Integrin-specific tissue-type plasminogen activator engineered by introduction of the Arg-Gly-Asp sequence.

To tailor tissue-type plasminogen activator (tPA) to possess an affinity for the integrins, several mutants were constructed by introducing the Arg-Gly-Asp (RGD) sequene into the tPA molecule. These mutants were expressed in COS-1 cells and partially purified by lysine-Sepharose chromatography. The RGD-dependent binding of the mutants to platelet integrin, integrin alpha IIb beta 3, was evaluated by subtracting the nonspecific binding in the presence of 10 mM EDTA (or 1 mg/ml GRGDSP). The binding assay showed that two tPA mutants possess high affinity for the integrin in an RGD-dependent manner. One mutant is 148RGD-tPA with RGDS in place of DRDS (residues 148 to 151) in the loop region of the kringle 1 domain of tPA, and the other is 270RGD-tPA with RGDS in place of SQPQ (residues 270 to 273) in the linker region between the kringle 2 and protease domains. Using the chromogenic substrate Spectrozyme tPA, the 148RGD-tPA mutant was shown to possess amidolytic activity comparable with that of native tPA, while the 270RGD-tPA mutant exhibited several-fold lower activity. In addition, the 148RGD-tPA exhibited full tPA activity even when interacting with the integrin alpha IIb beta 3. These results suggest that the bifunctional 148RGD-tPA molecule might be useful as an improved thrombolytic agent specific for the platelet integrin, the integrin alpha IIb beta 3.