title: Urokinase binding to laminin-nidogen. Structural requirements and interactions with heparin.
abstract: Recently we have shown that heparin and related sulfated polyanions are low-affinity ligands of the kringle domain in the amino-terminal region (ATF) of human urokinase (u-PA), and proposed that this may facilitate loading of u-PA onto its receptor at the focal contacts between adherent cells and their matrix. We have now tested other components of the cell matrix (fibronectin, vitronectin, thrombospondin and laminin-nidogen) for u-PA binding, and found that laminin-nidogen is also a ligand of the u-PA ATF. Direct binding assays and competition binding assays with defined fragments of laminin-nidogen showed that there are u-PA binding sites in fragment E4 of laminin as well as in nidogen. The long-arm terminal domain of laminin (fragment E3), which contains a heparin-binding site, competed for binding of u-PA to immobilised heparin. However nidogen, which does not bind to heparin, also inhibited binding of u-PA to heparin, and this effect was also observed with recombinant nidogen and with a fragment of nidogen lacking the carboxy-terminal domain. Direct binding assays confirmed that u-PA binds to nidogen through a site in the u-PA ATF. We conclude that u-PA binds to laminin-nidogen by interactions involving the ATF region of u-PA, the E4 domain of laminin and the rod or amino-terminal regions of nidogen. Since nidogen is suggested to be an important bridging molecule in the maintenance of the supramolecular organization in basement membranes, the presence of a binding site for u-PA in nidogen indicates a role for plasminogen activation in basement membrane remodelling.
authors.affiliation: Department of Virology, University of Helsinki, Finland.
authors.forename: R W
authors.initials: RW
authors.lastname: Stephens
authors.forename: M
authors.initials: M
authors.lastname: Aumailley
authors.forename: R
authors.initials: R
authors.lastname: Timpl
authors.forename: T
authors.initials: T
authors.lastname: Reisberg
authors.forename: H
authors.initials: H
authors.lastname: Tapiovaara
authors.forename: H
authors.initials: H
authors.lastname: Myöhänen
authors.forename: J
authors.initials: J
authors.lastname: Murphy-Ullrich
authors.forename: A
authors.initials: A
authors.lastname: Vaheri
mesh_terms: D001667:Binding, Competitive; D006493:Heparin / Q000378:metabolism*; D006801:Humans; D007797:Laminin / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism*; D014568:Urokinase-Type Plasminogen Activator / Q000378:metabolism*
chemical_list: D007797:Laminin; D008562:Membrane Glycoproteins; C029967:nidogen; D006493:Heparin; D014568:Urokinase-Type Plasminogen Activator