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 4 5-kDa protein could be inhibited by excess cold tPA. Morever, excess l ysine could inhibit the binding of tPA to the 45-kDa protein in both c oincubation and reversibility experiments. These studies indicated tha t like plasminogen, tPA interacts with the 45-kDa protein in a kringle -dependent and specific manner. To confirm that tPA and plasminogen ar e interacting with the same protein, we investigated the effect of exc ess 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 la tter properties distinguish the 45-kDa receptor from plasminogen/tPA-b inding proteins described by others. Therefore, the above studies sugg est that the 45-kDa protein represents a unique plasminogen/tPA recept or on human venous endothelial cells.