Common evolutionary origin of the fibrin‐binding structures of fibronectin and tissue‐type plasminogen activator

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Abstract
Comparison of the primary structures of high-M r urokinase and tissue-type plasminogen activator reveals a high degree of structural homology between the two proteins, except that tissue activator contains a 43 residue long amino-terminal region, which has no counterpart in urokinase. We show that this segment is homologous with the finger-domains responsible for the fibrin-affinity of fibronectin. Limited proteolysis of the amino-terminal region of plasminogen activator was found to lead to a loss of the fibrin-affinity of the enzyme. It is suggested that the finger-domains of fibronectin and tissue-types plasminogen activator have similar functions and that the finger-domains of the two proteins evolved from a common ancestral fibrin-binding domain.