Interaction of tissue plasminogen activator with a human endothelial cell 45-kilodalton plasminogen receptor

We have investigated the interaction of tissue plasminogen activator (tPA) with endothelial cell proteins of the human umbilical vein using the technique of ligand blotting. It was observed that tPA interacted with a 45-kilodalton (kDa) endothelial cell protein which appeared to be similar to the 45-kDa plasminogen receptor. Binding of tPA to the 45-kDa protein could be inhibited by excess cold tPA. Moreover, excess lysine could inhibit the binding of tPA to the 45-kDa protein in both coincubation and reversibility experiments. These studies indicated that like plasminogen, tPA interacts with the 45-kDa protein in a kringle-dependent and specific manner. To confirm that tPA and plasminogen are interacting with the same protein, we investigated the effect of excess cold plasminogen on tPA binding and excess cold tPA on plasminogen binding in reversibility experiments. It was observed that binding of tPA to the 45-kDa protein was reduced by plasminogen and vice versa. In addition, the 45-kDa protein did not cross-react with antibodies to annexin II, a 40-kDa protein that binds plasminogen and tPA. These latter properties distinguish the 45-kDa receptor from plasminogen/tPA-binding proteins described by others. Therefore, the above studies suggest that the 45-kDa protein represents a unique plasminogen/tPA receptor on human venous endothelial cells.