LOCALIZATION OF A CRITICAL DIPHTHERIA TOXIN-BINDING DOMAIN TO THE C-TERMINUS OF THE MATURE HEPARIN-BINDING EGF-LIKE GROWTH-FACTOR REGION OF THE DIPHTHERIA-TOXIN RECEPTOR

The monkey and human heparin-binding EGF-like growth factor precursors are diphtheria toxin receptors. To investigate which portion of this growth factor precursor is responsible for binding the toxin, the monkey precursor was compared to the mouse precursor which does not bind the toxin. Three clusters of differing residues were identified. The human mature growth factor (residues 63-148), which contains two of the three differing clusters, inhibited the binding of radiolabeled toxin to cell-surface receptors thus narrowing the binding domain to 86 of the 208 residue precursor. To define further the binding domain, chimeric mouse/monkey precursors were expressed and assayed for toxin sensitivity. The third cluster, residues 122-135, was found to be the most important region for toxin binding. Total replacement of monkey residues with mouse residues N-terminal to this region yields a highly toxin-sensitive cell. Collectively, the results suggest that the most critical residues for toxin binding lie between residues 122-148. (C) 1995 Academic Press, Inc.