INTERACTION OF SINGLE-CHAIN UROKINASE WITH ITS RECEPTOR INDUCES THE APPEARANCE AND DISAPPEARANCE OF BINDING EPITOPES WITHIN THE RESULTANT COMPLEX FOR OTHER CELL-SURFACE PROTEINS

Binding of urokinase-type plasminogen activator (uPA) to its glycosylp hosphatidylinositol-anchored receptor (uPAR) initiates signal transduc tion, adhesion, and migration in certain cell types. To determine whet her some of these activities may be mediated by associations between t he uPA/uPAR complex and other cell surface proteins, we studied the bi nding of complexes composed of recombinant soluble uPA receptor (suPAR ) and single chain uPA (scuPA) to a cell line (LM-TK fibroblasts) that does not express glycosylphosphatidylinositol (GPI)-anchored proteins to eliminate potential competition by endogenous uPA receptors. scuPA induced the binding of suPAR to LM-TK- cells. Binding of labeled suPA R/scuPA was inhibited by unlabeled complex, but not by scuPA or suPAR added separately, indicating cellular binding:sites had been formed th at are not present in either component. Binding of the complex was inh ibited by low molecular weight UPA (LMW-uPA) indicating exposure of an epitope found normally in the isolated B chain of two chain uPA (tcuP A), but hidden in soluble scuPA. Binding of LMW-uPA was independent of its catalytic site and was associated with retention of ifs enzymatic activity. Additional cell binding epitopes were generated within suPA R itself by the aminoterminal fragment of scuPA, which itself does not bind to LM-TK- cells. When scuPA bound to suPAR, a binding site for a lpha(2)-macroglobulin receptor/LDL receptor-related protein (alpha(2)M R/LRP) was rest, while binding sites for cell-associated vitronectin a nd thrombospondin were induced. In accord with this, the internalizati on and degradation of cell-associated tcuPA and tcuPA-PAI-1 complexes proceeded less efficiently in the presence of suPAR. Further, little d egradation of suPAR was detected, suggesting that cell-bound complex d issociated during the initial stages of endocytosis, Thus, the interac tion of scuPA with its receptor causes multiple functional changes wit hin the complex including the disappearance of an epitope in scuPA inv olved in its clearance from the cell surface and the generation of nov el epitopes that promote its binding to proteins involved in cell adhe sion and signal transduction. (C) 1996 by The American Society of Hema tology.