Structural domains of human tissue-type plasminogen activator that confer stimulation by heparin

Heparin has been shown recently to stimulate the activity of human tissue-type plasminogen activator (t-PA). To investigate this effect further, mutant proteins lacking various domains of t-PA were screened for the ability to be stimulated by heparin. Those mutants harboring either the finger domain or the 2nd kringle were found to have enhanced enzymatic activity in the presence of heparin. Only mutants containing these structures would bind to heparin-agarose beads; monoclonal antibodies directed against these domains blocked binding. The stimulatory effect of heparin was more pronounced in finger-containing mutants than kringle-2 proteins. Earlier results had localized the fibrin-binding domains to the same two structures. Unlike heparin, the 2nd kringle was shown to be more important than the finger for fibrin stimulation. Our results have implications for producing recombinant t-PA variants for use in thrombolytic therapy.