Characterization of a chimeric plasminogen activator consisting of amino acids 1 to 274 of tissue-type plasminogen activator and amino acids 138 to 411 of single-chain urokinase-type plasminogen activator.

A chimeric plasminogen activator (t-PA/scu-PA-s), consisting of amino acids 1-263 of tissue-type plasminogen activator (t-PA) and 144-411 of single-chain urokinase-type plasminogen activator (scu-PA), was previously shown to maintain the enzymatic properties of scu-PA but to have only partially acquired the fibrin affinity of t-PA, possibly as a result of steric interaction between the functional domains of t-PA and scu-PA (Nelles, L., Lijnen, H. R., Collen, D., and Holmes, W.E. (1987) J. Biol. Chem. 262, 10855-10862). Therefore, we now have constructed an extended chimeric t-PA/scu-PA protein, consisting of amino acids 1-274 of t-PA and 138-411 of scu-PA, which thus has an additional sequence of 17 residues in the region joining the two proteins. The highly purified extended chimeric protein (t-PA/scu-PA-e) was found to have similar specific activity on fibrin film (65,000 IU/mg), kinetic constants for the activation of plasminogen (Km = 1 microM, k2 = 0.0026 s-1), fibrin affinity (50% binding at a fibrin concentration of 3.3 g/liter), and fibrin specificity of clot lysis in a plasma environment (50% lysis in 2 h with 8 nM of the chimer) as the previously characterized chimeric protein (t-PA/scu-PA-s). Thus, unexpectedly, the fibrin affinity of t-PA is also only partially expressed in this extended chimeric protein. Therefore, the NH2-terminal chains (A-chains) of the plasmin-generated two-chain derivatives t-PA/tcu-PA-e, t-PA/tcu-PA-s, and of t-PA were isolated. These A-chain structures of the chimers were found to have lost most of their fibrin affinity, whereas the fibrin affinity of the A-chain of native t-PA was maintained. Differential reactivity of the A-chain structures of both chimeric molecules with monoclonal antibodies directed against the A-chain of t-PA suggested that they were conformationally altered. Sequential fibrin binding experiments with t-PA/scu-PA-e and t-PA/scu-PA-s yielded 45 +/- 8 (n = 11) and 43 +/- 5% (n = 8), respectively, binding in the first cycle and 44 +/- 7 (n = 11) and 27 +/- 10% (n = 8), respectively, binding in the second cycle. This suggests that the low affinity of the chimeric molecules for fibrin is not due to the occurrence of subpopulations of molecules with different fibrin affinity but, instead, to a uniformly decreased fibrin affinity in all molecules.