DIPHTHERIA TOXIN-RECEPTOR INTERACTION - ASSOCIATION, DISSOCIATION, AND EFFECT OF PH

Diphtheria toxin (DT) binds to a specific heparin-binding epidermal gr owth factor-like growth factor (HB-EGF) precursor that is expressed in DT-sensitive cells. DT binds to the cell-surface HB-EGF precursor wit h an apparent dissociation constant (K-D) of approximate to 1 x 10(-8) - 10(-9) M at 4 degrees C, a temperature at which toxin binds but is not internalized. The interaction of DT with the cell-surface receptor , however, may be influenced by other cell-surface components. We used a biosensor method to measure the binding of DT to immobilized recomb inant human HB-EGF (hHB-EGF) at 25 degrees C with no other cellular co mponents present. We observed that at pH 7.4, using this in vitro two component system, DT binds to hHB-EGF with an apparent K-D of 2.7 x 10 (-8) RI. We also observed that the dissociation of DT from hHB-EGF at pH values that approach those of the endosome occurs at a faster rate as the pH is decreased. These results suggest that the low pH of the e ndosome is sufficient to allow DT to dissociate from the HB-EGF precur sor, prior to the translocation of the enzymatically active fragment o f DT into the cytosol. (C) 1998 Academic Press.