binding to human umbilical vein endothelial cells with an IC50 of 41 nM. Using the crystalline structure of cystatin to computer model domain-3, LDC27 and CNA13 were located in the second hairpin loop of the reactive site of cystatin-like proteins (Bode, W., Engh, R., Musil, D., Thiele, U. Huber, R., Karshikov, A., Brzin, J., Kos, J., and Turk, V. (1988) EMBO J. 7, 2593-2599). These results indicate that the major endothelial cell attachment site on kininogen domain-3 is located on its carboxyl-terminal portion and that it overlaps its cysteine protease inhibitory region.