Endocytosis of a chimera between human pro-urokinase and the plant toxin saporin: an unusual internalization mechanism

A fluorescent derivative of a chimeric toxin between human pro-urokinase an d the plant ribosome-inactivating protein saporin (p-uPA-Sap(TRITC)), has b een prepared in order to study the endocytosis of this potentially antimeta static conjugate in the murine model cell line LB6 clone19 (Cl19) transfect ed with the human urokinase receptor gene. The physiological internalizatio n of urokinase-inhibitor complexes is triggered by the interaction of plasm inogen inhibitors (PAIs) with receptors belonging to the low density lipopr otein-related receptor protein (LRP) family, and involves a macro-quaternar y structure including uPAR, LRP, and PAIs. However, in contrast to this mec hanism, we observed a two-step process: first, the urokinase receptor (uPAR ) acts as the anchoring factor on the plasma membrane; subsequently, LRP ac ts as the endocytic trigger. Once the chimera is bound to the plasma membra ne by interaction with uPAR, we suggest that a possible exchange may occur to transfer the toxin to LRP via the saporin moiety and begin the internali zation. So an unusual endocytic process is described, where the toxin enter s the cell via a receptor different from that used to bind the plasma membr ane.