Processing of complex between urokinase and its type‐2 inhibitor on the cell surface

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Abstract
Complexes between the urokinase‐type plasminogen activator (uPA) and its type‐2 inhibitor (PAI‐2) are bound by a cell‐surface receptor for uPA and rapidly cleaved into two fragments of 70 and 22 kDa. The 70‐kDa fragment contains the active site of uPA and PAI‐2, while the 22‐kDa species was identified as the amino terminal fragment of uPA, that binds specifically to the receptor. When the experiment is performed at 4°C, both fragments remain bound to the cell surface and can be eluted by acid treatment. We therefore postulate that after the binding of the uPA‐PAI‐2 complex, a new binding site for the 70‐kDa species becomes available. This additional binding favours the cleavage of the complex into the 70‐and 22‐kDa fragments; the 70‐kDa species is endocytosed or released, while the 22‐kDa fragment remains on the cell surface to prevent the binding of intact uPA.